CVE-2021-34991
📋 TL;DR
This is a critical buffer overflow vulnerability in NETGEAR R6400v2 routers that allows network-adjacent attackers to execute arbitrary code as root without authentication. The flaw exists in the UPnP service when processing uuid headers, enabling remote code execution. All users of affected NETGEAR router models with vulnerable firmware versions are at risk.
💻 Affected Systems
- NETGEAR R6400v2
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router compromise with root-level access, allowing attacker to intercept all network traffic, install persistent malware, pivot to internal networks, and brick the device.
Likely Case
Router takeover leading to man-in-the-middle attacks, credential theft, DNS hijacking, and botnet recruitment.
If Mitigated
Limited to denial of service if UPnP is disabled or network segmentation prevents access to port 5000.
🎯 Exploit Status
ZDI published technical details and proof-of-concept. Exploitation requires network adjacency but no authentication.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware version 1.0.4.120 or later
Vendor Advisory: https://kb.netgear.com/000064361/Security-Advisory-for-Pre-Authentication-Buffer-Overflow-on-Multiple-Products-PSV-2021-0168
Restart Required: Yes
Instructions:
1. Log into router admin interface. 2. Navigate to Advanced > Administration > Firmware Update. 3. Check for updates and install latest firmware. 4. Reboot router after update completes.
🔧 Temporary Workarounds
Disable UPnP Service
allTurn off Universal Plug and Play service to prevent exploitation
Navigate to Advanced > Advanced Setup > UPnP and disable
Block Port 5000
linuxBlock external and internal access to UPnP service port
Add firewall rule: iptables -A INPUT -p tcp --dport 5000 -j DROP
🧯 If You Can't Patch
- Segment router onto isolated network segment with strict access controls
- Implement network monitoring for suspicious traffic to port 5000
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface under Advanced > Administration > Firmware UpdateCheck Version:
curl -s http://routerlogin.com/currentsetting.htm | grep FirmwareVerify Fix Applied:
Confirm firmware version is 1.0.4.120 or later and UPnP service is either disabled or patched📡 Detection & Monitoring
Log Indicators:
- Multiple malformed UPnP requests to port 5000
- Buffer overflow errors in router logs
- Unusual process execution
Network Indicators:
- Exploit traffic patterns to TCP port 5000
- Shellcode patterns in network captures
- Unusual outbound connections from router
SIEM Query:
source="router" dest_port=5000 AND (http_user_agent CONTAINS "uuid" OR payload_size>1024)🔗 References
- https://kb.netgear.com/000064361/Security-Advisory-for-Pre-Authentication-Buffer-Overflow-on-Multiple-Products-PSV-2021-0168
- https://www.zerodayinitiative.com/advisories/ZDI-21-1303/
- https://kb.netgear.com/000064361/Security-Advisory-for-Pre-Authentication-Buffer-Overflow-on-Multiple-Products-PSV-2021-0168
- https://www.zerodayinitiative.com/advisories/ZDI-21-1303/
