CVE-2025-67723: Discourse Math Plugin XSS Vulnerability

CVE-2025-67723 Overview

A content-security-policy-mitigated cross-site scripting (XSS) vulnerability has been identified in Discourse, an open source discussion platform. The vulnerability exists in the Discourse Math plugin when using the KaTeX variant for mathematical expression rendering.

Critical Impact
Attackers with low privileges could exploit this XSS vulnerability to compromise user sessions, manipulate page content, or perform unauthorized actions on behalf of authenticated users within the Discourse platform.

Affected Products

Discourse versions prior to 3.5.4
Discourse versions prior to 2025.11.2
Discourse versions prior to 2025.12.1
Discourse versions prior to 2026.1.0

Discovery Timeline

2026-01-28 - CVE CVE-2025-67723 published to NVD
2026-01-29 - Last updated in NVD database

Technical Details for CVE-2025-67723

Vulnerability Analysis

This vulnerability is classified as CWE-79 (Improper Neutralization of Input During Web Page Generation), commonly known as Cross-Site Scripting (XSS). The issue exists within the Discourse Math plugin's KaTeX rendering engine, which fails to properly sanitize user-supplied mathematical expressions before rendering them in the browser context.

While the vulnerability is mitigated by Content Security Policy (CSP) headers implemented by Discourse, the underlying XSS flaw remains exploitable in certain configurations or when CSP enforcement is weakened. The attack requires user interaction and low-level privileges (authenticated user), making it a stored or reflected XSS scenario where malicious mathematical notation could be injected into forum posts or comments.

Root Cause

The root cause stems from insufficient input validation and output encoding within the KaTeX mathematical rendering component of the Discourse Math plugin. When processing LaTeX-style mathematical expressions, the plugin fails to properly neutralize potentially dangerous characters and scripts before incorporating them into the rendered HTML output.

Attack Vector

The attack vector is network-based, requiring an authenticated attacker to submit specially crafted mathematical expressions through the Discourse forum interface. When other users view the content containing the malicious payload, the XSS code executes within their browser context. The exploitation scenario involves:

An authenticated attacker crafts a malicious mathematical expression containing XSS payload
The payload is submitted through the Discourse Math plugin's KaTeX renderer
Victim users viewing the content trigger the XSS execution
The attacker can potentially steal session tokens, modify page content, or perform actions as the victim

The vulnerability's impact is partially limited by Discourse's Content Security Policy implementation, which provides defense-in-depth against certain XSS attack patterns.

Detection Methods for CVE-2025-67723

Indicators of Compromise

Unusual mathematical expressions containing JavaScript-like syntax in forum posts
Unexpected script execution attempts logged in browser developer tools
CSP violation reports indicating blocked inline script execution attempts
Forum posts with malformed or suspicious LaTeX/KaTeX notation

Detection Strategies

Monitor CSP violation reports for patterns indicating XSS exploitation attempts
Implement web application firewall (WAF) rules to detect common XSS patterns in mathematical expression fields
Review Discourse application logs for unusual POST requests to math-rendering endpoints
Deploy browser-based XSS detection tools to identify malicious script injection attempts

Monitoring Recommendations

Enable detailed logging for the Discourse Math plugin
Configure CSP reporting endpoints to capture violation events
Monitor for unusual user behavior patterns following forum post views
Implement real-time alerting on detected XSS signatures in user-submitted content

How to Mitigate CVE-2025-67723

Immediate Actions Required

Upgrade Discourse to version 3.5.4, 2025.11.2, 2025.12.1, or 2026.1.0 immediately
If immediate patching is not possible, disable the Discourse Math plugin as a temporary workaround
Consider switching from KaTeX to the MathJax provider as an alternative mitigation
Review recent forum posts for potentially malicious mathematical expressions

Patch Information

Discourse has released security patches addressing this vulnerability across multiple release branches. Affected organizations should upgrade to one of the following patched versions:

Version 3.5.4 (stable branch)
Version 2025.11.2
Version 2025.12.1
Version 2026.1.0

For detailed patch information and upgrade instructions, refer to the GitHub Security Advisory.

Workarounds

Disable the Discourse Math plugin entirely through the admin panel until patching is possible
Switch the mathematical rendering provider from KaTeX to MathJax in plugin settings
Implement additional CSP headers to strengthen XSS protections
Enable strict input validation rules for mathematical expression fields

bash

# Discourse Admin Configuration - Disable Math Plugin
# Navigate to: Admin > Plugins > Discourse Math
# Set the following configuration:
discourse_math_enabled: false

# Alternative: Switch to MathJax provider
discourse_math_provider: mathjax