CVE-2021-35242
📋 TL;DR
This vulnerability in SolarWinds Serv-U is a Cross-Site Request Forgery (CSRF) flaw where the server improperly validates CSRF tokens when requests contain only session information. Attackers can trick authenticated users into performing unintended actions on the Serv-U server. This affects all organizations running vulnerable versions of SolarWinds Serv-U.
💻 Affected Systems
- SolarWinds Serv-U
📦 What is this software?
Serv U by Solarwinds
⚠️ Risk & Real-World Impact
Worst Case
Attackers could perform administrative actions on the Serv-U server, potentially compromising the entire file transfer infrastructure, stealing sensitive data, or deploying malware.
Likely Case
Attackers could modify user permissions, upload/download files without authorization, or disrupt file transfer operations.
If Mitigated
With proper network segmentation and access controls, impact is limited to the Serv-U server itself, though unauthorized file transfers could still occur.
🎯 Exploit Status
Exploitation requires the attacker to trick an authenticated user into visiting a malicious webpage. No authentication bypass is required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 15.2.5 and later
Vendor Advisory: https://www.solarwinds.com/trust-center/security-advisories/cve-2021-35242
Restart Required: Yes
Instructions:
1. Download Serv-U 15.2.5 or later from SolarWinds website. 2. Backup current configuration. 3. Run the installer to upgrade. 4. Restart Serv-U service. 5. Verify version is 15.2.5 or higher.
🔧 Temporary Workarounds
Network Segmentation
allRestrict access to Serv-U management interface to trusted networks only
Browser Security Headers
allImplement Content Security Policy and SameSite cookie attributes
🧯 If You Can't Patch
- Implement strict network access controls to limit Serv-U management interface access
- Monitor Serv-U logs for unusual administrative actions and file transfers
🔍 How to Verify
Check if Vulnerable:
Check Serv-U version via web interface or command line. Versions below 15.2.5 are vulnerable.Check Version:
On Windows: sc query "Serv-U" | findstr /i version. On Linux: serv-u --version or check web interface.Verify Fix Applied:
Verify Serv-U version is 15.2.5 or higher and test CSRF protection by attempting to submit requests without valid tokens.📡 Detection & Monitoring
Log Indicators:
- Multiple failed CSRF token validations
- Administrative actions from unexpected IP addresses
- Unusual file transfer patterns
Network Indicators:
- HTTP POST requests to Serv-U management endpoints without proper referrer headers
- Requests with missing or invalid CSRF tokens
SIEM Query:
source="serv-u" AND (event_type="csrf_failure" OR action="admin_*" AND src_ip NOT IN [trusted_ips])🔗 References
- https://documentation.solarwinds.com/en/success_center/servu/content/release_notes/servu_15-2-5_release_notes.htm
- https://www.solarwinds.com/trust-center/security-advisories/cve-2021-35242
- https://documentation.solarwinds.com/en/success_center/servu/content/release_notes/servu_15-2-5_release_notes.htm
- https://www.solarwinds.com/trust-center/security-advisories/cve-2021-35242
