CVE-2025-23207: KaTeX JavaScript Library XSS Vulnerability

CVE-2025-23207 Overview

CVE-2025-23207 is a Cross-Site Scripting (XSS) vulnerability affecting KaTeX, a fast, easy-to-use JavaScript library for TeX math rendering on the web. The vulnerability allows attackers to execute arbitrary JavaScript or generate invalid HTML when users render untrusted mathematical expressions using the renderToString function in combination with malicious \htmlData commands.

Critical Impact

Attackers can inject malicious JavaScript through specially crafted LaTeX input, potentially leading to session hijacking, data theft, or unauthorized actions in the context of affected web applications that render untrusted math expressions.

Affected Products

• KaTeX versions prior to v0.16.21
• Web applications using KaTeX with the trust option enabled
• Applications rendering untrusted mathematical expressions via renderToString

Discovery Timeline

• 2025-01-17 - CVE CVE-2025-23207 published to NVD
• 2025-09-08 - Last updated in NVD database

Technical Details for CVE-2025-23207

Vulnerability Analysis

This vulnerability stems from improper encoding of output in the KaTeX library (CWE-116) and classic Cross-Site Scripting (CWE-79). When users render untrusted mathematical expressions using the renderToString function, malicious input leveraging the \htmlData command can bypass sanitization and execute arbitrary JavaScript code in the browser context.

The attack is network-accessible and requires no authentication or user interaction to exploit. The vulnerability has a changed scope, meaning a successful exploit can impact resources beyond the vulnerable component's security scope. While confidentiality and integrity are affected, availability is not impacted by this vulnerability.

Root Cause

The root cause lies in improper output encoding within KaTeX's \htmlData command processing. The library fails to adequately sanitize or escape user-controlled input before incorporating it into the rendered HTML output. This allows specially crafted LaTeX expressions containing the \htmlData command to inject malicious content that executes when the rendered output is displayed in a browser.

Attack Vector

The attack vector involves providing malicious mathematical expressions to applications that use KaTeX's renderToString function with the trust option enabled. An attacker crafts a LaTeX expression containing a malicious \htmlData command payload. When the expression is rendered, the unsanitized content is included in the HTML output, allowing JavaScript execution.

The security patch addresses this by adding input validation controls:

import type Options from "./Options";
import {DocumentFragment} from "./tree";
import {makeEm} from "./units";
+import ParseError from "./ParseError";

import type {VirtualNode} from "./tree";

Source: GitHub Commit Update

The patch also includes ESLint configuration updates to support the security fix:

         "no-array-constructor": 2,
         "no-console": 2,
         "no-const-assign": 2,
+        "no-control-regex": 0,
         "no-debugger": 2,
         "no-dupe-class-members": 2,
         "no-dupe-keys": 2,

Source: GitHub Commit Update

Detection Methods for CVE-2025-23207

Indicators of Compromise

• Unexpected JavaScript execution originating from pages containing KaTeX-rendered content
• HTTP requests or logs containing the substring \\htmlData in mathematical expression inputs
• User reports of suspicious behavior on pages with math rendering functionality
• Anomalous DOM modifications following KaTeX render operations

Detection Strategies

• Implement Web Application Firewall (WAF) rules to detect and block inputs containing \\htmlData patterns in LaTeX expressions
• Monitor client-side JavaScript errors and unexpected script execution on pages using KaTeX
• Audit application logs for suspicious mathematical expression submissions
• Deploy Content Security Policy (CSP) headers to detect and mitigate inline script injection attempts

Monitoring Recommendations

• Enable verbose logging for applications processing user-submitted mathematical expressions
• Monitor for CSP violation reports indicating attempted script injection
• Set up alerts for unusual patterns in LaTeX input processing
• Review KaTeX usage across your application portfolio to identify potentially vulnerable implementations

How to Mitigate CVE-2025-23207

Immediate Actions Required

• Upgrade KaTeX to version v0.16.21 or later immediately
• Audit all applications using KaTeX to identify instances where untrusted input is rendered
• Disable or restrict the trust option in KaTeX configurations until patching is complete
• Implement input validation to reject expressions containing \\htmlData

Patch Information

The vulnerability has been addressed in KaTeX version v0.16.21. The fix includes proper input validation through the integration of ParseError handling in src/domTree.js to prevent malicious \htmlData commands from bypassing sanitization. Users should upgrade to this version or later to eliminate the vulnerability. For detailed patch information, see the GitHub Commit Update and the GitHub Security Advisory.

Workarounds

• Disable the trust option entirely in KaTeX configuration
• Configure the trust option to explicitly forbid \htmlData commands
• Implement server-side input filtering to reject inputs containing the substring "\\htmlData"
• Sanitize all HTML output from KaTeX before rendering in the browser

# Example: Configuring KaTeX to disable trust option
# In your JavaScript configuration:
# katex.render(expression, element, {
#     trust: false,  // Disable trust to prevent htmlData exploitation
#     strict: true   // Enable strict mode for additional security
# });

# Alternative: Filter inputs before rendering
# if (userInput.includes('\\htmlData')) {
#     throw new Error('Potentially malicious input detected');
# }