CVE-2021-41738
📋 TL;DR
ZeroShell 3.9.5 contains a command injection vulnerability in the /cgi-bin/kerbynet IP parameter that allows authenticated attackers to execute arbitrary system commands. This affects organizations using ZeroShell 3.9.5 for network management. Attackers with valid credentials can achieve remote code execution.
💻 Affected Systems
- ZeroShell
📦 What is this software?
Zeroshell by Zeroshell
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attacker to install persistent backdoors, steal sensitive data, pivot to other network segments, and disrupt network operations.
Likely Case
Attacker gains shell access to the ZeroShell appliance, potentially compromising network configurations, intercepting traffic, or using the system as a foothold for lateral movement.
If Mitigated
With proper network segmentation and authentication controls, impact limited to the isolated ZeroShell appliance with minimal data exposure.
🎯 Exploit Status
Exploitation requires authentication but is straightforward once credentials are obtained. Public technical details available.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.9.6 or later
Vendor Advisory: https://www.zeroshell.net/
Restart Required: Yes
Instructions:
1. Backup current configuration. 2. Download ZeroShell 3.9.6 or later from official site. 3. Follow upgrade instructions in documentation. 4. Restart services or appliance.
🔧 Temporary Workarounds
Network Access Restriction
linuxRestrict access to ZeroShell web interface to trusted IP addresses only
iptables -A INPUT -p tcp --dport 443 -s TRUSTED_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP
Authentication Hardening
allImplement strong password policies and multi-factor authentication if supported
🧯 If You Can't Patch
- Isolate ZeroShell appliance in dedicated VLAN with strict firewall rules
- Implement network monitoring and alerting for suspicious commands or connections from ZeroShell
🔍 How to Verify
Check if Vulnerable:
Check ZeroShell version via web interface or SSH: cat /etc/zeroshell/versionCheck Version:
cat /etc/zeroshell/versionVerify Fix Applied:
Verify version is 3.9.6 or later and test IP parameter input validation📡 Detection & Monitoring
Log Indicators:
- Unusual command execution in system logs
- Multiple failed authentication attempts followed by successful login
- Suspicious processes spawned from web server
Network Indicators:
- Unexpected outbound connections from ZeroShell appliance
- Traffic to unusual ports from ZeroShell
SIEM Query:
source="zeroshell" AND (process="sh" OR process="bash" OR process="python") AND user="www-data"