CVE-2021-3254
📋 TL;DR
This vulnerability in Asus DSL-N14U-B1 routers allows remote attackers to cause a Denial of Service (DoS) by sending TCP SYN packets, which can crash or disrupt the device's network services. It affects users of this specific router model with vulnerable firmware. The attack requires no authentication and can be performed using common network scanning tools like nmap.
💻 Affected Systems
- Asus DSL-N14U-B1
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete router crash requiring physical power cycle, disrupting all network connectivity for connected devices.
Likely Case
Temporary network service disruption affecting internet connectivity and local network services.
If Mitigated
Minimal impact with proper network segmentation and firewall rules blocking external SYN scans.
🎯 Exploit Status
Exploit uses standard nmap SYN scan: 'nmap -sS [target_ip]'. No special tools or authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Unknown
Vendor Advisory: No official vendor advisory found
Restart Required: No
Instructions:
No official patch available. Check Asus support site for firmware updates. If newer firmware exists, download from Asus website and flash via router admin interface.
🔧 Temporary Workarounds
Firewall SYN Flood Protection
linuxConfigure firewall to limit SYN packets per second to prevent DoS
iptables -A INPUT -p tcp --syn -m limit --limit 1/s -j ACCEPT
iptables -A INPUT -p tcp --syn -j DROP
Network Segmentation
allPlace router behind another firewall or in DMZ to limit direct exposure
🧯 If You Can't Patch
- Replace router with supported model that receives security updates
- Implement network monitoring to detect SYN flood attacks and alert on suspicious activity
🔍 How to Verify
Check if Vulnerable:
Check router firmware version in admin interface. If version is 1.1.2.3_805, device is vulnerable.Check Version:
Login to router admin interface (typically 192.168.1.1) and check Firmware Version in System StatusVerify Fix Applied:
After applying workarounds, test with nmap SYN scan: 'nmap -sS [router_ip]' should not cause service disruption.📡 Detection & Monitoring
Log Indicators:
- High volume of TCP SYN packets in router logs
- Router reboot events
- Connection resets
Network Indicators:
- Unusual high rate of SYN packets to router IP
- SYN packets without subsequent ACK completion
SIEM Query:
source="router_logs" AND "TCP SYN" AND count > 1000 per 60s