CVE-2021-26569
📋 TL;DR
A race condition vulnerability in Synology DiskStation Manager's iSCSI snapshot component allows remote attackers to execute arbitrary code via crafted web requests. This affects Synology NAS devices running DSM before version 6.2.3-25426-3, potentially giving attackers full system control.
💻 Affected Systems
- Synology DiskStation Manager (DSM)
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Remote unauthenticated attacker gains root privileges and full control of the NAS device, potentially compromising all stored data and using the device as a pivot point into the network.
Likely Case
Remote attacker executes arbitrary code with elevated privileges, potentially installing malware, exfiltrating data, or disrupting services.
If Mitigated
Attack prevented by network segmentation, patched systems, or disabled vulnerable services.
🎯 Exploit Status
Exploitation requires race condition timing but is remotely accessible via web interface. ZDI advisory suggests exploitation is feasible.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: DSM 6.2.3-25426-3 or 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 needed. 4. Click 'Update Now' to install DSM 6.2.3-25426-3 or later. 5. System will restart automatically.
🔧 Temporary Workarounds
Disable iSCSI Service
allTemporarily disable iSCSI functionality if not required
Login to DSM > Storage Manager > iSCSI Manager > Disable iSCSI service
Network Segmentation
allRestrict access to DSM web interface to trusted networks only
Configure firewall rules to limit DSM port (default 5000/5001) access
🧯 If You Can't Patch
- Isolate affected NAS devices from internet and untrusted networks
- Implement strict network access controls to DSM management interface
🔍 How to Verify
Check if Vulnerable:
Check DSM version in Control Panel > Info Center. If version is earlier than 6.2.3-25426-3, system is vulnerable.Check Version:
ssh admin@nas 'cat /etc.defaults/VERSION'Verify Fix Applied:
Confirm DSM version is 6.2.3-25426-3 or later in Control Panel > Info Center.📡 Detection & Monitoring
Log Indicators:
- Unusual iSCSI snapshot activity
- Multiple rapid requests to iSCSI endpoints
- Process execution from web service context
Network Indicators:
- Multiple HTTP requests to /webapi/entry.cgi with iSCSI parameters
- Unusual outbound connections from NAS after web requests
SIEM Query:
source="synology" AND (uri_path="/webapi/entry.cgi" AND query="api=SYNO.Core.ISCSISnapshot" AND count>10 within 1s)