CVE-2025-23013: Yubico pam-u2f Privilege Escalation Flaw

CVE-2025-23013 Overview

CVE-2025-23013 is an authentication bypass vulnerability in Yubico pam-u2f before version 1.3.1 that can lead to local privilege escalation on macOS and Linux systems. The pam-u2f software package implements a Pluggable Authentication Module (PAM) designed to support authentication using YubiKey or other FIDO compliant authenticators. This vulnerability allows attackers with unprivileged local access to bypass authentication controls under certain configurations.

Critical Impact

Local attackers can exploit this authentication bypass to escalate privileges on affected macOS and Linux systems using pam-u2f for FIDO authentication. Depending on configuration, an attacker may also need knowledge of the user's password.

Affected Products

• Yubico pam-u2f versions prior to 1.3.1
• macOS systems utilizing pam-u2f for FIDO authentication
• Linux systems utilizing pam-u2f for FIDO authentication

Discovery Timeline

• January 15, 2025 - CVE-2025-23013 published to NVD
• February 3, 2025 - Last updated in NVD database

Technical Details for CVE-2025-23013

Vulnerability Analysis

This vulnerability is classified under CWE-394 (Unexpected Status Code or Return Value), indicating that the pam-u2f module improperly handles certain status codes or return values during the authentication process. The authentication bypass occurs because the software fails to properly validate expected status conditions, allowing the authentication flow to succeed when it should fail.

The vulnerability requires local access to the system with an unprivileged user account. The attack complexity is low once the attacker has local access, though it depends on specific PAM configuration settings. In some configurations, the attacker may need to know the target user's password in addition to exploiting the bypass.

The impact is significant: successful exploitation allows complete compromise of confidentiality, integrity, and availability on the local system through privilege escalation.

Root Cause

The root cause stems from improper handling of status codes or return values within the pam-u2f authentication module (CWE-394). When certain unexpected conditions occur during the FIDO authentication process, the module fails to properly interpret the return status, potentially allowing authentication to succeed without proper credential verification.

Attack Vector

The attack requires local access to the target system. An attacker must first gain access as an unprivileged user on a macOS or Linux system where pam-u2f is deployed for authentication. The attacker then exploits the authentication bypass by triggering conditions that cause the PAM module to incorrectly process status codes during authentication.

The exploitation mechanism involves manipulating the authentication flow to reach a code path where the unexpected status handling allows the bypass. Depending on how pam-u2f is configured in the PAM stack, the attacker may leverage this bypass to escalate privileges to other user accounts or administrative access.

For detailed technical information on exploitation, refer to the Yubico Security Advisory YSA-2025-01.

Detection Methods for CVE-2025-23013

Indicators of Compromise

• Unexpected authentication successes in PAM logs without corresponding valid FIDO authenticator events
• Privilege escalation attempts from unprivileged user accounts on systems using pam-u2f
• Anomalous sudo or su commands from users without physical access to their registered FIDO authenticators

Detection Strategies

• Monitor PAM authentication logs for pam-u2f module entries that indicate authentication success without proper FIDO device interaction
• Implement file integrity monitoring on PAM configuration files (/etc/pam.d/) to detect unauthorized modifications
• Track privilege escalation events and correlate with expected FIDO device usage patterns
• Deploy endpoint detection to identify suspicious authentication sequences

Monitoring Recommendations

• Enable verbose logging for pam-u2f to capture detailed authentication flow information
• Monitor system authentication logs (/var/log/auth.log or /var/log/secure) for unusual pam-u2f entries
• Implement alerting on privilege escalation attempts from accounts configured to require FIDO authentication

How to Mitigate CVE-2025-23013

Immediate Actions Required

• Upgrade pam-u2f to version 1.3.1 or later immediately on all affected systems
• Audit PAM configurations to understand which services rely on pam-u2f for authentication
• Review authentication logs for any signs of exploitation prior to patching
• Consider temporarily implementing additional authentication controls until patching is complete

Patch Information

Yubico has released pam-u2f version 1.3.1 which addresses this vulnerability. Organizations should update through their package manager or download directly from Yubico. Debian LTS users can obtain the patched version through the official Debian repositories as announced in the Debian LTS Announcement.

For complete patch details and guidance, consult the Yubico Security Advisory YSA-2025-01.

Workarounds

• If immediate patching is not possible, consider temporarily removing pam-u2f from critical PAM configurations and using alternative authentication mechanisms
• Implement additional authentication layers in the PAM stack to provide defense in depth
• Restrict local access to systems using pam-u2f to only essential personnel until patching is complete

# Verify installed pam-u2f version on Debian/Ubuntu
dpkg -l libpam-u2f

# Update pam-u2f on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade libpam-u2f

# Verify pam-u2f version after update (should be 1.3.1 or later)
dpkg -l libpam-u2f | grep -E "^ii"