CVE-2021-39175
📋 TL;DR
CVE-2021-39175 is a cross-site scripting (XSS) vulnerability in HedgeDoc that allows unauthenticated attackers to inject malicious JavaScript into slide-mode speaker notes. This affects all HedgeDoc instances running versions before 1.9.0, potentially compromising user sessions and data.
💻 Affected Systems
- HedgeDoc
📦 What is this software?
Hedgedoc by Hedgedoc
⚠️ Risk & Real-World Impact
Worst Case
Attackers could steal session cookies, redirect users to malicious sites, perform actions on behalf of users, or install malware through drive-by downloads.
Likely Case
Session hijacking, credential theft, and unauthorized actions within the HedgeDoc instance.
If Mitigated
Limited impact with proper content security policies and network segmentation, but XSS could still affect users within the application.
🎯 Exploit Status
Exploitation is straightforward via iframe embedding or page embedding techniques. Public proof-of-concept exists in the advisory references.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.9.0
Vendor Advisory: https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-j748-779h-9697
Restart Required: Yes
Instructions:
1. Backup your HedgeDoc instance and database. 2. Update to HedgeDoc version 1.9.0 or later. 3. Restart the HedgeDoc service. 4. Verify the update was successful.
🔧 Temporary Workarounds
Disable slide-mode feature
allTemporarily disable the slide-mode feature to prevent exploitation via speaker notes.
Modify HedgeDoc configuration to disable slide-mode (specific commands depend on deployment method)
🧯 If You Can't Patch
- Implement strict Content Security Policy (CSP) headers to restrict script execution
- Use network segmentation to isolate HedgeDoc instances and limit attack surface
🔍 How to Verify
Check if Vulnerable:
Check HedgeDoc version - if below 1.9.0, the system is vulnerable.Check Version:
Check HedgeDoc web interface footer or run appropriate version command for your deployment methodVerify Fix Applied:
Confirm HedgeDoc version is 1.9.0 or higher and test slide-mode functionality.📡 Detection & Monitoring
Log Indicators:
- Unusual iframe embedding patterns in slide content
- Multiple failed attempts to access slide-mode features
Network Indicators:
- Suspicious external script loading in slide-mode requests
- Unusual cross-origin requests from HedgeDoc
SIEM Query:
Search for HedgeDoc logs containing 'slide-mode' or 'speaker-notes' with suspicious script tags or iframe references🔗 References
- https://github.com/hedgedoc/hedgedoc/pull/1369
- https://github.com/hedgedoc/hedgedoc/pull/1375
- https://github.com/hedgedoc/hedgedoc/pull/1513
- https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-j748-779h-9697
- https://github.com/hedgedoc/hedgedoc/pull/1369
- https://github.com/hedgedoc/hedgedoc/pull/1375
- https://github.com/hedgedoc/hedgedoc/pull/1513
- https://github.com/hedgedoc/hedgedoc/security/advisories/GHSA-j748-779h-9697
