CVE-2021-36798
📋 TL;DR
A Denial-of-Service vulnerability in Cobalt Strike Team Server allows remote attackers to crash the C2 server thread, blocking beacon communications. This affects Cobalt Strike 4.2 and 4.3 installations, potentially disrupting red team operations and adversary infrastructure.
💻 Affected Systems
- HelpSystems Cobalt Strike
📦 What is this software?
Cobalt Strike by Helpsystems
Cobalt Strike by Helpsystems
⚠️ Risk & Real-World Impact
Worst Case
Complete disruption of C2 operations, making all beacons unreachable and halting ongoing campaigns until server restart.
Likely Case
Temporary C2 server thread crash requiring manual restart, causing beacon communication interruptions during attacks.
If Mitigated
Minimal impact with proper network segmentation and monitoring allowing quick detection and recovery.
🎯 Exploit Status
Exploit requires sending specially crafted packets to the Team Server port (default 50050). Public exploit code exists in security research publications.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.4
Vendor Advisory: https://www.cobaltstrike.com/releasenotes.txt
Restart Required: Yes
Instructions:
1. Download Cobalt Strike 4.4 or later from HelpSystems portal. 2. Stop the vulnerable Team Server. 3. Deploy the updated version. 4. Restart Team Server with updated binaries.
🔧 Temporary Workarounds
Network Segmentation
linuxRestrict access to Team Server port using firewall rules to only allow connections from trusted IPs.
iptables -A INPUT -p tcp --dport 50050 -s TRUSTED_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 50050 -j DROP
Port Change
allChange default Team Server port from 50050 to non-standard port to reduce automated attack surface.
Edit teamserver configuration file to change port binding
🧯 If You Can't Patch
- Implement strict network ACLs allowing only beacon IPs to connect to Team Server
- Deploy redundant Team Servers with load balancing to maintain availability during attacks
🔍 How to Verify
Check if Vulnerable:
Check Cobalt Strike version: cat /path/to/cobaltstrike/version.txt or examine startup logs for version 4.2 or 4.3.Check Version:
grep -i version /path/to/cobaltstrike/*.txt || java -jar cobaltstrike.jar --versionVerify Fix Applied:
Confirm version is 4.4 or later and test Team Server stability under simulated attack conditions.📡 Detection & Monitoring
Log Indicators:
- Team Server thread crash logs
- Unexpected server restarts
- Beacon connection failures in logs
Network Indicators:
- Multiple malformed packets to Team Server port
- Sudden drop in beacon check-ins
- Unusual traffic patterns to C2 port
SIEM Query:
source="cobaltstrike.log" AND ("thread crash" OR "server restart" OR "connection reset")🔗 References
- https://labs.sentinelone.com/hotcobalt-new-cobalt-strike-dos-vulnerability-that-lets-you-halt-operations/
- https://www.cobaltstrike.com/releasenotes.txt
- https://labs.sentinelone.com/hotcobalt-new-cobalt-strike-dos-vulnerability-that-lets-you-halt-operations/
- https://www.cobaltstrike.com/releasenotes.txt
