CVE-2013-2179 in xdm
Summary
by MITRE
X.Org xdm 1.1.10, 1.1.11, and possibly other versions, when performing authentication using certain implementations of the crypt API function that can return NULL, allows remote attackers to cause a denial of service (NULL pointer dereference and crash) by attempting to log into an account whose password field contains invalid characters, as demonstrated using the crypt function from glibc 2.17 and later with (1) the "!" character in the salt portion of a password field or (2) a password that has been encrypted using DES or MD5 in FIPS-140 mode.
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Analysis
by VulDB Data Team • 12/27/2024
The vulnerability CVE-2013-2179 represents a critical denial of service flaw in X.Org xdm version 1.1.10 and 1.1.11, with potential impacts extending to other versions in the affected series. This issue manifests when the display manager attempts to authenticate users through the crypt API function, which can legitimately return NULL values under specific conditions. The vulnerability arises from inadequate error handling within the authentication process, creating a scenario where remote attackers can exploit malformed password entries to crash the xdm service. The flaw specifically targets implementations of the crypt API that may return NULL pointers, making it particularly dangerous in environments where authentication mechanisms are heavily relied upon for system security.
The technical exploitation of this vulnerability occurs through careful manipulation of password field contents in user account entries. Attackers can trigger the NULL pointer dereference by attempting to log into accounts where the password field contains invalid characters, specifically leveraging the crypt function behavior in glibc 2.17 and later versions. The two primary attack vectors involve using the "!" character within the salt portion of a password field, or employing passwords that have been encrypted using either DES or MD5 algorithms while operating in FIPS-140 mode. These conditions cause the crypt function to return NULL, which the xdm authentication code fails to properly handle, resulting in an immediate crash of the display manager service. This behavior directly violates the principle of robust error handling that should be implemented in security-critical systems.
The operational impact of CVE-2013-2179 extends beyond simple service disruption to potentially compromise system availability and user access. When the xdm service crashes, users lose access to the graphical login interface, effectively preventing legitimate authentication attempts and creating a denial of service condition for all users attempting to access the system through the display manager. This vulnerability is particularly concerning in multi-user environments where the display manager serves as the primary authentication gateway, as it can be exploited remotely without requiring any special privileges or prior access to the system. The flaw essentially creates an attack surface where an unauthenticated remote user can cause a persistent disruption to the system's authentication infrastructure.
From a cybersecurity perspective, this vulnerability aligns with CWE-476, which describes NULL pointer dereference conditions that can lead to application crashes and service disruptions. The attack pattern follows typical denial of service methodologies documented in the MITRE ATT&CK framework under the T1499 category for network denial of service attacks. The vulnerability demonstrates poor input validation and error handling practices that are fundamental to secure software development. Organizations should prioritize patching this vulnerability as it represents a straightforward exploitation path that can be executed by anyone with network access to the affected system. The fix typically involves implementing proper NULL pointer checks in the authentication code and ensuring that the display manager gracefully handles cases where the crypt function returns invalid results.
The remediation approach for CVE-2013-2179 requires immediate patching of affected xdm versions, with administrators verifying that their systems are running patched versions of X.Org xdm. Additionally, system administrators should consider implementing monitoring for authentication service disruptions and establish procedures for rapid response to potential exploitation attempts. The vulnerability serves as a reminder of the critical importance of robust error handling in authentication systems, particularly when dealing with cryptographic functions that may return NULL values under specific conditions. Organizations should also review their authentication infrastructure to ensure that similar NULL pointer dereference vulnerabilities do not exist in other components of their security architecture.