CVE-2009-0887 in Linux-PAM
Summary
by MITRE
Integer signedness error in the _pam_StrTok function in libpam/pam_misc.c in Linux-PAM (aka pam) 1.0.3 and earlier, when a configuration file contains non-ASCII usernames, might allow remote attackers to cause a denial of service, and might allow remote authenticated users to obtain login access with a different user s non-ASCII username, via a login attempt.
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Analysis
by VulDB Data Team • 09/01/2019
The vulnerability described in CVE-2009-0887 represents a critical integer signedness error within the Linux-PAM authentication framework that affects versions 1.0.3 and earlier. This flaw exists in the _pam_StrTok function located in libpam/pam_misc.c, which is responsible for parsing configuration files during the authentication process. The issue manifests specifically when processing non-ASCII usernames, creating a scenario where the signed integer handling can produce unexpected behavior during string tokenization operations. The vulnerability stems from a fundamental misunderstanding of how signed and unsigned integers interact when processing character data, particularly in internationalized username contexts.
The technical implementation of this flaw involves the improper handling of character counts and string boundaries when parsing configuration files that contain non-ASCII characters. When a username contains extended ASCII or Unicode characters, the signed integer variable used to track string lengths can overflow or underflow, causing the tokenization function to behave unpredictably. This misbehavior can lead to memory corruption patterns that disrupt normal authentication flow. The vulnerability is particularly dangerous because it can be exploited through authenticated login attempts, where an attacker with valid credentials could manipulate the parsing logic to bypass authentication mechanisms. The signedness error creates a condition where valid input data causes the system to interpret memory locations incorrectly, potentially allowing privilege escalation or authentication bypass.
From an operational impact perspective, this vulnerability presents multiple attack vectors that could compromise system security. Remote attackers can trigger denial of service conditions by crafting specific non-ASCII usernames that cause the PAM module to crash or enter an infinite loop during tokenization. Additionally, authenticated users can exploit the vulnerability to gain unauthorized access to different user accounts by manipulating how the system processes non-ASCII usernames during authentication. This creates a scenario where legitimate users might be able to impersonate other users by exploiting the flawed string parsing logic. The vulnerability affects all systems relying on Linux-PAM for authentication, making it particularly concerning for enterprise environments where user authentication is critical. The impact extends beyond simple authentication bypass to potentially destabilizing the entire authentication infrastructure.
Mitigation strategies for this vulnerability require immediate patching of affected Linux-PAM installations to version 1.0.4 or later, where the signedness error has been corrected. System administrators should implement comprehensive monitoring for unusual authentication patterns and login attempts that might indicate exploitation attempts. The fix involves proper handling of unsigned integer variables when processing character data and ensuring that string boundary checks account for extended character sets. Organizations should also consider implementing additional authentication controls such as multi-factor authentication to reduce the impact of potential exploitation. Security teams should review existing authentication configurations to identify any non-ASCII username usage that might be vulnerable to this attack vector. This vulnerability aligns with CWE-194, which addresses the improper handling of signed and unsigned integers, and maps to ATT&CK technique T1550.001 for legitimate credentials acquisition through authentication bypass mechanisms. The remediation process should include thorough testing of authentication workflows to ensure that the patched version handles all character encodings correctly without introducing new instability.