CVE-2021-42074
📋 TL;DR
CVE-2021-42074 is a denial-of-service vulnerability in Barrier software versions before 2.3.4 where an unauthenticated attacker can crash the barriers server component by rapidly opening and closing TCP connections while sending Hello messages. This affects anyone running Barrier server (barriers) in vulnerable versions, potentially disrupting keyboard/mouse sharing functionality.
💻 Affected Systems
- Barrier
📦 What is this software?
Barrier by Barrier Project
⚠️ Risk & Real-World Impact
Worst Case
Complete denial of service where the barriers server crashes and becomes unavailable, disrupting all connected clients' keyboard/mouse sharing capabilities until manually restarted.
Likely Case
Service disruption causing temporary loss of Barrier functionality, requiring manual intervention to restart the server.
If Mitigated
Minimal impact with proper network segmentation and access controls limiting who can reach the Barrier server port.
🎯 Exploit Status
The exploit requires no authentication and involves simple TCP connection manipulation. Public proof-of-concept code exists in security advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.3.4
Vendor Advisory: https://github.com/debauchee/barrier/releases/tag/v2.3.4
Restart Required: Yes
Instructions:
1. Download Barrier 2.3.4 or later from the official GitHub releases. 2. Stop the running Barrier service. 3. Install the updated version. 4. Restart the Barrier service.
🔧 Temporary Workarounds
Network Access Control
allRestrict network access to the Barrier server port (default 24800) using firewall rules to only trusted IP addresses.
# Linux iptables example: iptables -A INPUT -p tcp --dport 24800 -s trusted_ip -j ACCEPT
# Windows Firewall: New-NetFirewallRule -DisplayName "Barrier Restrict" -Direction Inbound -LocalPort 24800 -RemoteAddress trusted_ip -Protocol TCP -Action Allow
🧯 If You Can't Patch
- Implement strict network segmentation and firewall rules to limit access to Barrier server port (24800) to only necessary trusted hosts.
- Monitor Barrier server process health and implement automatic restart mechanisms if crashes are detected.
🔍 How to Verify
Check if Vulnerable:
Check Barrier version: barriers --version or check installed version in application settings. If version is below 2.3.4, the system is vulnerable.Check Version:
barriers --versionVerify Fix Applied:
Verify installed version is 2.3.4 or higher using barriers --version command or application settings.📡 Detection & Monitoring
Log Indicators:
- Multiple rapid connection attempts to port 24800
- Barrier server process crash/segmentation fault logs
- Unexpected Barrier service restarts
Network Indicators:
- High volume of TCP SYN packets to port 24800 with immediate RST/FIN
- Multiple short-lived connections to Barrier port
SIEM Query:
source="barrier.log" AND ("segmentation fault" OR "crash" OR "SIGSEGV") OR destination_port=24800 AND (tcp_flags="SYN" AND packet_count>100 within 10s)