[Bug]: Strange behavior of html sanitizer #5293

Closed
Closed
[Bug]: Strange behavior of html sanitizer#5293
Labels
bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.

Description

@mbeckenbach
mbeckenbach
opened on Aug 3, 2026

Before submitting

  • I searched existing issues and did not find a duplicate.
  • I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.

Expected behavior

I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.

If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.

Actual behavior

I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.

Impact

Minor bug or occasional failure

Version or commit

No response

Environment

macOS 26.x

Logs or stack traces

Screenshots, recordings, or supporting files
Screenshot 2026-08-03 at 20.06.20.png
Workaround
Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
Metadata
Metadata
Assignees
No one assigned
    Labels
    bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
    Type
    No type
    Projects
    No projects
      Milestone
      No milestone
      Relationships
      None yet
      Development
      No branches or pull requests
      Issue actions
      
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Add copy buttons to all 
       blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n 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;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

      Skip to content
      [Bug]: Strange behavior of html sanitizer #5293
      Closed
      Closed
      [Bug]: Strange behavior of html sanitizer#5293
      Labels
      bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
      Description
      @mbeckenbach
      mbeckenbach
      opened on Aug 3, 2026
      Before submitting
      •  I searched existing issues and did not find a duplicate.
      •  I included enough detail to reproduce or investigate the problem.
      Area
      apps/web
      Steps to reproduce
      Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.
      Expected behavior
      I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.
      If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.
      Actual behavior
      I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.
      Impact
      Minor bug or occasional failure
      Version or commit
      No response
      Environment
      macOS 26.x
      Logs or stack traces
      Screenshots, recordings, or supporting files
      Screenshot 2026-08-03 at 20.06.20.png
      Workaround
      Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
      Metadata
      Metadata
      Assignees
      No one assigned
        Labels
        bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
        Type
        No type
        Projects
        No projects
          Milestone
          No milestone
          Relationships
          None yet
          Development
          No branches or pull requests
          Issue actions
          
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

          Skip to content
          [Bug]: Strange behavior of html sanitizer #5293
          Closed
          Closed
          [Bug]: Strange behavior of html sanitizer#5293
          Labels
          bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
          Description
          @mbeckenbach
          mbeckenbach
          opened on Aug 3, 2026
          Before submitting
          •  I searched existing issues and did not find a duplicate.
          •  I included enough detail to reproduce or investigate the problem.
          Area
          apps/web
          Steps to reproduce
          Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.
          Expected behavior
          I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.
          If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.
          Actual behavior
          I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.
          Impact
          Minor bug or occasional failure
          Version or commit
          No response
          Environment
          macOS 26.x
          Logs or stack traces
          Screenshots, recordings, or supporting files
          Screenshot 2026-08-03 at 20.06.20.png
          Workaround
          Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
          Metadata
          Metadata
          Assignees
          No one assigned
            Labels
            bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
            Type
            No type
            Projects
            No projects
              Milestone
              No milestone
              Relationships
              None yet
              Development
              No branches or pull requests
              Issue actions
              
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

              Skip to content
              [Bug]: Strange behavior of html sanitizer #5293
              Closed
              Closed
              [Bug]: Strange behavior of html sanitizer#5293
              Labels
              bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
              Description
              @mbeckenbach
              mbeckenbach
              opened on Aug 3, 2026
              Before submitting
              •  I searched existing issues and did not find a duplicate.
              •  I included enough detail to reproduce or investigate the problem.
              Area
              apps/web
              Steps to reproduce
              Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.
              Expected behavior
              I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.
              If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.
              Actual behavior
              I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.
              Impact
              Minor bug or occasional failure
              Version or commit
              No response
              Environment
              macOS 26.x
              Logs or stack traces
              Screenshots, recordings, or supporting files
              Screenshot 2026-08-03 at 20.06.20.png
              Workaround
              Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
              Metadata
              Metadata
              Assignees
              No one assigned
                Labels
                bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                Type
                No type
                Projects
                No projects
                  Milestone
                  No milestone
                  Relationships
                  None yet
                  Development
                  No branches or pull requests
                  Issue actions
                  
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

                  Skip to content
                  [Bug]: Strange behavior of html sanitizer #5293
                  Closed
                  Closed
                  [Bug]: Strange behavior of html sanitizer#5293
                  Labels
                  bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                  Description
                  @mbeckenbach
                  mbeckenbach
                  opened on Aug 3, 2026
                  Before submitting
                  •  I searched existing issues and did not find a duplicate.
                  •  I included enough detail to reproduce or investigate the problem.
                  Area
                  apps/web
                  Steps to reproduce
                  Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.
                  Expected behavior
                  I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.
                  If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.
                  Actual behavior
                  I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.
                  Impact
                  Minor bug or occasional failure
                  Version or commit
                  No response
                  Environment
                  macOS 26.x
                  Logs or stack traces
                  Screenshots, recordings, or supporting files
                  Screenshot 2026-08-03 at 20.06.20.png
                  Workaround
                  Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
                  Metadata
                  Metadata
                  Assignees
                  No one assigned
                    Labels
                    bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                    Type
                    No type
                    Projects
                    No projects
                      Milestone
                      No milestone
                      Relationships
                      None yet
                      Development
                      No branches or pull requests
                      Issue actions
                      
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                      Skip to content
                      [Bug]: Strange behavior of html sanitizer #5293
                      Closed
                      Closed
                      [Bug]: Strange behavior of html sanitizer#5293
                      Labels
                      bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                      Description
                      @mbeckenbach
                      mbeckenbach
                      opened on Aug 3, 2026
                      Before submitting
                      •  I searched existing issues and did not find a duplicate.
                      •  I included enough detail to reproduce or investigate the problem.
                      Area
                      apps/web
                      Steps to reproduce
                      Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.
                      Expected behavior
                      I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.
                      If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.
                      Actual behavior
                      I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.
                      Impact
                      Minor bug or occasional failure
                      Version or commit
                      No response
                      Environment
                      macOS 26.x
                      Logs or stack traces
                      Screenshots, recordings, or supporting files
                      Screenshot 2026-08-03 at 20.06.20.png
                      Workaround
                      Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
                      Metadata
                      Metadata
                      Assignees
                      No one assigned
                        Labels
                        bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                        Type
                        No type
                        Projects
                        No projects
                          Milestone
                          No milestone
                          Relationships
                          None yet
                          Development
                          No branches or pull requests
                          Issue actions
                          
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                          Skip to content
                          [Bug]: Strange behavior of html sanitizer #5293
                          Closed
                          Closed
                          [Bug]: Strange behavior of html sanitizer#5293
                          Labels
                          bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                          Description
                          @mbeckenbach
                          mbeckenbach
                          opened on Aug 3, 2026
                          Before submitting
                          •  I searched existing issues and did not find a duplicate.
                          •  I included enough detail to reproduce or investigate the problem.
                          Area
                          apps/web
                          Steps to reproduce
                          Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.
                          Expected behavior
                          I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.
                          If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.
                          Actual behavior
                          I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.
                          Impact
                          Minor bug or occasional failure
                          Version or commit
                          No response
                          Environment
                          macOS 26.x
                          Logs or stack traces
                          Screenshots, recordings, or supporting files
                          Screenshot 2026-08-03 at 20.06.20.png
                          Workaround
                          Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
                          Metadata
                          Metadata
                          Assignees
                          No one assigned
                            Labels
                            bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                            Type
                            No type
                            Projects
                            No projects
                              Milestone
                              No milestone
                              Relationships
                              None yet
                              Development
                              No branches or pull requests
                              Issue actions
                              
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

                              Skip to content
                              [Bug]: Strange behavior of html sanitizer #5293
                              Closed
                              Closed
                              [Bug]: Strange behavior of html sanitizer#5293
                              Labels
                              bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
                              Description
                              @mbeckenbach
                              mbeckenbach
                              opened on Aug 3, 2026
                              Before submitting
                              •  I searched existing issues and did not find a duplicate.
                              •  I included enough detail to reproduce or investigate the problem.
                              Area
                              apps/web
                              Steps to reproduce
                              Submit a prompt that contains some simple html markup like strong. Then submit a prompt that contains a script tag with an alert() call inside.
                              Expected behavior
                              I would expect that t3code does an encoding/sanitizing on submit and not at render of my message. I would expect both cases to simply output using encoding like html entities or even in quotes like these.
                              If the input fields value should really not be sanitized on submit that raises the question if a user could somehow cause a code execution within t3code or on harness side. Like somewhere on the way between the input field and the rendered user input message bubble.
                              Actual behavior
                              I understand it is just some markdown renderer, so it might be partially expected behavior. The user message gets rendered as formatted html for common html tags. The strange part is that something like script tags are filtered out and not displayed at all instead of at least encoded.
                              Impact
                              Minor bug or occasional failure
                              Version or commit
                              No response
                              Environment
                              macOS 26.x
                              Logs or stack traces
                              Screenshots, recordings, or supporting files
                              Screenshot 2026-08-03 at 20.06.20.png
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                              Copy pasted code seams to be auto surrounded with markdown code block quotes. Manually typed code in prompts is not.
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                                bugSomething is broken or behaving incorrectly.needs-triageIssue needs maintainer review and initial categorization.
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