Security Vulnerability in PAM-u2f: Bypassing Two-Factor Authentication

PAM-u2f is a module designed to support two-factor authentication using U2F (Universal 2nd Factor) technology. It is based on the FIDO standard and ensures secure system access with devices like YubiKey, Feitian, or other U2F-compatible tokens.

If you are using openSUSE or other systems with PAM (Pluggable Authentication Module) and rely on PAM-u2f for two-factor authentication, take note! A critical vulnerability was recently discovered in this module. It allows bypassing authentication even without a physical token like YubiKey. This vulnerability is registered under the identifier CVE-2025-23013.

What Causes the Vulnerability?

The root cause lies in incorrect handling within the pam_sm_authenticate() function. Simply put, if memory errors or other technical issues occur during the two-factor authentication process, the module returns a special code called PAM_IGNORE.

The problem is that the system misinterprets this code as a successful authentication. As a result, operations requiring two-factor authentication, such as sudo or su, can be executed without using a token.

Why Is This Dangerous?

If an attacker has local access to the system, they can exploit this vulnerability to bypass security measures and gain elevated privileges. This poses a significant threat to system security.

How to Fix the Issue?

To secure your system, you should immediately update the PAM-u2f module. The issue has been resolved in PAM-u2f version 1.3.1. Follow these steps to update:

  1. Update your system:
sudo zypper update pam-u2f

or on other distributions:

sudo apt update && sudo apt upgrade pam-u2f

2. Review your configurations: After the update, verify that the PAM-u2f configuration complies with your security policies.

    The Importance of Two-Factor Authentication

    Two-factor authentication is a vital component of modern security, playing a key role in protecting user accounts. However, to ensure its effectiveness, it is crucial to:

    • Configure it properly;
    • Keep the modules up to date;
    • Implement backup mechanisms and regularly test the system.

    Remember: security is not just about good technology, but also about using it correctly!

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