Title: Yubico Addresses 2FA Bypass Vulnerability in Security Advisory

Title: Yubico Addresses 2FA Bypass Vulnerability in Security Advisory

Latest Update: January 18, 2025. This article now includes additional information concerning CVE-2025-23013 and clarifications provided by Yubico regarding the severity rating.

Two-factor authentication has emerged as a crucial security measure in recent years, so when news of potential 2FA bypasses arises, it warrants attention. Whether it's long-lasting hack attempts targeting Google users, malicious Chrome extensions, or the Rockstar bypass kit affecting Microsoft users, recent events remind us that cybersecurity is an ongoing battle. Joining the fray, Yubico has issued a security advisory confirming a bypass vulnerability in their software that supports 2FA on Linux or macOS using a YubiKey or FIDO authenticator. Here's what you should know.

Yubico Security Advisory: YSA-2025-01

When we think about two-factor authentication hardware keys and reliable security solutions, Yubico often comes to mind. It's been a significant player in the hardware key market for decades, and it's no surprise that when Yubico releases a security advisory, it's worth paying attention.

Yubico's security advisory entitled YSA-2025-01 relates to a bypass vulnerability in their pam-u2f package, which supports YubiKey on macOS and Linux platforms. The pam-u2f package, prior to version 1.3.1, can be susceptible to a vulnerability that allows an authentication bypass under certain circumstances. As per the advisory, "An attacker would require the ability to access the system as an unprivileged user," while in some configurations, "the attacker may also need to know the user's password."

Yubico Provides Example Scenarios for Attacks

"A key differentiator between scenarios is the location of the authfile," Yubico explained, referring to the argument used to store the pam-u2f configuration in the PAM stack. Depending on the configuration, if an attacker can tamper with the authfile, they may be able to bypass authentication, potentially leading to local privilege escalation.

To illustrate this, Yubico provided a few example scenarios involving the management of the authfile, including:

• In a scenario involving user-managed authfiles, an attacker could remove or corrupt the file if pam-u2f is used as a single-factor authentication method and the "nouserok" option is enabled. This could lead to local privilege escalation if the user is allowed to sudo.
• With centrally managed authfiles, an attacker could attempt to exhaust system resources by allocating large amounts of memory, potentially leading to a memory allocation error within pam-u2f. If successful, the second factor would no longer be verified during an authentication event.

Yubico Confirms No Impact on YubiKey Hardware

Yubico confirmed that this vulnerability has no impact on any of their previous or current generation YubiKey Series, YubiKey FIPS Series, Security Key Series, YubiHSM, or YubiHSM FIPS devices.

Understanding CVE-2025-23013

The vulnerability in question, CVE-2025-23013, is classified as high-severity. This means that under certain conditions, when memory cannot be allocated or the module cannot change privileges, the module does not contribute to the final authentication decision performed by PAM. This, in turn, can disable the verification of primary or secondary authentication factors.

To address this vulnerability, Yubico recommends that affected users upgrade to the latest version of pam-u2f either by directly downloading from GitHub or by getting the update from Yubico PPA.

A spokesperson for Yubico offered the following statement regarding the issue: "We can confirm that Yubico was informed of this issue by researchers. This software issue does not impact YubiKeys or YubiHSMs."

Enrichment Data:

Overall:

The CVE-2025-23013 vulnerability impacts the pam-u2f package, a Pluggable Authentication Module (PAM) used to support 2FA using YubiKeys or other FIDO-compliant authenticators on macOS and Linux systems. This vulnerability is classified as high-severity, enabling an attacker with unprivileged user access to bypass authentication under certain conditions.

Details of the Vulnerability

1. Affected Software: The pam-u2f package, which is used to support YubiKeys or other FIDO-compliant authenticators on macOS and Linux systems, is affected.
2. Function Involved: The vulnerability lies within the implementation of the function, which can return a response under specific error conditions, like memory allocation failures or missing configuration files.
3. Impact: When a module returns , it does not contribute to the final authentication decision performed by PAM, potentially enabling attackers to bypass the verification of primary or secondary authentication factors. In some scenarios, this may lead to local privilege escalation.
4. Configuration-Specific Impact:
5. User-Managed Authfile: An attacker could tamper with the authfile, leading to an authentication bypass if pam-u2f is used as a single-factor authentication method with the option enabled.
6. Centrally Managed Authfile: An attacker could exploit memory allocation errors to disable second-factor verification if pam-u2f is used as a second-factor authentication method along with a user password.
7. Single-Factor with Non-Auth Modules: In cases where pam-u2f is used as a single-factor method alongside non-authentication PAM modules, an attacker could use a response to bypass all authentication checks.

Severity

Yubico has rated this vulnerability as High with a CVSS score of 7.3.

Mitigation Strategies

To safeguard against the CVE-2025-23013 vulnerability, Yubico recommends the following measures:

1. Upgrade to Latest Version: Update to the latest version of the pam-u2f package, version 1.3.1 or higher, which addresses the vulnerability.
2. Alternative Mitigation Strategies (for those who cannot upgrade immediately):
3. Disable the option in pam-u2f configurations.
4. Set console control flags ( and or ) in the PAM stack to prevent responses from pam-u2f modules.

By implementing these measures, users can protect their systems against the CVE-2025-23013 vulnerability and uphold strong two-factor authentication.

1. The security advisory Yubico released, titled YSA-2025-01, highlights a bypass vulnerability in their pam-u2f package, which supports 2FA on Linux or macOS using a YubiKey or FIDO authenticator.
2. The YSA-2025-01 vulnerability, classified as high-severity, allows for a partial authentication bypass in the pam-u2f software package when used under certain circumstances, potentially leading to local privilege escalation.
3. To mitigate this vulnerability, Yubico advises affected users to upgrade to the latest version of pam-u2f, either by downloading it directly from GitHub or by getting the update from Yubico PPA.
4. Yubico has confirmed that the vulnerability does not impact any of their previous or current generation YubiKey hardware, including their YubiKey Series, FIPS Series, Security Key Series, YubiHSM, or YubiHSM FIPS devices.

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