CVE-2020-8295
📋 TL;DR
A logic flaw in Nextcloud Server's password reset functionality allows attackers to trigger a denial of service condition. This affects Nextcloud Server versions 19 and earlier, potentially impacting all users of vulnerable instances.
💻 Affected Systems
- Nextcloud Server
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability for all users due to resource exhaustion from repeated password reset requests.
Likely Case
Temporary service degradation or unavailability affecting user productivity.
If Mitigated
Minimal impact with proper rate limiting and monitoring in place.
🎯 Exploit Status
Exploit requires no authentication and can be performed via simple HTTP requests.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Nextcloud Server 20.0.0 and later
Vendor Advisory: https://nextcloud.com/security/advisory/?id=NC-SA-2021-003
Restart Required: No
Instructions:
1. Update Nextcloud Server to version 20.0.0 or later. 2. No restart required as it's a web application.
🔧 Temporary Workarounds
Disable password reset functionality
allTemporarily disable the password reset feature to prevent exploitation.
Edit config/config.php and set 'lost_password_link' => false
Implement rate limiting
linuxAdd rate limiting for password reset requests at the web server or application level.
# Example nginx rate limiting: limit_req_zone $binary_remote_addr zone=reset:10m rate=1r/m;
# Then apply to password reset endpoint
🧯 If You Can't Patch
- Implement strict rate limiting on password reset endpoints
- Monitor for unusual patterns of password reset requests
🔍 How to Verify
Check if Vulnerable:
Check Nextcloud version via admin panel or by examining the version.php file.Check Version:
grep 'OC_VersionString' /path/to/nextcloud/version.phpVerify Fix Applied:
Confirm Nextcloud version is 20.0.0 or later and test password reset functionality.📡 Detection & Monitoring
Log Indicators:
- Unusually high volume of POST requests to /index.php/lostpassword/ endpoints
- Multiple failed password reset attempts from single IP
Network Indicators:
- High volume of HTTP POST requests to password reset endpoint
- Spike in server resource usage
SIEM Query:
source="nextcloud.log" AND "lostpassword" | stats count by src_ip