How to remediate – Remote Authentication Message Check

1. Introduction

The Remote Authentication Message Check vulnerability concerns whether a system correctly indicates if remote login attempts are valid. This can lead to false positives during security assessments, giving an inaccurate view of risk. SSH servers are commonly affected. A successful check could reveal valid account names, potentially aiding brute force attacks on confidentiality.

2. Technical Explanation

• Root cause: The SSH implementation does not properly differentiate between a valid login attempt and an invalid one, returning success signals for both.
• Exploit mechanism: An attacker sends multiple login attempts to the remote system with different usernames and monitors the server’s responses. A positive response indicates a potentially valid username.
• Scope: SSH servers running on various platforms including Linux, Unix, and Windows are affected. Specific versions depend on the implementation details of the SSH software.

3. Detection and Assessment

• Scanning: Nmap with the ssh-auth script can be used (example only). nmap -p 22 --script ssh-auth
• Logs and evidence: Examine SSH server logs for authentication attempts, looking for patterns indicating acceptance of invalid credentials. Log files are typically located in /var/log/auth.log or similar depending on the distribution.

ssh -o StrictHostKeyChecking=no invaliduser@

4. Solution / Remediation Steps

Fixing this issue requires ensuring the SSH server correctly handles authentication attempts and does not provide false positives. This often involves updating the SSH software or configuring it to be more strict with login responses.

4.1 Preparation

• A change window may be needed depending on your organisation’s policies and the impact of downtime. Approval from a senior administrator might be required.

4.2 Implementation

1. Step 1: Update the SSH server software to the latest version using your distribution’s package manager (e.g., apt update && apt upgrade for Debian/Ubuntu).
2. Step 3: Verify that the updated SSH server correctly handles invalid login attempts as described in section 5.

4.3 Config or Code Example

Before

# No specific configuration related to authentication message checks is present.

After

StrictModes yes #Enforce strict mode for better security. This may be the default already, but check.

4.4 Security Practices Relevant to This Vulnerability

Practices that directly address this vulnerability type include least privilege and input validation. Least privilege reduces the impact if an attacker identifies a valid account. Input validation can block unsafe data used in login attempts.

• Practice 1: Implement least privilege by granting users only the necessary permissions to reduce potential damage from compromised accounts.
• Practice 2: Use input validation on all user-supplied data, including usernames and passwords, to prevent malicious inputs.

4.5 Automation (Optional)

# Example Ansible playbook to update SSH server on Debian/Ubuntu systems:
- name: Update SSH server
  apt:
    name: openssh-server
    state: latest
  become: true

5. Verification / Validation

• Post-fix check: Attempt to connect via SSH using an invalid username (e.g., 'invaliduser'). The connection should timeout without accepting any input.
• Re-test: Run nmap -p 22 --script ssh-auth again; the script should not report any valid usernames.
• Smoke test: Verify that legitimate users can still connect to the SSH server using their correct credentials.
• Monitoring: Monitor SSH logs for unexpected authentication attempts or error messages (example only). grep "Failed password" /var/log/auth.log

ssh -o StrictHostKeyChecking=no invaliduser@ # Should timeout without accepting input.

6. Preventive Measures and Monitoring

Update security baselines to include the latest SSH server versions. Implement checks in CI/CD pipelines to prevent vulnerable configurations from being deployed. Establish a regular patch review cycle for critical systems. For example, CIS benchmarks provide guidance on secure SSH configuration.

• Baselines: Update your security baseline or policy to require the latest SSH server version and strict authentication settings (e.g., CIS control 1.2).
• Asset and patch process: Implement a monthly patch review cycle for critical systems, including SSH servers.

7. Risks, Side Effects, and Roll Back

Updating the SSH server could potentially cause compatibility issues with older clients. Ensure you have tested the update in a non-production environment first. Roll back by restoring the system snapshot or reverting configuration changes.

• Risk or side effect 2: Temporary service interruption during restart. Mitigation: Schedule the update during a maintenance window and ensure alternative access methods are available.

8. References and Resources

• Vendor advisory or bulletin: Check your SSH server vendor’s website for security advisories related to authentication handling.
• NVD or CVE entry: No specific CVE currently exists for this check, but search the NVD database for vulnerabilities in your SSH server version.
• Product or platform documentation relevant to the fix: Refer to your SSH server's official documentation for configuration options and security best practices