CVE-2021-27649
📋 TL;DR
This is a critical use-after-free vulnerability in Synology DiskStation Manager's file transfer protocol component that allows remote attackers to execute arbitrary code on affected systems. Attackers can potentially gain full control of Synology NAS devices without authentication. All Synology DSM users with vulnerable versions are affected.
💻 Affected Systems
- Synology DiskStation Manager (DSM)
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, ransomware deployment, or device takeover for botnet participation.
Likely Case
Remote code execution allowing attackers to install malware, exfiltrate data, or pivot to internal networks.
If Mitigated
Limited impact if systems are isolated behind firewalls with strict network controls and no internet exposure.
🎯 Exploit Status
While no public PoC exists, the high CVSS score and remote unauthenticated nature make weaponization likely. Attackers need to craft specific network packets to trigger the use-after-free condition.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: DSM 6.2.3-25426-3 and later
Vendor Advisory: https://www.synology.com/security/advisory/Synology_SA_20_26
Restart Required: Yes
Instructions:
1. Log into DSM web interface as administrator. 2. Go to Control Panel > Update & Restore. 3. Click 'Download' for DSM update. 4. Click 'Install' when download completes. 5. System will restart automatically after installation.
🔧 Temporary Workarounds
Disable File Services
allTemporarily disable vulnerable file transfer protocols if patching cannot be done immediately
Network Segmentation
allIsolate Synology devices from internet and restrict internal network access
🧯 If You Can't Patch
- Immediately isolate affected devices from internet and restrict network access to trusted IPs only
- Implement strict firewall rules blocking all unnecessary inbound connections to Synology devices
🔍 How to Verify
Check if Vulnerable:
Check DSM version in Control Panel > Info Center. If version is below 6.2.3-25426-3, system is vulnerable.Check Version:
ssh admin@synology-nas 'cat /etc.defaults/VERSION'Verify Fix Applied:
After update, verify DSM version is 6.2.3-25426-3 or higher in Control Panel > Info Center.📡 Detection & Monitoring
Log Indicators:
- Unusual file transfer protocol activity
- Unexpected process execution
- Failed authentication attempts to DSM services
Network Indicators:
- Unusual outbound connections from Synology device
- Suspicious inbound traffic to file transfer ports (20, 21, 22, 139, 445, 873)
SIEM Query:
source="synology*" AND (event="failed_login" OR event="unusual_activity")