CVE-2025-66514
📋 TL;DR
This vulnerability allows authenticated Nextcloud Mail users to inject HTML into email subject lines displayed in the message list. While JavaScript execution is prevented by Nextcloud's Content Security Policy, HTML injection can still enable phishing attacks and UI manipulation. Only Nextcloud instances with the Mail app installed and using versions before 5.5.3 are affected.
💻 Affected Systems
- Nextcloud Mail
📦 What is this software?
Mail by Nextcloud
⚠️ Risk & Real-World Impact
Worst Case
An authenticated attacker could craft malicious email subjects containing HTML that appears legitimate, potentially tricking users into clicking malicious links or revealing sensitive information through sophisticated phishing attacks.
Likely Case
Limited HTML injection in email subjects that could be used for minor UI manipulation or basic phishing attempts, but without JavaScript execution capabilities.
If Mitigated
With proper CSP and input validation, the impact is limited to visual manipulation of email subjects without code execution or data compromise.
🎯 Exploit Status
Exploitation requires authenticated access to a Nextcloud Mail account. The HTML injection is straightforward and doesn't require advanced technical skills.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.5.3
Vendor Advisory: https://github.com/nextcloud/security-advisories/security/advisories/GHSA-v394-8gpc-6fv5
Restart Required: No
Instructions:
1. Update Nextcloud Mail app to version 5.5.3 or later via Nextcloud App Store. 2. Navigate to Nextcloud admin panel. 3. Go to Apps section. 4. Find Mail app and click update. 5. No server restart required.
🔧 Temporary Workarounds
Disable Mail App
linuxTemporarily disable the Nextcloud Mail app until patching is possible
occ app:disable mail
🧯 If You Can't Patch
- Restrict user permissions to minimize authenticated attack surface
- Implement additional monitoring for unusual HTML content in email subjects
🔍 How to Verify
Check if Vulnerable:
Check Mail app version in Nextcloud admin panel under Apps sectionCheck Version:
occ app:list | grep mailVerify Fix Applied:
Verify Mail app version is 5.5.3 or higher in Nextcloud admin panel📡 Detection & Monitoring
Log Indicators:
- Unusual HTML patterns in email subject lines
- Multiple failed attempts to send emails with HTML in subjects
Network Indicators:
- Unusual patterns in email synchronization traffic
SIEM Query:
source="nextcloud" AND (message="mail" OR message="email") AND (subject CONTAINS "<" OR subject CONTAINS ">")