CVE-2021-29087
📋 TL;DR
This path traversal vulnerability in Synology DiskStation Manager's webapi component allows remote attackers to write arbitrary files to restricted directories. It affects Synology DSM versions before 6.2.3-25426-3, potentially enabling attackers to modify system files or upload malicious content.
💻 Affected Systems
- Synology DiskStation Manager (DSM)
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise through arbitrary file write leading to remote code execution, data theft, or ransomware deployment.
Likely Case
Unauthorized file modification, configuration changes, or malware upload to the NAS system.
If Mitigated
Limited impact if proper network segmentation and access controls prevent exploitation attempts.
🎯 Exploit Status
Vectors unspecified in advisory; requires web access but authentication status unclear.
🛠️ 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. 2. Go to Control Panel > Update & Restore. 3. Click 'Download DSM Update' if available. 4. Click 'Install Now' for DSM 6.2.3-25426-3 or later. 5. System will restart automatically.
🔧 Temporary Workarounds
Network Access Restriction
allRestrict access to DSM web interface to trusted networks only
Firewall Rules
allBlock external access to DSM web ports (default 5000/5001)
🧯 If You Can't Patch
- Isolate Synology device on separate VLAN with strict access controls
- Implement web application firewall (WAF) with path traversal protection rules
🔍 How to Verify
Check if Vulnerable:
Check DSM version in Control Panel > Info Center > DSM versionCheck Version:
ssh admin@synology 'cat /etc.defaults/VERSION'Verify Fix Applied:
Verify DSM version is 6.2.3-25426-3 or later after update📡 Detection & Monitoring
Log Indicators:
- Unusual file write operations in webapi logs
- Path traversal patterns in web access logs
Network Indicators:
- HTTP requests with directory traversal sequences (../, ..\) to webapi endpoints
SIEM Query:
source="dsm_logs" AND ("..\" OR "../" OR "%2e%2e") AND "webapi"