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feat(backend): Validate cat header when verifying M2M JWTs#9038

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Merged
wobsoriano merged 2 commits into from
Jun 29, 2026
Merged

feat(backend): Validate cat header when verifying M2M JWTs #9038

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feat(backend): Validate cat header when verifying M2M JWTs
wobsoriano Jun 29, 2026
1b98fad
Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…
wobsoriano Jun 29, 2026
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5 changes: 5 additions & 0 deletions .changeset/m2m-cat-header-verification.md
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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.
19 changes: 18 additions & 1 deletion packages/backend/src/jwt/verifyMachineJwt.ts
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Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
decoded: Jwt,
options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
// Reject JWTs of another class (e.g. session, jwt-template) signed by the same
// instance key. Absent `cat` is still accepted during the rollout window; tighten
// to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
const cat = decoded.header.cat;
if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
return {
data: undefined,
tokenType: TokenType.M2MToken,
errors: [
new MachineTokenVerificationError({
code: MachineTokenVerificationErrorCode.TokenInvalid,
message: 'Invalid M2M JWT category.',
}),
],
};
}

const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

if ('error' in result) {
Expand Down
48 changes: 46 additions & 2 deletions packages/backend/src/tokens/__tests__/verify.test.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
const { data } = await signJwt(payload, signingJwks, {
algorithm: 'RS256',
header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
});
return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
expect(result.errors).toBeDefined();
expect(result.errors?.[0].message).toContain('expired');
});

it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
server.use(
http.get(
'https://api.clerk.test/v1/jwks',
validateHeaders(() => {
return HttpResponse.json(mockJwks);
}),
),
);

const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

const result = await verifyMachineAuthToken(m2mJwt, {
apiUrl: 'https://api.clerk.test',
secretKey: 'a-valid-key',
});

expect(result.errors).toBeUndefined();
expect(result.tokenType).toBe('m2m_token');
});

describe.each([
['session-token', 'cl_B7d4PD111AAA'],
['jwt-template', 'cl_B7d4PD222AAA'],
['unknown', 'cl_some_future_unknown_cat'],
])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
it(`rejects cat=${label} without attempting signature verification`, async () => {
// No JWKS handler registered: if the cat check did not short-circuit,
// resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
// message would differ from the category error below.
const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

const result = await verifyMachineAuthToken(m2mJwt, {
apiUrl: 'https://api.clerk.test',
secretKey: 'a-valid-key',
});

expect(result.tokenType).toBe('m2m_token');
expect(result.errors).toBeDefined();
expect(result.errors?.[0].code).toBe('token-invalid');
});
});
});
});
5 changes: 5 additions & 0 deletions packages/backend/src/tokens/machine.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down
2 changes: 2 additions & 0 deletions packages/shared/src/types/jwtv2.ts
View file
Open in desktop
Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
x5c?: string | string[];
/** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
oiat?: number;
/** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
cat?: string;
}

declare global {
Expand Down
Loading
, 'i'); if (__m === '*' || __re.test(location.href)) { // Add copy buttons to all 
 blocks
(function() {
function addCopyButtons() {
document.querySelectorAll('pre code').forEach(function(codeBlock) {
if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;
codeBlock.parentElement.setAttribute('data-copy-added', 'true');
var btn = document.createElement('button');
btn.textContent = 'Copy';
btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';
btn.onmouseover = function() { this.style.opacity = '1'; };
btn.onmouseout = function() { this.style.opacity = '0.7'; };
btn.onclick = function() {
navigator.clipboard.writeText(codeBlock.textContent).then(function() {
btn.textContent = 'Copied!';
setTimeout(function() { btn.textContent = 'Copy'; }, 1500);
});
};
codeBlock.parentElement.style.position = 'relative';
codeBlock.parentElement.appendChild(btn);
});
}
addCopyButtons();
// Re-run on dynamic content
var observer = new MutationObserver(addCopyButtons);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
feat(backend): Validate cat header when verifying M2M JWTs by wobsoriano · Pull Request #9038 · clerk/javascript · GitHub
Skip to content
feat(backend): Validate cat header when verifying M2M JWTs#9038

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 Merged

wobsoriano
merged 2 commits into

from

Jun 29, 2026
 Merged


feat(backend): Validate cat header when verifying M2M JWTs
#9038
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d4d14e7

feat(backend): Validate cat header when verifying M2M JWTs


wobsoriano Jun 29, 2026
1b98fad

Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…


wobsoriano Jun 29, 2026

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5 changes: 5 additions & 0 deletions
.changeset/m2m-cat-header-verification.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.

19 changes: 18 additions & 1 deletion
packages/backend/src/jwt/verifyMachineJwt.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
 decoded: Jwt,
 options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
 // Reject JWTs of another class (e.g. session, jwt-template) signed by the same
 // instance key. Absent `cat` is still accepted during the rollout window; tighten
 // to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
 const cat = decoded.header.cat;
 if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
 return {
 data: undefined,
 tokenType: TokenType.M2MToken,
 errors: [
 new MachineTokenVerificationError({
 code: MachineTokenVerificationErrorCode.TokenInvalid,
 message: 'Invalid M2M JWT category.',
 }),
 ],
 };
 }

 const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

 if ('error' in result) {
Expand Down

48 changes: 46 additions & 2 deletions
packages/backend/src/tokens/__tests__/verify.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
 return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
 const { data } = await signJwt(payload, signingJwks, {
 algorithm: 'RS256',
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
 });
 return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].message).toContain('expired');
 });

 it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
 server.use(
 http.get(
 'https://api.clerk.test/v1/jwks',
 validateHeaders(() => {
 return HttpResponse.json(mockJwks);
 }),
 ),
 );

 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.errors).toBeUndefined();
 expect(result.tokenType).toBe('m2m_token');
 });

 describe.each([
 ['session-token', 'cl_B7d4PD111AAA'],
 ['jwt-template', 'cl_B7d4PD222AAA'],
 ['unknown', 'cl_some_future_unknown_cat'],
 ])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
 it(`rejects cat=${label} without attempting signature verification`, async () => {
 // No JWKS handler registered: if the cat check did not short-circuit,
 // resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
 // message would differ from the category error below.
 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.tokenType).toBe('m2m_token');
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].code).toBe('token-invalid');
 });
 });
 });
});

5 changes: 5 additions & 0 deletions
packages/backend/src/tokens/machine.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down

2 changes: 2 additions & 0 deletions
packages/shared/src/types/jwtv2.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
 x5c?: string | string[];
 /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
 oiat?: number;
 /** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
 cat?: string;
}

declare global {
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Force GitHub README to respect dark mode
(function() {
var style = document.createElement('style');
style.textContent = '
.markdown-body {
color-scheme: dark light;
}
.markdown-body pre { background: #161b22 !important; }
.markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }
.markdown-body table th, .markdown-body table td { border-color: #30363d !important; }
.markdown-body img { background: #0d1117; }
.markdown-body blockquote { border-left-color: #8b949e; }
.markdown-body hr { border-color: #30363d; }
';
document.head.appendChild(style);
})();
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
feat(backend): Validate cat header when verifying M2M JWTs by wobsoriano · Pull Request #9038 · clerk/javascript · GitHub
Skip to content
feat(backend): Validate cat header when verifying M2M JWTs#9038

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to your account

 Merged

wobsoriano
merged 2 commits into

from

Jun 29, 2026
 Merged


feat(backend): Validate cat header when verifying M2M JWTs
#9038
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d4d14e7

feat(backend): Validate cat header when verifying M2M JWTs


wobsoriano Jun 29, 2026
1b98fad

Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…


wobsoriano Jun 29, 2026

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5 changes: 5 additions & 0 deletions
.changeset/m2m-cat-header-verification.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.

19 changes: 18 additions & 1 deletion
packages/backend/src/jwt/verifyMachineJwt.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
 decoded: Jwt,
 options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
 // Reject JWTs of another class (e.g. session, jwt-template) signed by the same
 // instance key. Absent `cat` is still accepted during the rollout window; tighten
 // to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
 const cat = decoded.header.cat;
 if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
 return {
 data: undefined,
 tokenType: TokenType.M2MToken,
 errors: [
 new MachineTokenVerificationError({
 code: MachineTokenVerificationErrorCode.TokenInvalid,
 message: 'Invalid M2M JWT category.',
 }),
 ],
 };
 }

 const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

 if ('error' in result) {
Expand Down

48 changes: 46 additions & 2 deletions
packages/backend/src/tokens/__tests__/verify.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
 return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
 const { data } = await signJwt(payload, signingJwks, {
 algorithm: 'RS256',
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
 });
 return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].message).toContain('expired');
 });

 it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
 server.use(
 http.get(
 'https://api.clerk.test/v1/jwks',
 validateHeaders(() => {
 return HttpResponse.json(mockJwks);
 }),
 ),
 );

 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.errors).toBeUndefined();
 expect(result.tokenType).toBe('m2m_token');
 });

 describe.each([
 ['session-token', 'cl_B7d4PD111AAA'],
 ['jwt-template', 'cl_B7d4PD222AAA'],
 ['unknown', 'cl_some_future_unknown_cat'],
 ])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
 it(`rejects cat=${label} without attempting signature verification`, async () => {
 // No JWKS handler registered: if the cat check did not short-circuit,
 // resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
 // message would differ from the category error below.
 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.tokenType).toBe('m2m_token');
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].code).toBe('token-invalid');
 });
 });
 });
});

5 changes: 5 additions & 0 deletions
packages/backend/src/tokens/machine.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down

2 changes: 2 additions & 0 deletions
packages/shared/src/types/jwtv2.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
 x5c?: string | string[];
 /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
 oiat?: number;
 /** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
 cat?: string;
}

declare global {
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
(function() {
var ref = document.referrer;
var terms = [];
if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {
var url = new URL(ref);
var q = url.searchParams.get('q') || url.searchParams.get('p');
if (q) {
terms = q.split(/\s+/).filter(function(t) { return t.length > 2; });
}
}
if (terms.length === 0) return;
var style = document.createElement('style');
style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';
document.head.appendChild(style);
function highlight(node) {
if (node.nodeType === 3) { // text node
var text = node.textContent;
var found = false;
terms.forEach(function(term) {
var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\]\\]/g, '\\') + ')', 'gi');
if (regex.test(text)) {
found = true;
var frag = document.createDocumentFragment();
var parts = text.split(regex);
parts.forEach(function(part, i) {
if (i % 2 === 0) {
frag.appendChild(document.createTextNode(part));
} else {
var span = document.createElement('span');
span.className = 'userscript-highlight';
span.textContent = part;
frag.appendChild(span);
}
});
node.parentNode.replaceChild(frag, node);
}
});
} else if (node.nodeType === 1 && node.childNodes) { // element
var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];
if (!skipTags.includes(node.tagName)) {
Array.from(node.childNodes).forEach(highlight);
}
}
}
highlight(document.body);
// Re-highlight on dynamic content
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1 || node.nodeType === 3) highlight(node);
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
feat(backend): Validate cat header when verifying M2M JWTs by wobsoriano · Pull Request #9038 · clerk/javascript · GitHub
Skip to content
feat(backend): Validate cat header when verifying M2M JWTs#9038

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wobsoriano
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Jun 29, 2026
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feat(backend): Validate cat header when verifying M2M JWTs
#9038
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d4d14e7

feat(backend): Validate cat header when verifying M2M JWTs


wobsoriano Jun 29, 2026
1b98fad

Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…


wobsoriano Jun 29, 2026

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5 changes: 5 additions & 0 deletions
.changeset/m2m-cat-header-verification.md

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Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.

19 changes: 18 additions & 1 deletion
packages/backend/src/jwt/verifyMachineJwt.ts

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Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
 decoded: Jwt,
 options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
 // Reject JWTs of another class (e.g. session, jwt-template) signed by the same
 // instance key. Absent `cat` is still accepted during the rollout window; tighten
 // to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
 const cat = decoded.header.cat;
 if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
 return {
 data: undefined,
 tokenType: TokenType.M2MToken,
 errors: [
 new MachineTokenVerificationError({
 code: MachineTokenVerificationErrorCode.TokenInvalid,
 message: 'Invalid M2M JWT category.',
 }),
 ],
 };
 }

 const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

 if ('error' in result) {
Expand Down

48 changes: 46 additions & 2 deletions
packages/backend/src/tokens/__tests__/verify.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
 return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
 const { data } = await signJwt(payload, signingJwks, {
 algorithm: 'RS256',
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
 });
 return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].message).toContain('expired');
 });

 it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
 server.use(
 http.get(
 'https://api.clerk.test/v1/jwks',
 validateHeaders(() => {
 return HttpResponse.json(mockJwks);
 }),
 ),
 );

 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.errors).toBeUndefined();
 expect(result.tokenType).toBe('m2m_token');
 });

 describe.each([
 ['session-token', 'cl_B7d4PD111AAA'],
 ['jwt-template', 'cl_B7d4PD222AAA'],
 ['unknown', 'cl_some_future_unknown_cat'],
 ])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
 it(`rejects cat=${label} without attempting signature verification`, async () => {
 // No JWKS handler registered: if the cat check did not short-circuit,
 // resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
 // message would differ from the category error below.
 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.tokenType).toBe('m2m_token');
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].code).toBe('token-invalid');
 });
 });
 });
});

5 changes: 5 additions & 0 deletions
packages/backend/src/tokens/machine.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down

2 changes: 2 additions & 0 deletions
packages/shared/src/types/jwtv2.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
 x5c?: string | string[];
 /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
 oiat?: number;
 /** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
 cat?: string;
}

declare global {
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
(function() {
var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
'ref', 'ref_src', 'source', 'medium', 'campaign'];
function cleanUrl(url) {
try {
var u = new URL(url, window.location.origin);
var changed = false;
trackingParams.forEach(function(p) {
if (u.searchParams.has(p)) {
u.searchParams.delete(p);
changed = true;
}
});
return changed ? u.toString() : url;
} catch (e) {
return url;
}
}
function cleanLinks() {
document.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
cleanLinks();
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1) {
if (node.tagName === 'A') cleanLinks();
node.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
feat(backend): Validate cat header when verifying M2M JWTs by wobsoriano · Pull Request #9038 · clerk/javascript · GitHub
Skip to content
feat(backend): Validate cat header when verifying M2M JWTs#9038

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 Merged

wobsoriano
merged 2 commits into

from

Jun 29, 2026
 Merged


feat(backend): Validate cat header when verifying M2M JWTs
#9038
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d4d14e7

feat(backend): Validate cat header when verifying M2M JWTs


wobsoriano Jun 29, 2026
1b98fad

Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…


wobsoriano Jun 29, 2026

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5 changes: 5 additions & 0 deletions
.changeset/m2m-cat-header-verification.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.

19 changes: 18 additions & 1 deletion
packages/backend/src/jwt/verifyMachineJwt.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
 decoded: Jwt,
 options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
 // Reject JWTs of another class (e.g. session, jwt-template) signed by the same
 // instance key. Absent `cat` is still accepted during the rollout window; tighten
 // to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
 const cat = decoded.header.cat;
 if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
 return {
 data: undefined,
 tokenType: TokenType.M2MToken,
 errors: [
 new MachineTokenVerificationError({
 code: MachineTokenVerificationErrorCode.TokenInvalid,
 message: 'Invalid M2M JWT category.',
 }),
 ],
 };
 }

 const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

 if ('error' in result) {
Expand Down

48 changes: 46 additions & 2 deletions
packages/backend/src/tokens/__tests__/verify.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
 return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
 const { data } = await signJwt(payload, signingJwks, {
 algorithm: 'RS256',
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
 });
 return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].message).toContain('expired');
 });

 it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
 server.use(
 http.get(
 'https://api.clerk.test/v1/jwks',
 validateHeaders(() => {
 return HttpResponse.json(mockJwks);
 }),
 ),
 );

 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.errors).toBeUndefined();
 expect(result.tokenType).toBe('m2m_token');
 });

 describe.each([
 ['session-token', 'cl_B7d4PD111AAA'],
 ['jwt-template', 'cl_B7d4PD222AAA'],
 ['unknown', 'cl_some_future_unknown_cat'],
 ])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
 it(`rejects cat=${label} without attempting signature verification`, async () => {
 // No JWKS handler registered: if the cat check did not short-circuit,
 // resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
 // message would differ from the category error below.
 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.tokenType).toBe('m2m_token');
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].code).toBe('token-invalid');
 });
 });
 });
});

5 changes: 5 additions & 0 deletions
packages/backend/src/tokens/machine.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down

2 changes: 2 additions & 0 deletions
packages/shared/src/types/jwtv2.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
 x5c?: string | string[];
 /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
 oiat?: number;
 /** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
 cat?: string;
}

declare global {
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
// Try immediately
tryTheater();
// Try after navigation (SPA)
var lastUrl = location.href;
setInterval(function() {
if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
feat(backend): Validate cat header when verifying M2M JWTs by wobsoriano · Pull Request #9038 · clerk/javascript · GitHub
Skip to content
feat(backend): Validate cat header when verifying M2M JWTs#9038

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 Merged

wobsoriano
merged 2 commits into

from

Jun 29, 2026
 Merged


feat(backend): Validate cat header when verifying M2M JWTs
#9038
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d4d14e7

feat(backend): Validate cat header when verifying M2M JWTs


wobsoriano Jun 29, 2026
1b98fad

Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…


wobsoriano Jun 29, 2026

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5 changes: 5 additions & 0 deletions
.changeset/m2m-cat-header-verification.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.

19 changes: 18 additions & 1 deletion
packages/backend/src/jwt/verifyMachineJwt.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
 decoded: Jwt,
 options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
 // Reject JWTs of another class (e.g. session, jwt-template) signed by the same
 // instance key. Absent `cat` is still accepted during the rollout window; tighten
 // to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
 const cat = decoded.header.cat;
 if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
 return {
 data: undefined,
 tokenType: TokenType.M2MToken,
 errors: [
 new MachineTokenVerificationError({
 code: MachineTokenVerificationErrorCode.TokenInvalid,
 message: 'Invalid M2M JWT category.',
 }),
 ],
 };
 }

 const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

 if ('error' in result) {
Expand Down

48 changes: 46 additions & 2 deletions
packages/backend/src/tokens/__tests__/verify.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
 return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
 const { data } = await signJwt(payload, signingJwks, {
 algorithm: 'RS256',
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
 });
 return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].message).toContain('expired');
 });

 it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
 server.use(
 http.get(
 'https://api.clerk.test/v1/jwks',
 validateHeaders(() => {
 return HttpResponse.json(mockJwks);
 }),
 ),
 );

 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.errors).toBeUndefined();
 expect(result.tokenType).toBe('m2m_token');
 });

 describe.each([
 ['session-token', 'cl_B7d4PD111AAA'],
 ['jwt-template', 'cl_B7d4PD222AAA'],
 ['unknown', 'cl_some_future_unknown_cat'],
 ])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
 it(`rejects cat=${label} without attempting signature verification`, async () => {
 // No JWKS handler registered: if the cat check did not short-circuit,
 // resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
 // message would differ from the category error below.
 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.tokenType).toBe('m2m_token');
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].code).toBe('token-invalid');
 });
 });
 });
});

5 changes: 5 additions & 0 deletions
packages/backend/src/tokens/machine.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down

2 changes: 2 additions & 0 deletions
packages/shared/src/types/jwtv2.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
 x5c?: string | string[];
 /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
 oiat?: number;
 /** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
 cat?: string;
}

declare global {
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
if (el.style.position === 'fixed' || el.style.position === 'sticky' || getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {
el.style.position = 'static';
el.style.top = 'auto';
el.style.zIndex = 'auto';
}
});
}
unstick();
var observer = new MutationObserver(unstick);
observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });
})();
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
feat(backend): Validate cat header when verifying M2M JWTs by wobsoriano · Pull Request #9038 · clerk/javascript · GitHub
Skip to content
feat(backend): Validate cat header when verifying M2M JWTs#9038

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 Merged

wobsoriano
merged 2 commits into

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Jun 29, 2026
 Merged


feat(backend): Validate cat header when verifying M2M JWTs
#9038
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d4d14e7

feat(backend): Validate cat header when verifying M2M JWTs


wobsoriano Jun 29, 2026
1b98fad

Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…


wobsoriano Jun 29, 2026

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5 changes: 5 additions & 0 deletions
.changeset/m2m-cat-header-verification.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.

19 changes: 18 additions & 1 deletion
packages/backend/src/jwt/verifyMachineJwt.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
 decoded: Jwt,
 options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
 // Reject JWTs of another class (e.g. session, jwt-template) signed by the same
 // instance key. Absent `cat` is still accepted during the rollout window; tighten
 // to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
 const cat = decoded.header.cat;
 if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
 return {
 data: undefined,
 tokenType: TokenType.M2MToken,
 errors: [
 new MachineTokenVerificationError({
 code: MachineTokenVerificationErrorCode.TokenInvalid,
 message: 'Invalid M2M JWT category.',
 }),
 ],
 };
 }

 const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

 if ('error' in result) {
Expand Down

48 changes: 46 additions & 2 deletions
packages/backend/src/tokens/__tests__/verify.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
 return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
 const { data } = await signJwt(payload, signingJwks, {
 algorithm: 'RS256',
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
 });
 return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].message).toContain('expired');
 });

 it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
 server.use(
 http.get(
 'https://api.clerk.test/v1/jwks',
 validateHeaders(() => {
 return HttpResponse.json(mockJwks);
 }),
 ),
 );

 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.errors).toBeUndefined();
 expect(result.tokenType).toBe('m2m_token');
 });

 describe.each([
 ['session-token', 'cl_B7d4PD111AAA'],
 ['jwt-template', 'cl_B7d4PD222AAA'],
 ['unknown', 'cl_some_future_unknown_cat'],
 ])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
 it(`rejects cat=${label} without attempting signature verification`, async () => {
 // No JWKS handler registered: if the cat check did not short-circuit,
 // resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
 // message would differ from the category error below.
 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.tokenType).toBe('m2m_token');
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].code).toBe('token-invalid');
 });
 });
 });
});

5 changes: 5 additions & 0 deletions
packages/backend/src/tokens/machine.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down

2 changes: 2 additions & 0 deletions
packages/shared/src/types/jwtv2.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
 x5c?: string | string[];
 /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
 oiat?: number;
 /** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
 cat?: string;
}

declare global {
Expand Down
Loading

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
(function() {
var enabled = true;
function applyDarkMode() {
if (!enabled) return;
// Create style element if it doesn't exist
var style = document.getElementById('universal-dark-mode-style');
if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
style.textContent = '
/* Invert everything except media */
html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
.ace_editor, .ace_editor *,
.CodeMirror, .CodeMirror *,
.monaco-editor, .monaco-editor *,
.markdown-body pre, .markdown-body pre *,
.highlight, .highlight *,
pre code, pre code * {
filter: none !important;
}
/* Fix common UI elements */
.modal, .popup, .dropdown-menu, .tooltip, .popover {
filter: invert(1) hue-rotate(180deg) !important;
background: #2d2d44 !important;
border-color: #444 !important;
}
/* Scrollbars */
::-webkit-scrollbar { background: #1a1a2e !important; }
::-webkit-scrollbar-thumb { background: #444 !important; }
::-webkit-scrollbar-thumb:hover { background: #555 !important; }
/* Selection */
::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
';
}
function removeDarkMode() {
var style = document.getElementById('universal-dark-mode-style');
if (style) style.remove();
}
// Toggle with Alt+Shift+D
document.addEventListener('keydown', function(e) {
if (e.altKey && e.shiftKey && e.key === 'D') {
e.preventDefault();
enabled = !enabled;
if (enabled) {
applyDarkMode();
console.log('[Universal Dark Mode] Enabled');
} else {
removeDarkMode();
console.log('[Universal Dark Mode] Disabled');
}
}
});
// Apply on load
applyDarkMode();
// Re-apply on dynamic content
var observer = new MutationObserver(function(mutations) {
if (enabled && !document.getElementById('universal-dark-mode-style')) {
applyDarkMode();
}
});
observer.observe(document.head, { childList: true });
console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');
})();
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();
feat(backend): Validate cat header when verifying M2M JWTs by wobsoriano · Pull Request #9038 · clerk/javascript · GitHub
Skip to content
feat(backend): Validate cat header when verifying M2M JWTs#9038

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 Merged

wobsoriano
merged 2 commits into

from

Jun 29, 2026
 Merged


feat(backend): Validate cat header when verifying M2M JWTs
#9038
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d4d14e7

feat(backend): Validate cat header when verifying M2M JWTs


wobsoriano Jun 29, 2026
1b98fad

Merge branch 'main' into rob/user-5437-require-m2m-cat-header-for-sdk…


wobsoriano Jun 29, 2026

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5 changes: 5 additions & 0 deletions
.changeset/m2m-cat-header-verification.md

View file


Open in desktop

Original file line numberDiff line numberDiff line change
@@ -0,0 +1,5 @@
---
'@clerk/backend': patch
---

M2M JWT verification now validates the token-category (`cat`) header and rejects M2M JWTs tagged as a different token class. M2M JWTs minted by Clerk carry the correct category and are unaffected; M2M JWTs without the header continue to verify.

19 changes: 18 additions & 1 deletion
packages/backend/src/jwt/verifyMachineJwt.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -11,7 +11,7 @@ import type { MachineTokenReturnType } from '../jwt/types';
import { verifyJwt } from '../jwt/verifyJwt';
import type { LoadClerkJWKFromRemoteOptions } from '../tokens/keys';
import { loadClerkJwkFromPem, loadClerkJWKFromRemote } from '../tokens/keys';
import { OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { JWT_CATEGORY_M2M_TOKEN, OAUTH_ACCESS_TOKEN_TYPES } from '../tokens/machine';
import { TokenType } from '../tokens/tokenTypes';

export type JwtMachineVerifyOptions = Pick<LoadClerkJWKFromRemoteOptions, 'secretKey' | 'apiUrl' | 'skipJwksCache'> & {
Expand DownExpand Up@@ -86,6 +86,23 @@ export async function verifyM2MJwt(
 decoded: Jwt,
 options: JwtMachineVerifyOptions,
): Promise<MachineTokenReturnType<M2MToken, MachineTokenVerificationError>> {
 // Reject JWTs of another class (e.g. session, jwt-template) signed by the same
 // instance key. Absent `cat` is still accepted during the rollout window; tighten
 // to strict equality once pre-rollout M2M JWTs have expired (USER-5437).
 const cat = decoded.header.cat;
 if (cat !== undefined && cat !== JWT_CATEGORY_M2M_TOKEN) {
 return {
 data: undefined,
 tokenType: TokenType.M2MToken,
 errors: [
 new MachineTokenVerificationError({
 code: MachineTokenVerificationErrorCode.TokenInvalid,
 message: 'Invalid M2M JWT category.',
 }),
 ],
 };
 }

 const result = await resolveKeyAndVerifyJwt(token, decoded.header.kid, options);

 if ('error' in result) {
Expand Down

48 changes: 46 additions & 2 deletions
packages/backend/src/tokens/__tests__/verify.test.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -18,6 +18,7 @@ import {
} from '../../fixtures/machine';
import { signJwt } from '../../jwt/signJwt';
import { server, validateHeaders } from '../../mock-server';
import { JWT_CATEGORY_M2M_TOKEN } from '../machine';
import { verifyMachineAuthToken, verifyToken } from '../verify';

async function createSignedOAuthJwt(
Expand All@@ -31,10 +32,10 @@ async function createSignedOAuthJwt(
 return data!;
}

async function createSignedM2MJwt(payload = mockM2MJwtPayload) {
async function createSignedM2MJwt(payload = mockM2MJwtPayload, cat: string | undefined = JWT_CATEGORY_M2M_TOKEN) {
 const { data } = await signJwt(payload, signingJwks, {
 algorithm: 'RS256',
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD' },
 header: { typ: 'JWT', kid: 'ins_2GIoQhbUpy0hX7B2cVkuTMinXoD', ...(cat !== undefined ? { cat } : {}) },
 });
 return data!;
}
Expand DownExpand Up@@ -640,5 +641,48 @@ describe('tokens.verifyMachineAuthToken(token, options)', () => {
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].message).toContain('expired');
 });

 it('verifies a valid M2M JWT with no cat header (rollout window)', async () => {
 server.use(
 http.get(
 'https://api.clerk.test/v1/jwks',
 validateHeaders(() => {
 return HttpResponse.json(mockJwks);
 }),
 ),
 );

 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, undefined);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.errors).toBeUndefined();
 expect(result.tokenType).toBe('m2m_token');
 });

 describe.each([
 ['session-token', 'cl_B7d4PD111AAA'],
 ['jwt-template', 'cl_B7d4PD222AAA'],
 ['unknown', 'cl_some_future_unknown_cat'],
 ])('rejects M2M JWT masquerading with a non-M2M cat', (label, cat) => {
 it(`rejects cat=${label} without attempting signature verification`, async () => {
 // No JWKS handler registered: if the cat check did not short-circuit,
 // resolveKeyAndVerifyJwt would attempt a JWKS fetch and the failure
 // message would differ from the category error below.
 const m2mJwt = await createSignedM2MJwt(mockM2MJwtPayload, cat);

 const result = await verifyMachineAuthToken(m2mJwt, {
 apiUrl: 'https://api.clerk.test',
 secretKey: 'a-valid-key',
 });

 expect(result.tokenType).toBe('m2m_token');
 expect(result.errors).toBeDefined();
 expect(result.errors?.[0].code).toBe('token-invalid');
 });
 });
 });
});

5 changes: 5 additions & 0 deletions
packages/backend/src/tokens/machine.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -8,6 +8,11 @@ export const M2M_SUBJECT_PREFIX = 'mch_';
export const OAUTH_TOKEN_PREFIX = 'oat_';
export const API_KEY_PREFIX = 'ak_';

// Token-category tag in the protected JOSE header of instance-signed M2M JWTs,
// used to distinguish them from other JWT classes signed by the same instance
// key. Kept in sync with clerk_go (pkg/jwt) and cloudflare-workers.
export const JWT_CATEGORY_M2M_TOKEN = 'cl_B7d4PD333AAA';

const MACHINE_TOKEN_PREFIXES = [M2M_TOKEN_PREFIX, OAUTH_TOKEN_PREFIX, API_KEY_PREFIX] as const;

export const JwtFormatRegExp = /^[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]+$/;
Expand Down

2 changes: 2 additions & 0 deletions
packages/shared/src/types/jwtv2.ts

View file


Open in desktop

Original file line numberDiff line numberDiff line change
Expand Up@@ -27,6 +27,8 @@ export interface JwtHeader {
 x5c?: string | string[];
 /** @internal - used by Session Minter for monotonic token freshness checks. Do not depend on this field. */
 oiat?: number;
 /** @internal - Clerk token-category tag written by the token minter. Do not depend on this field. */
 cat?: string;
}

declare global {
Expand Down
Loading