CVE-2021-31439
📋 TL;DR
This is a heap-based buffer overflow vulnerability in Netatalk's DSI structure processing that allows unauthenticated attackers on the same network to execute arbitrary code on Synology DiskStation Manager installations. The vulnerability affects Synology NAS devices running vulnerable versions of DSM. Attackers can gain control of the affected system without authentication.
💻 Affected Systems
- Synology DiskStation Manager
- Netatalk
📦 What is this software?
Netatalk by Netatalk
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with root-level code execution, data theft, ransomware deployment, and lateral movement to other network systems.
Likely Case
Remote code execution leading to data exfiltration, installation of backdoors, or cryptomining malware on affected Synology NAS devices.
If Mitigated
Limited impact if systems are isolated from untrusted networks, though internal attackers could still exploit if present on the same network segment.
🎯 Exploit Status
Exploit requires network adjacency but no authentication. The vulnerability is well-documented with public advisories and likely incorporated into attack frameworks.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Synology DSM 6.2.4-25556-2, 7.0-41890, or later; Netatalk 3.1.12 or later
Vendor Advisory: https://www.synology.com/zh-hk/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 available. 4. Click 'Install Now' and follow prompts. 5. System will restart automatically after update.
🔧 Temporary Workarounds
Disable AFP Service
linuxDisable Apple Filing Protocol service if not required, reducing attack surface.
ssh admin@nas-ip
sudo synoservice --disable netatalk
Network Segmentation
allIsolate Synology devices to trusted VLANs and restrict access to AFP port 548/tcp.
🧯 If You Can't Patch
- Implement strict network access controls to limit access to Synology devices to trusted IPs only
- Deploy intrusion detection/prevention systems to monitor for exploitation attempts on port 548
🔍 How to Verify
Check if Vulnerable:
Check DSM version via web interface: Control Panel > Info Center > DSM version. If version is below 6.2.4-25556-2 (for DSM 6.x) or 7.0-41890 (for DSM 7.x), system is vulnerable.Check Version:
ssh admin@nas-ip 'cat /etc.defaults/VERSION' | grep productversionVerify Fix Applied:
Confirm DSM version is 6.2.4-25556-2 or higher (DSM 6.x) or 7.0-41890 or higher (DSM 7.x) in Control Panel > Info Center.📡 Detection & Monitoring
Log Indicators:
- Unusual connections to port 548/tcp
- Multiple failed AFP connection attempts
- Process creation anomalies from netatalk service
Network Indicators:
- Unusual traffic patterns to/from port 548
- Large or malformed DSI packets to AFP service
SIEM Query:
source="synology-nas" AND (dest_port=548 OR protocol="afp") AND (bytes_out>100000 OR packet_size>1500)🔗 References
- https://lists.debian.org/debian-lts-announce/2023/05/msg00018.html
- https://security.gentoo.org/glsa/202311-02
- https://www.debian.org/security/2023/dsa-5503
- https://www.synology.com/zh-hk/security/advisory/Synology_SA_20_26
- https://www.zerodayinitiative.com/advisories/ZDI-21-492/
- https://lists.debian.org/debian-lts-announce/2023/05/msg00018.html
- https://security.gentoo.org/glsa/202311-02
- https://www.debian.org/security/2023/dsa-5503
- https://www.synology.com/zh-hk/security/advisory/Synology_SA_20_26
- https://www.zerodayinitiative.com/advisories/ZDI-21-492/
