CVE-2021-43928
📋 TL;DR
This CVE allows remote authenticated users to execute arbitrary operating system commands on Synology Mail Station servers through OS command injection in the mail sending/receiving component. Attackers with valid credentials can potentially gain full system control. Affects Synology Mail Station installations before version 20211105-10315.
💻 Affected Systems
- Synology Mail Station
📦 What is this software?
Mail Station by Synology
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attackers to execute arbitrary commands with system privileges, potentially leading to data theft, ransomware deployment, or persistent backdoor installation.
Likely Case
Authenticated attackers gaining shell access to the underlying operating system, enabling privilege escalation, lateral movement, and data exfiltration.
If Mitigated
Limited impact if proper network segmentation, least privilege access, and command execution restrictions are in place.
🎯 Exploit Status
Exploitation requires authenticated access but the vulnerability itself is straightforward OS command injection.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 20211105-10315 or later
Vendor Advisory: https://www.synology.com/security/advisory/Synology_SA_21_28
Restart Required: Yes
Instructions:
1. Log into DSM as administrator. 2. Open Package Center. 3. Find Synology Mail Station. 4. Click Update to version 20211105-10315 or later. 5. Restart the service when prompted.
🔧 Temporary Workarounds
Disable Mail Station
linuxTemporarily disable the vulnerable component until patching is possible
sudo synopkg stop MailStation
Restrict Access
linuxLimit network access to Mail Station using firewall rules
sudo iptables -A INPUT -p tcp --dport 443 -s trusted_networks -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 443 -j DROP
🧯 If You Can't Patch
- Implement strict network segmentation to isolate Mail Station from critical systems
- Enforce strong authentication policies and monitor for suspicious authenticated sessions
🔍 How to Verify
Check if Vulnerable:
Check Mail Station version in DSM Package Center or run: sudo synopkg version MailStationCheck Version:
sudo synopkg version MailStationVerify Fix Applied:
Verify version is 20211105-10315 or later: sudo synopkg version MailStation | grep -E '20211105-10315|^[0-9]{8}-[0-9]{5}$'📡 Detection & Monitoring
Log Indicators:
- Unusual command execution patterns in system logs
- Multiple failed authentication attempts followed by successful login
- Suspicious process creation from Mail Station service
Network Indicators:
- Unexpected outbound connections from Mail Station server
- Unusual traffic patterns to/from mail service ports
SIEM Query:
source="mailstation" AND (process_execution OR command_injection OR suspicious_shell_activity)