CVE-2019-18839
📋 TL;DR
CVE-2019-18839 is a stored cross-site scripting (XSS) vulnerability in FUDForum 3.0.9 that allows authenticated attackers to inject malicious scripts via the nlogin parameter. When an administrator views the compromised user's information, the payload executes, potentially leading to remote code execution and full system compromise. All FUDForum 3.0.9 installations are affected.
💻 Affected Systems
- FUDForum
📦 What is this software?
Fudforum by Fudforum
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining remote code execution, writing PHP files to web root, and taking complete control of the server.
Likely Case
Attacker gains administrative privileges, executes arbitrary code, and potentially compromises the entire FUDForum installation and underlying server.
If Mitigated
Attack limited to authenticated user context with proper input validation and output encoding preventing payload execution.
🎯 Exploit Status
Exploit requires authenticated user account. Public proof-of-concept code available on GitHub and Packet Storm.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.0.10 or later
Vendor Advisory: https://github.com/fudforum/FUDforum/security/advisories
Restart Required: No
Instructions:
1. Backup current installation. 2. Download latest FUDForum version. 3. Replace vulnerable files. 4. Verify proper input validation and output encoding in user profile handling.
🔧 Temporary Workarounds
Input Validation Filter
allAdd server-side validation to sanitize nlogin parameter before processing.
# Add to user profile processing code:
# $nlogin = htmlspecialchars($nlogin, ENT_QUOTES, 'UTF-8');
Content Security Policy
allImplement CSP headers to restrict script execution from untrusted sources.
# Add to .htaccess or server config:
# Header set Content-Security-Policy "default-src 'self'; script-src 'self'"
# For Nginx:
# add_header Content-Security-Policy "default-src 'self'; script-src 'self';"
🧯 If You Can't Patch
- Implement web application firewall (WAF) rules to block XSS payloads in POST requests.
- Restrict user registration and require admin approval for new accounts to limit attack surface.
🔍 How to Verify
Check if Vulnerable:
Check if FUDForum version is 3.0.9. Review source code for lack of input validation on nlogin parameter in user profile handling.Check Version:
Check FUDForum version in admin panel or review CHANGELOG.txt file in installation directory.Verify Fix Applied:
Test that nlogin parameter is properly sanitized and XSS payloads no longer execute when admin views user profiles.📡 Detection & Monitoring
Log Indicators:
- POST requests to user profile endpoints containing script tags or JavaScript in nlogin parameter
- Admin user accessing modified user profiles with suspicious content
Network Indicators:
- HTTP POST requests with encoded JavaScript in parameters
- Unexpected file writes to web root directory
SIEM Query:
source="web_logs" AND (method="POST" AND uri="/forum/profile.php" AND param="nlogin" AND (content="<script>" OR content="javascript:"))