CVE-2026-21881
📋 TL;DR
This critical authentication bypass vulnerability in Kanboard allows attackers to impersonate any user, including administrators, by sending spoofed HTTP headers when REVERSE_PROXY_AUTH is enabled. The application fails to verify that authentication headers originate from a trusted reverse proxy. All Kanboard instances running versions 1.2.48 or below with reverse proxy authentication enabled are affected.
💻 Affected Systems
- Kanboard
📦 What is this software?
Kanboard by Kanboard
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise where attackers gain administrative access, modify all projects, steal sensitive data, and potentially execute arbitrary code through subsequent attacks.
Likely Case
Unauthorized access to sensitive project data, manipulation of Kanban boards, privilege escalation, and potential data exfiltration.
If Mitigated
Limited impact if proper network segmentation and proxy validation are implemented, though risk remains if reverse proxy authentication is enabled.
🎯 Exploit Status
Exploitation requires only HTTP header manipulation. Public proof-of-concept exists in advisory references.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.2.49
Vendor Advisory: https://github.com/kanboard/kanboard/security/advisories/GHSA-wwpf-3j4p-739w
Restart Required: Yes
Instructions:
1. Backup your Kanboard installation and database. 2. Download version 1.2.49 from GitHub releases. 3. Replace existing files with new version. 4. Restart web server. 5. Verify functionality.
🔧 Temporary Workarounds
Disable REVERSE_PROXY_AUTH
allDisable reverse proxy authentication feature if not required
Edit config.php: define('REVERSE_PROXY_AUTH', false);
Configure Trusted Proxies
allRestrict which IP addresses can send reverse proxy headers
Edit config.php: define('REVERSE_PROXY_TRUSTED_PROXIES', '192.168.1.0/24');
🧯 If You Can't Patch
- Disable REVERSE_PROXY_AUTH configuration immediately
- Implement network controls to restrict access to Kanboard instance
🔍 How to Verify
Check if Vulnerable:
Check if REVERSE_PROXY_AUTH is enabled in config.php and version is ≤1.2.48Check Version:
Check Kanboard version in footer or via 'grep APP_VERSION app/constants.php'Verify Fix Applied:
Verify version is 1.2.49+ and test authentication with invalid proxy headers📡 Detection & Monitoring
Log Indicators:
- Authentication attempts with unusual HTTP headers
- User logins from unexpected IP addresses
- Administrative actions from non-admin users
Network Indicators:
- HTTP requests with X-Forwarded-User or similar headers from untrusted sources
- Authentication bypass attempts
SIEM Query:
source="web_logs" AND (header="X-Forwarded-User" OR header="REMOTE_USER") AND NOT src_ip IN [trusted_proxies]