CVE-2011-4120 in Yubico PAM Module
Summary
by MITRE
Yubico PAM Module before 2.10 performed user authentication when 'use_first_pass' PAM configuration option was not used and the module was configured as 'sufficient' in the PAM configuration. A remote attacker could use this flaw to circumvent common authentication process and obtain access to the account in question by providing a NULL value (pressing Ctrl-D keyboard sequence) as the password string.
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Analysis
by VulDB Data Team • 02/28/2024
The vulnerability identified as CVE-2011-4120 affects the Yubico PAM module version 2.10 and earlier, representing a critical authentication bypass flaw within Unix authentication frameworks. This issue arises from improper handling of authentication flows when specific PAM configuration parameters are not properly set, creating a security gap that can be exploited by remote attackers to gain unauthorized access to user accounts. The flaw specifically manifests when the module operates in a 'sufficient' configuration context without the 'use_first_pass' directive being enabled, allowing attackers to manipulate the authentication sequence through crafted input.
The technical root cause of this vulnerability stems from the module's failure to properly validate authentication inputs when operating in certain PAM configurations. When the 'use_first_pass' option is absent from the PAM configuration, the module incorrectly processes authentication requests, particularly when the password field receives a NULL value through the Ctrl-D keyboard sequence. This behavior violates fundamental PAM security principles and creates an unintended authentication path that bypasses normal credential validation mechanisms. The flaw essentially allows a malicious actor to submit an empty password string and still gain successful authentication, undermining the entire authentication framework's integrity.
From an operational impact perspective, this vulnerability presents a significant risk to system security as it enables remote attackers to bypass standard authentication controls without requiring valid credentials. The attack vector is particularly concerning because it can be executed through simple keyboard input sequences, making it accessible to attackers with minimal technical expertise. The vulnerability affects any system utilizing the affected Yubico PAM module version in 'sufficient' configuration mode, potentially compromising multiple user accounts simultaneously. This flaw directly violates security principle of least privilege and can lead to unauthorized system access, data breaches, and privilege escalation scenarios.
Security professionals should recognize this vulnerability as aligning with CWE-287, which addresses improper authentication issues in authentication systems. The flaw also corresponds to ATT&CK technique T1078.004, which covers valid accounts through default passwords or authentication bypasses. Organizations should implement immediate mitigations including updating to Yubico PAM module version 2.10 or later, ensuring proper PAM configuration with 'use_first_pass' directives enabled, and conducting comprehensive security audits of all authentication modules. Additionally, system administrators should review PAM configuration files to eliminate 'sufficient' module placements without proper authentication controls and consider implementing additional authentication layers such as two-factor authentication to reduce the attack surface.