CVE-2021-27561
📋 TL;DR
CVE-2021-27561 is an unauthenticated command injection vulnerability in Yealink Device Management that allows remote attackers to execute arbitrary commands as root. This affects Yealink DM version 3.6.0.20, potentially compromising the entire device management system. Organizations using vulnerable Yealink device management servers are at risk.
💻 Affected Systems
- Yealink Device Management
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attacker to gain root access, install persistent backdoors, pivot to other network systems, and exfiltrate sensitive data.
Likely Case
Remote code execution leading to device takeover, credential theft, and deployment of malware or ransomware on managed devices.
If Mitigated
Limited impact if isolated in segmented network with strict firewall rules and no internet exposure.
🎯 Exploit Status
Exploit requires no authentication and minimal technical skill; CISA lists as known exploited.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Update to version later than 3.6.0.20 (check Yealink for specific fixed version)
Vendor Advisory: https://www.yealink.com/en/security-advisory
Restart Required: Yes
Instructions:
1. Check current DM version. 2. Download latest firmware from Yealink support portal. 3. Backup configuration. 4. Apply update via web interface or CLI. 5. Verify update and restart services.
🔧 Temporary Workarounds
Network Segmentation
linuxIsolate Yealink DM server from internet and restrict access to trusted IPs only.
iptables -A INPUT -p tcp --dport 80 -s TRUSTED_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s TRUSTED_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP
Disable Vulnerable Endpoint
allBlock access to the vulnerable /sm/api/v1/firewall/zone/services URI via web server configuration.
location /sm/api/v1/firewall/zone/services { deny all; }
🧯 If You Can't Patch
- Immediately isolate the DM server from internet and implement strict network access controls
- Monitor for suspicious activity on the DM server and implement application-level WAF rules
🔍 How to Verify
Check if Vulnerable:
Check if Yealink DM version is 3.6.0.20 via web interface or SSH, or test for vulnerable endpoint with curl: curl -X POST http://DM_IP/sm/api/v1/firewall/zone/servicesCheck Version:
ssh admin@DM_IP 'cat /etc/version' or check web interface System InformationVerify Fix Applied:
Verify version is updated beyond 3.6.0.20 and test endpoint no longer accepts malicious payloads.📡 Detection & Monitoring
Log Indicators:
- Unusual POST requests to /sm/api/v1/firewall/zone/services
- Suspicious command execution in system logs
- Unexpected root user activity
Network Indicators:
- POST requests to vulnerable URI with shell metacharacters
- Outbound connections from DM server to unknown IPs
SIEM Query:
source="yealink-dm" AND (uri="/sm/api/v1/firewall/zone/services" OR cmd="*;*" OR cmd="*|*")