1. Introduction
The ASUS RT-N13U Router has a built-in admin telnet account with known, unchangeable credentials. This means an attacker could gain complete control of your router if they can connect to it via Telnet. This affects routers that have not had their default settings changed and are accessible from the internet or internal networks. Successful exploitation results in full device compromise – confidentiality, integrity, and availability are all at risk.
2. Technical Explanation
The vulnerability stems from a hardcoded admin account with the username ‘admin’ and password ‘admin’. This service is enabled by default on many ASUS RT-N13U routers. An attacker can use standard telnet clients to connect to the router using these credentials, bypassing normal authentication mechanisms. There is no known CVE associated with this specific issue but it has been documented in security advisories. A simple example of an attack involves connecting to the router’s Telnet port (typically 23) and logging in with ‘admin/admin’.
- Root cause: The device ships with a default, unchangeable admin account enabled via telnet.
- Exploit mechanism: An attacker connects to the router’s telnet service using the hardcoded credentials (admin/admin) and gains shell access.
- Scope: ASUS RT-N13U routers running the default firmware configuration are affected.
3. Detection and Assessment
You can confirm if your system is vulnerable by checking for an active telnet service with the default credentials. A quick check involves attempting to connect via Telnet using ‘admin/admin’. More thorough assessment includes network scanning tools looking for open port 23.
- Quick checks: Attempt a telnet connection to the router’s IP address using username ‘admin’ and password ‘admin’.
- Scanning: Nmap can be used with the following command: `nmap -p 23
`. Look for open port 23. Nessus plugin ID 63394 may also detect this vulnerability. - Logs and evidence: Check router logs for successful telnet logins from unexpected sources, though logging may be limited.
telnet 4. Solution / Remediation Steps
Since there is no available fix, the only workaround is to restrict access to the Telnet service. These steps aim to minimize exposure and prevent unauthorized access.
4.1 Preparation
- There are no dependencies, but ensure you have console access to the router in case of issues. Change windows may be needed depending on network usage.
4.2 Implementation
- Step 1: Disable Telnet access through the router’s web interface (if possible). Navigate to Administration > System and disable Telnet service.
- Step 2: Block incoming connections to port 23 on your firewall. This prevents external access to the telnet service.
- Step 3: If you cannot disable Telnet, restrict access to specific trusted IP addresses only using firewall rules.
4.3 Config or Code Example
Before
(Telnet service enabled by default)After
(Telnet service disabled, or access restricted via firewall rules)4.4 Security Practices Relevant to This Vulnerability
Several security practices can help mitigate this type of vulnerability. Least privilege reduces the impact if exploited. Safe defaults prevent insecure configurations. Patch cadence ensures timely updates and fixes.
- Practice 1: Implement least privilege by limiting user access rights on your router.
4.5 Automation (Optional)
Automation is limited for this specific issue, but you can use scripting to check firewall rules and telnet service status across multiple routers.
# Example Bash script to check if port 23 is open:
#!/bin/bash
target_ip=$1
if nc -z $target_ip 23; then
echo "Port 23 is open on $target_ip"
else
echo "Port 23 is closed on $target_ip"
fi5. Verification / Validation
Confirm the fix by verifying that Telnet access is blocked or restricted. Attempt to connect via Telnet again and confirm you cannot log in with ‘admin/admin’. A smoke test involves checking basic internet connectivity.
- Post-fix check: Attempt a telnet connection to the router’s IP address using username ‘admin’ and password ‘admin’. Expected output: Connection refused or timeout.
- Re-test: Re-run the Nmap scan from step 3. Verify that port 23 is closed or only accessible from trusted IPs.
- Smoke test: Confirm you can still browse the internet and access basic network services.
telnet (Expected output: Connection refused) 6. Preventive Measures and Monitoring
Update security baselines to include a requirement for changing default credentials on all routers. Implement CI/CD pipelines with SAST tools to detect hardcoded credentials in firmware images. A sensible patch or config review cycle should be established based on the risk assessment.
- Baselines: Update your router security baseline to require immediate password changes and disabling unnecessary services like Telnet.
- Pipelines: Integrate SAST tools into your CI/CD pipeline to scan firmware images for hardcoded credentials.
7. Risks, Side Effects, and Roll Back
Disabling Telnet may impact remote management capabilities if you rely on it. Blocking port 23 could affect other services using that port (though rare). To roll back, re-enable the telnet service in the router’s web interface or remove firewall rules blocking port 23.
- Risk or side effect 1: Loss of remote management access if Telnet was used for administration.
- Risk or side effect 2: Potential disruption to other services using port 23 (unlikely).
- Roll back: 1) Re-enable the telnet service in the router’s web interface. 2) Remove any firewall rules blocking incoming connections on port 23.
8. References and Resources
- Vendor advisory or bulletin: https://seclists.org/fulldisclosure/2013/Oct/271
- NVD or CVE entry: No specific CVE is associated with this issue.
- Product or platform documentation relevant to the fix: ASUS Support Website