CVE-2025-66512
📋 TL;DR
This vulnerability allows malicious users to bypass Nextcloud's Content Security Policy (CSP) by tricking users into viewing specially crafted SVG files outside the Nextcloud web interface. This affects Nextcloud Server and Server Enterprise installations where users can upload files. Attackers could potentially execute malicious scripts in the victim's browser context.
💻 Affected Systems
- Nextcloud Server
- Nextcloud Server Enterprise
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Malicious actors could execute arbitrary JavaScript in victim browsers, potentially leading to session hijacking, data theft, or further exploitation of the Nextcloud instance.
Likely Case
Attackers could bypass CSP protections to run limited scripts, potentially stealing session cookies or performing actions as the victim within Nextcloud.
If Mitigated
With proper CSP configurations and user education about opening untrusted files, impact is limited to potential script execution within the SVG viewer context.
🎯 Exploit Status
Exploitation requires authenticated user with file upload permissions and social engineering to trick victims into viewing malicious SVG files outside the Nextcloud web interface.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 31.0.12 or 32.0.3
Vendor Advisory: https://github.com/nextcloud/security-advisories/security/advisories/GHSA-qcw2-p26m-9gc5
Restart Required: No
Instructions:
1. Update Nextcloud Server to version 31.0.12 or 32.0.3 or later. 2. Update via Nextcloud's web updater or manually via command line. 3. Clear browser cache after update.
🔧 Temporary Workarounds
Disable SVG file uploads
allPrevent users from uploading SVG files to eliminate the attack vector
Edit config/config.php and add: 'enabledPreviewProviders' => array_diff($CONFIG['enabledPreviewProviders'], ['OC\Preview\SVG'])
Restrict file upload permissions
allLimit which users can upload files to trusted individuals only
🧯 If You Can't Patch
- Educate users about the risks of opening SVG files from untrusted sources
- Implement strict Content Security Policy headers and monitor for violations
🔍 How to Verify
Check if Vulnerable:
Check Nextcloud version via Admin Settings > Overview or run: sudo -u www-data php occ statusCheck Version:
sudo -u www-data php occ status | grep 'versionstring'Verify Fix Applied:
Confirm version is 31.0.12 or 32.0.3 or higher, and check that the Viewer app is updated to version containing commit 5044a27d📡 Detection & Monitoring
Log Indicators:
- Multiple SVG file uploads from single user
- CSP violation logs showing SVG-related bypass attempts
- Unusual file access patterns to SVG files
Network Indicators:
- Requests for SVG files with suspicious parameters
- CSP violation reports in browser console
SIEM Query:
source="nextcloud.log" AND ("svg" OR "CSP violation") AND ("upload" OR "access")