How to remediate – ZyXEL Router Default Telnet Password Present

1. Introduction

2. Technical Explanation

The vulnerability exists because ZyXEL routers ship with a default username and password for Telnet access. An attacker can connect to the router using these credentials without needing valid user account details. CVE-1999-0571 describes this issue. A simple example is an attacker using a basic Telnet client to log in, then changing the administrator password or enabling remote management features. Affected devices are ZyXEL routers with default credentials enabled.

• Root cause: The router uses insecure default credentials upon initial installation.
• Exploit mechanism: An attacker attempts to connect using Telnet with the default username and password, gaining access to the router’s command line interface.
• Scope: ZyXEL routers that have not had their default passwords changed are affected. Specific models were not identified in available documentation.

3. Detection and Assessment

You can check if a system is vulnerable by attempting to connect via Telnet using the default credentials. A more thorough method involves reviewing router configurations for default settings.

• Quick checks: Attempt a Telnet connection with username ‘admin’ and password ‘1234’. If successful, the device is likely vulnerable.
• Scanning: Nessus plugin ID 10865 can detect this vulnerability as an example.
• Logs and evidence: Check router logs for successful logins using default credentials. Log locations vary by ZyXEL model.

telnet <router_ip>

4. Solution / Remediation Steps

The following steps will fix the issue by changing the default password on the router.

4.1 Preparation

• Dependencies: Access to the router’s web interface or command line. Roll back involves restoring the backed-up configuration if issues occur.
• A change window may be needed during busy periods, and approval from a senior network administrator might be necessary.

4.2 Implementation

1. Step 1: Log in to the router’s web interface using the default credentials (if possible).
2. Step 2: Navigate to the System or Administration section of the web interface.
3. Step 3: Locate the password change option and enter a strong, unique password.
4. Step 4: Save the changes and reboot the router if prompted.

4.3 Config or Code Example

Before

Username: admin
Password: 1234

After

Username: <your_username>
Password: <your_strong_password>

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue. Least privilege reduces the impact of a compromised account. Safe defaults ensure systems start in a secure state. A regular patch cadence keeps devices up-to-date with the latest security fixes.

• Practice 1: Implement least privilege to limit access rights and reduce potential damage from compromised accounts.
• Practice 2: Enforce safe defaults by requiring all new systems to have strong passwords set during initial configuration.

4.5 Automation (Optional)

Automation is not recommended for this specific vulnerability due to the risk of locking yourself out of the device if incorrectly configured.

5. Verification / Validation

Confirm the fix by attempting to log in with the old default credentials, which should now fail. Then verify that you can log in with the new password. Finally, perform a basic service smoke test.

• Post-fix check: Attempt to Telnet into the router using username ‘admin’ and password ‘1234’. The connection attempt should be refused.
• Re-test: Repeat the quick check from section 3; it should no longer succeed with default credentials.

telnet <router_ip>

6. Preventive Measures and Monitoring

Update security baselines to include strong password requirements for all network devices. Implement checks in your CI/CD pipelines to ensure new configurations do not use default credentials. Establish a regular patch or configuration review cycle.

• Baselines: Update your security baseline to require strong passwords and disable Telnet where possible.
• Pipelines: Add automated checks during deployment to flag any configurations using default usernames or passwords.
• Asset and patch process: Review router configurations quarterly for compliance with password policies.

7. Risks, Side Effects, and Roll Back

Changing the password incorrectly could lock you out of the router. Always have a backup configuration available. If issues occur, restore the backed-up configuration.

• Risk or side effect 1: Incorrectly entering the new password can result in loss of access to the router.
• Risk or side effect 2: Rebooting the router may temporarily interrupt internet connectivity.
• Roll back:
  1. Step 1: If possible, use the web interface to restore the backed-up configuration file.
  2. Step 2: If unable to access the web interface, attempt a factory reset (consult ZyXEL documentation for your specific model).

8. References and Resources

• Vendor advisory or bulletin: ZyXEL Security Advisory
• NVD or CVE entry: CVE-1999-0571
• Product or platform documentation relevant to the fix: ZyXEL Router Password Reset Guide