CVE-2021-26562
📋 TL;DR
This vulnerability allows man-in-the-middle attackers to execute arbitrary code on Synology DiskStation Manager (DSM) systems by exploiting an out-of-bounds write in the synoagentregisterd service via a crafted HTTP header. It affects DSM versions before 6.2.3-25426-3, potentially compromising Synology NAS devices. Attackers can achieve remote code execution with high privileges.
💻 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 on Synology NAS devices.
Likely Case
Remote code execution leading to data exfiltration, lateral movement within the network, and deployment of malware or cryptominers.
If Mitigated
Limited impact due to network segmentation, proper TLS configuration, and intrusion detection systems blocking malicious traffic.
🎯 Exploit Status
Exploitation requires man-in-the-middle position to inject malicious syno_finder_site HTTP header. Technical details and proof-of-concept are publicly available.
🛠️ 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 after update.
🔧 Temporary Workarounds
Network Segmentation
allIsolate Synology NAS devices from untrusted networks and limit access to trusted IPs only.
Disable Unnecessary Services
linuxDisable synoagentregisterd service if not required for your use case.
ssh admin@synology-nas
sudo synoservice --disable pkgctl-synoagentregisterd
🧯 If You Can't Patch
- Implement strict network access controls to limit connections to Synology devices from trusted sources only.
- Deploy intrusion detection/prevention systems to monitor for malicious HTTP headers and block exploitation attempts.
🔍 How to Verify
Check if Vulnerable:
Check DSM version via web interface: Control Panel > Info Center > DSM Version. If version is below 6.2.3-25426-3, system is vulnerable.Check Version:
ssh admin@synology-nas 'cat /etc.defaults/VERSION' | grep productversionVerify Fix Applied:
After updating, verify DSM version is 6.2.3-25426-3 or higher in Control Panel > Info Center.📡 Detection & Monitoring
Log Indicators:
- Unusual process creation from synoagentregisterd
- Crash logs for synoagentregisterd service
- HTTP requests with unusually long or malformed syno_finder_site headers
Network Indicators:
- HTTP traffic to Synology devices containing manipulated syno_finder_site headers
- Unexpected outbound connections from Synology devices post-exploitation
SIEM Query:
source="synology_logs" AND (process="synoagentregisterd" AND event="crash") OR (http_header="syno_finder_site" AND length(http_value)>100)