diff --git a/backend/src/modules/auth/auth.service.ts b/backend/src/modules/auth/auth.service.ts
index b5f5dba..a3e51fb 100644
--- a/backend/src/modules/auth/auth.service.ts
+++ b/backend/src/modules/auth/auth.service.ts
@@ -69,7 +69,7 @@ export class AuthService {
     try {
       await emailService.sendWelcome({
         email: user.email,
-        name: user.displayName ?? user.username,
+        name: user.displayName ?? user.username ?? "Usuário",
       });
     } catch (error) {
       logError("Falha ao disparar e-mail de boas-vindas no login social.", {
diff --git a/backend/src/modules/auth/connections.controller.ts b/backend/src/modules/auth/connections.controller.ts
index df3cd74..f3f536f 100644
--- a/backend/src/modules/auth/connections.controller.ts
+++ b/backend/src/modules/auth/connections.controller.ts
@@ -18,7 +18,10 @@ export class ConnectionsController {
     const userId = req.session.userId as string;
     const provider = req.params.provider;
 
-    if (!(SUPPORTED_PROVIDERS as readonly string[]).includes(provider)) {
+    if (
+      typeof provider !== "string" ||
+      !(SUPPORTED_PROVIDERS as readonly string[]).includes(provider)
+    ) {
       throw AppError.validation("Provider inválido.");
     }
 
diff --git a/backend/src/modules/auth/credentials.service.ts b/backend/src/modules/auth/credentials.service.ts
index ef50c67..a774280 100644
--- a/backend/src/modules/auth/credentials.service.ts
+++ b/backend/src/modules/auth/credentials.service.ts
@@ -86,8 +86,8 @@ export class CredentialsService {
     // E-mail de boas-vindas: falha nunca derruba o registro (EMAIL-08).
     try {
       await emailService.sendWelcome({
-        email: user.email,
-        name: user.displayName ?? user.username,
+        email: normalizedEmail,
+        name: name?.trim() || normalizedEmail.split("@")[0],
       });
     } catch (error) {
       logError("Falha ao disparar e-mail de boas-vindas.", {
diff --git a/backend/tests/integration/routes/connections.routes.test.ts b/backend/tests/integration/routes/connections.routes.test.ts
index 6347ae4..1b9b65e 100644
--- a/backend/tests/integration/routes/connections.routes.test.ts
+++ b/backend/tests/integration/routes/connections.routes.test.ts
@@ -84,4 +84,20 @@ describe("connections routes", () => {
     expect(res.body.code).toBe("VALIDATION_ERROR");
     expect(mocks.disconnectProvider).not.toHaveBeenCalled();
   });
+
+  it("recusa provider não escalar antes de desconectar", async () => {
+    const controller = new ConnectionsController();
+
+    await expect(
+      controller.disconnect(
+        {
+          session: { userId: "user-A" },
+          params: { provider: ["google", "github"] },
+        } as any,
+        { json: vi.fn() } as any,
+      ),
+    ).rejects.toMatchObject({ code: "VALIDATION_ERROR" });
+
+    expect(mocks.disconnectProvider).not.toHaveBeenCalled();
+  });
 });
diff --git a/backend/tests/unit/modules/auth/auth.service.test.ts b/backend/tests/unit/modules/auth/auth.service.test.ts
index 9382037..e919c55 100644
--- a/backend/tests/unit/modules/auth/auth.service.test.ts
+++ b/backend/tests/unit/modules/auth/auth.service.test.ts
@@ -216,5 +216,20 @@ describe("AuthService", () => {
         profile: mockProfile,
       });
     });
+
+    it("uses a fallback name when the social profile has no name", async () => {
+      mocks.exchangeCode.mockResolvedValueOnce(mockProfile);
+      mocks.findOrCreateUser.mockResolvedValueOnce({
+        user: { ...mockUser, displayName: null, username: null },
+        isNewUser: true,
+      });
+
+      await service.handleCallback(validCallbackParams);
+
+      expect(mocks.sendWelcome).toHaveBeenCalledWith({
+        email: mockUser.email,
+        name: "Usuário",
+      });
+    });
   });
 });
diff --git a/backend/tests/unit/modules/auth/credentials.service.test.ts b/backend/tests/unit/modules/auth/credentials.service.test.ts
index 5ff325f..2583b1f 100644
--- a/backend/tests/unit/modules/auth/credentials.service.test.ts
+++ b/backend/tests/unit/modules/auth/credentials.service.test.ts
@@ -213,8 +213,8 @@ describe("CredentialsService", () => {
       await service.register(registerInput);
 
       expect(mocks.sendWelcome).toHaveBeenCalledWith({
-        email: mockUser.email,
-        name: mockUser.displayName,
+        email: registerInput.email,
+        name: registerInput.name,
       });
     });