CVE-2026-25642: HedgeDoc XSS Vulnerability

CVE-2026-25642 Overview

HedgeDoc is an open source, real-time, collaborative markdown notes application. A security vulnerability was discovered in versions prior to 1.10.6 where files served below the /uploads/ endpoint did not implement a strict Content-Security-Policy. This misconfiguration resulted in an overly permissive CSP that enabled attackers to host malicious interactive web content, such as fake login forms, using crafted SVG files uploaded to the platform.

Critical Impact

Attackers can leverage uploaded SVG files to execute phishing attacks, credential harvesting, or other social engineering attacks against HedgeDoc users by hosting interactive malicious content within the trusted application domain.

Affected Products

• HedgeDoc versions prior to 1.10.6
• Self-hosted HedgeDoc instances with user upload functionality enabled
• Deployments allowing SVG file uploads

Discovery Timeline

• 2026-02-06 - CVE-2026-25642 published to NVD
• 2026-02-06 - Last updated in NVD database

Technical Details for CVE-2026-25642

Vulnerability Analysis

This vulnerability stems from a Cross-Site Scripting (CWE-79) weakness in the file upload handling mechanism. The /uploads/ endpoint in HedgeDoc lacked proper Content-Security-Policy headers, allowing uploaded files—particularly SVG files—to execute interactive content within the browser context.

SVG files are unique in that they can contain embedded JavaScript and interactive HTML elements. When served without restrictive security headers, these files can render as fully interactive web pages, enabling threat actors to create convincing phishing pages or execute script-based attacks within the trusted HedgeDoc domain.

Root Cause

The root cause was the absence of security headers on the /uploads/ route. Files served from this endpoint were delivered without:

1. A restrictive Content-Security-Policy header to prevent script execution
2. A Content-Disposition: attachment header to force downloads instead of inline rendering
3. A sandbox directive to isolate uploaded content from the main application context

Without these protections, the browser would render uploaded SVG files with the same privileges as the main application, allowing embedded scripts and forms to operate as if they were part of HedgeDoc itself.

Attack Vector

The attack requires user interaction where a victim must be convinced to access a malicious SVG file hosted on the HedgeDoc instance. An attacker would:

1. Upload a specially crafted SVG file containing malicious content (fake login forms, JavaScript, etc.)
2. Distribute the URL to the uploaded file to potential victims
3. When accessed, the SVG renders interactively within the HedgeDoc domain
4. Victims may unknowingly submit credentials or interact with malicious elements

The security patches address this by implementing proper CSP headers and forcing attachment disposition:

// security headers for uploads
app.use('/uploads', (req, res, next) => {
  res.set('Content-Disposition', 'attachment')
  res.set('Content-Security-Policy', "default-src 'none'")
  next()
})

Source: GitHub Commit Update

A follow-up patch further strengthened the protection by adding a sandbox directive:

// security headers for uploads
app.use('/uploads', (req, res, next) => {
  res.set('Content-Disposition', 'attachment')
  res.set('Content-Security-Policy', "default-src 'none'; sandbox")
  next()
})

Source: GitHub Commit Update

Detection Methods for CVE-2026-25642

Indicators of Compromise

• SVG files in the /uploads/ directory containing embedded <script> tags or JavaScript event handlers
• SVG files with <foreignObject> elements containing HTML form elements
• User reports of unexpected login prompts or interactive content when viewing uploads
• Web server logs showing unusual access patterns to uploaded SVG files

Detection Strategies

• Implement Content-Type monitoring on uploaded files to identify SVG uploads with suspicious embedded content
• Deploy web application firewall (WAF) rules to inspect SVG file contents for script elements
• Monitor authentication logs for credential submissions originating from upload URLs
• Use browser developer tools or security scanners to verify CSP headers are properly applied to /uploads/ endpoints

Monitoring Recommendations

• Enable verbose logging for the /uploads/ endpoint to track file access patterns
• Implement file integrity monitoring for the uploads directory
• Configure alerting for SVG files containing known malicious patterns or excessive JavaScript
• Regularly audit uploaded content for suspicious interactive elements

How to Mitigate CVE-2026-25642

Immediate Actions Required

• Upgrade HedgeDoc to version 1.10.6 or later immediately
• Review existing uploaded SVG files for malicious content
• Consider temporarily disabling SVG uploads if immediate patching is not possible
• Notify users about potential phishing attempts originating from the HedgeDoc instance

Patch Information

The vulnerability is fixed in HedgeDoc version 1.10.6. The patch implements strict Content-Security-Policy headers with a sandbox directive and forces Content-Disposition: attachment for all files served from the /uploads/ endpoint. Update via your preferred deployment method:

• GitHub Release: HedgeDoc 1.10.6
• Security Advisory: GHSA-x74j-jmf9-534w

Workarounds

• Configure a reverse proxy (nginx, Apache) to add restrictive CSP headers for the /uploads/ path
• Implement file type restrictions to block SVG uploads entirely
• Deploy CSP headers at the infrastructure level if application-level patching is delayed
• Use a separate domain or subdomain for serving uploaded content to isolate from the main application

# Nginx configuration workaround for /uploads/ endpoint
location /uploads/ {
    add_header Content-Security-Policy "default-src 'none'; sandbox" always;
    add_header Content-Disposition "attachment" always;
    add_header X-Content-Type-Options "nosniff" always;
}