CVE-2011-3149 in Linux-PAM
Summary
by MITRE
The _expand_arg function in the pam_env module (modules/pam_env/pam_env.c) in Linux-PAM (aka pam) before 1.1.5 does not properly handle when environment variable expansion can overflow, which allows local users to cause a denial of service (CPU consumption).
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Analysis
by VulDB Data Team • 12/06/2021
The vulnerability identified as CVE-2011-3149 resides within the Linux-PAM authentication framework, specifically in the pam_env module's _expand_arg function. This flaw represents a classic buffer overflow condition that occurs during environment variable expansion processes, where the module fails to adequately validate the length of expanded variables against allocated memory boundaries. The issue affects Linux-PAM versions prior to 1.1.5, making it a significant concern for systems running older authentication frameworks that remain widely deployed across enterprise environments.
The technical implementation of this vulnerability stems from improper input validation within the _expand_arg function, which processes environment variable expansions during PAM session initialization. When a malicious user provides environment variables containing overly long expansion sequences or recursive references, the function attempts to allocate memory for the expanded result without sufficient bounds checking. This memory management flaw creates a condition where CPU resources become consumed excessively as the system attempts to process increasingly complex expansion scenarios, ultimately leading to denial of service conditions that can render authentication services unavailable to legitimate users.
From an operational impact perspective, this vulnerability enables local attackers to consume excessive system resources through carefully crafted environment variable configurations, effectively creating a denial of service scenario that affects the entire PAM authentication infrastructure. The vulnerability is particularly concerning because it operates at the authentication layer, potentially affecting multiple services that depend on PAM for user authentication and authorization. Attackers can leverage this weakness to disrupt system availability, particularly in environments where PAM is heavily utilized for authentication across various network services and applications.
The vulnerability aligns with CWE-121, which categorizes buffer overflow conditions in stack-based buffers, and represents a specific instance of improper input validation that leads to resource exhaustion. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation and denial of service tactics, as it allows local users to consume system resources and potentially disrupt authentication services. The attack surface is broad since PAM is fundamental to Unix-like system authentication, making this vulnerability particularly dangerous in multi-tenant environments where authentication service availability directly impacts system integrity and user access.
Mitigation strategies should focus on immediate patching of affected Linux-PAM installations to version 1.1.5 or later, where the memory handling has been corrected to properly validate expansion lengths. System administrators should also implement monitoring for unusual CPU consumption patterns during authentication processes and consider implementing environment variable validation policies that prevent overly complex expansion sequences. Additionally, organizations should conduct comprehensive vulnerability assessments to identify other potential buffer overflow conditions in authentication modules and establish robust patch management processes to prevent similar issues from arising in the future. The remediation process should include thorough testing of updated PAM configurations to ensure that authentication services remain functional while addressing the specific resource exhaustion vulnerability that was present in earlier versions.