CVE-2021-27239
📋 TL;DR
This vulnerability allows attackers on the same network to execute arbitrary code as root on NETGEAR R6400 and R6700 routers without authentication. The flaw exists in the upnpd service, where a crafted SSDP message with a malicious MX header triggers a stack-based buffer overflow. Affected users are those running vulnerable firmware versions on these router models.
💻 Affected Systems
- NETGEAR R6400
- NETGEAR R6700
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router compromise with root-level code execution, allowing attackers to intercept traffic, install persistent malware, pivot to internal networks, or brick the device.
Likely Case
Router takeover leading to man-in-the-middle attacks, credential theft, network surveillance, and potential lateral movement to connected devices.
If Mitigated
Limited impact if routers are isolated from untrusted networks, though internal threats could still exploit if on same network segment.
🎯 Exploit Status
Exploitation requires sending crafted SSDP packets to UDP 1900. Public exploit code exists, making this easily weaponizable by attackers with network access.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware versions after 1.0.4.98
Vendor Advisory: https://kb.netgear.com/000062820/Security-Advisory-for-Stack-based-Buffer-Overflow-Remote-Code-Execution-Vulnerability-on-Some-Routers-PSV-2020-0432
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 via SSDP messages.
Log into router admin interface, navigate to Advanced > Advanced Setup > UPnP, disable UPnP
Block UDP Port 1900
linuxBlock incoming SSDP traffic at network perimeter or on router firewall.
Add firewall rule: iptables -A INPUT -p udp --dport 1900 -j DROP
🧯 If You Can't Patch
- Isolate routers on separate VLANs to limit network adjacency attack surface
- Implement network segmentation to restrict lateral movement from compromised routers
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface under Advanced > Administration > Firmware Update. If version is 1.0.4.98, device is vulnerable.Check Version:
From router admin interface or via SSH if enabled: cat /proc/version or check web interface firmware versionVerify Fix Applied:
Verify firmware version is updated beyond 1.0.4.98. Test by attempting to connect to UDP 1900 - service should still respond but crafted SSDP packets should not cause crashes.📡 Detection & Monitoring
Log Indicators:
- Multiple SSDP requests with unusual MX header values
- upnpd service crashes or restarts
- Unusual outbound connections from router
Network Indicators:
- SSDP packets with abnormally long MX headers (> typical 1-5 range)
- UDP port 1900 traffic from unexpected internal sources
SIEM Query:
source_port:1900 AND (mx_header_length > 10 OR packet_size > 1500)🔗 References
- https://kb.netgear.com/000062820/Security-Advisory-for-Stack-based-Buffer-Overflow-Remote-Code-Execution-Vulnerability-on-Some-Routers-PSV-2020-0432
- https://www.zerodayinitiative.com/advisories/ZDI-21-206/
- https://kb.netgear.com/000062820/Security-Advisory-for-Stack-based-Buffer-Overflow-Remote-Code-Execution-Vulnerability-on-Some-Routers-PSV-2020-0432
- https://www.zerodayinitiative.com/advisories/ZDI-21-206/
