CVE-2021-27647
📋 TL;DR
This vulnerability allows remote attackers to execute arbitrary code on Synology DiskStation Manager (DSM) systems by sending specially crafted web requests. It affects DSM versions before 6.2.3-25426-3 and can be exploited without authentication. The out-of-bounds read in the iSCSI snapshot component enables remote code execution.
💻 Affected Systems
- Synology DiskStation Manager (DSM)
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with root-level access, data theft, ransomware deployment, and persistent backdoor installation.
Likely Case
Remote code execution leading to data exfiltration, lateral movement within the network, and service disruption.
If Mitigated
Limited impact if systems are patched, isolated from internet, and have proper network segmentation.
🎯 Exploit Status
ZDI advisory suggests weaponization is likely given the high CVSS score and unauthenticated nature.
🛠️ 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 DSM Update' if not already downloaded. 4. Click 'Install Now' and follow prompts. 5. System will restart automatically.
🔧 Temporary Workarounds
Disable iSCSI Service
linuxTemporarily disable iSCSI functionality to block exploitation vector
ssh admin@synology-nas
sudo synoservice --disable pkgctl-iSCSITarget
sudo synoservice --disable pkgctl-iSCSILUN
Network Isolation
linuxRestrict access to DSM web interface using firewall rules
iptables -A INPUT -p tcp --dport 5000 -s trusted_ip -j ACCEPT
iptables -A INPUT -p tcp --dport 5000 -j DROP
🧯 If You Can't Patch
- Isolate affected systems from internet using firewall rules
- Implement strict network segmentation to limit lateral movement
🔍 How to Verify
Check if Vulnerable:
Check DSM version in Control Panel > Info Center > DSM VersionCheck Version:
ssh admin@synology-nas 'cat /etc.defaults/VERSION'Verify Fix Applied:
Verify version is 6.2.3-25426-3 or higher in Control Panel > Info Center📡 Detection & Monitoring
Log Indicators:
- Unusual iSCSI-related requests in /var/log/messages
- Multiple failed authentication attempts followed by successful iSCSI operations
Network Indicators:
- Unusual outbound connections from DSM system
- Traffic to unexpected ports from DSM
SIEM Query:
source="synology*" AND ("iscsi_snapshot" OR "iSCSI" AND "snapshot") AND status=200