CVE-2021-44158
📋 TL;DR
ASUS RT-AX56U Wi-Fi routers have a stack-based buffer overflow vulnerability in their httpd service. An authenticated attacker on the local network can exploit this to execute arbitrary code, potentially taking full control of the router or disrupting its service. Only users of this specific router model are affected.
💻 Affected Systems
- ASUS RT-AX56U Wi-Fi Router
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of the router allowing attacker to intercept all network traffic, install persistent malware, or use the router as a pivot point into the internal network.
Likely Case
Router becomes unresponsive requiring physical reset, or attacker gains limited control to modify DNS settings or capture credentials.
If Mitigated
With proper network segmentation and monitoring, impact is limited to the router itself without lateral movement to other devices.
🎯 Exploit Status
Exploitation requires authentication but buffer overflow techniques for embedded devices are well understood. The vulnerability is in parameter handling of the httpd service.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.0.0.4.386.45956 or later
Vendor Advisory: https://www.asus.com/Networking-IoT-Servers/WiFi-Routers/ASUS-WiFi-Routers/RT-AX56U/HelpDesk_BIOS/
Restart Required: Yes
Instructions:
1. Log into router web interface. 2. Navigate to Administration > Firmware Upgrade. 3. Click 'Check' for updates or manually upload firmware. 4. Download firmware version 3.0.0.4.386.45956 or later from ASUS support site. 5. Upload and apply the firmware update. 6. Router will reboot automatically.
🔧 Temporary Workarounds
Restrict LAN Access
linuxLimit which devices can access the router's administration interface
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.100 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.100 -j DROP
Disable Remote Administration
allEnsure router web interface is only accessible from LAN
🧯 If You Can't Patch
- Segment the router on isolated VLAN to limit attack surface
- Implement strict access controls allowing only trusted devices to communicate with router management interface
🔍 How to Verify
Check if Vulnerable:
Log into router web interface, navigate to Administration > Firmware Upgrade, check current firmware version. If version is below 3.0.0.4.386.45956, device is vulnerable.Check Version:
curl -s http://router.asus.com/ | grep -i firmware || ssh admin@router 'nvram get buildno'Verify Fix Applied:
After updating, verify firmware version shows 3.0.0.4.386.45956 or higher in Administration > Firmware Upgrade page.📡 Detection & Monitoring
Log Indicators:
- Multiple failed authentication attempts followed by successful login
- Unusual httpd process crashes or restarts
- Large HTTP POST requests to router management interface
Network Indicators:
- Unusual outbound connections from router to external IPs
- DNS queries to suspicious domains from router
- Port scanning originating from router
SIEM Query:
source="router.logs" AND (http_status=500 OR process="httpd" AND event="crash")