CVE-2016-2145 in mod_auth_mellon
Summary
by MITRE
The am_read_post_data function in mod_auth_mellon before 0.11.1 does not check if the ap_get_client_block function returns an error, which allows remote attackers to cause a denial of service (segmentation fault and process crash) via a crafted POST data.
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Analysis
by VulDB Data Team • 07/25/2022
The vulnerability identified as CVE-2016-2145 resides within the mod_auth_mellon Apache module, specifically within the am_read_post_data function that handles authentication data processing. This module serves as an authentication handler for SAML (Security Assertion Markup Language) protocols, enabling single sign-on functionality for web applications. The flaw manifests in the function's inadequate error handling when processing client POST data, creating a critical security gap that can be exploited by remote attackers to disrupt service availability.
The technical implementation of this vulnerability stems from the am_read_post_data function's failure to validate the return value of the ap_get_client_block function, which is a core Apache HTTP Server API call responsible for reading client request data. When ap_get_client_block encounters malformed or excessive POST data, it returns an error condition that the vulnerable code does not properly handle. This oversight leads to the function proceeding with invalid data processing, ultimately causing segmentation faults and subsequent process crashes that result in complete denial of service for the affected web server instance.
The operational impact of this vulnerability extends beyond simple service disruption, as it represents a classic denial of service attack vector that can be executed without authentication or specialized privileges. Attackers can craft malicious POST requests containing malformed data structures that trigger the error condition in the authentication module, causing the Apache server process to terminate unexpectedly. This vulnerability affects all versions of mod_auth_mellon prior to 0.11.1, making it particularly concerning for organizations that rely on SAML-based authentication and have not yet applied the necessary security patches.
The vulnerability aligns with CWE-248, which addresses "Uncaught Exception" conditions in software applications, and represents a failure in proper error handling within the Apache module framework. From an adversarial perspective, this vulnerability maps to ATT&CK technique T1499.004, specifically "Toggle Service State," as it enables attackers to disrupt service availability through process termination. Organizations utilizing mod_auth_mellon for SAML authentication should prioritize patching to version 0.11.1 or later, which implements proper error checking for the ap_get_client_block function. Additionally, implementing network-level protections such as rate limiting and input validation at the web application firewall level can provide additional defense-in-depth measures against exploitation attempts.
The broader implications of this vulnerability highlight the critical importance of robust error handling in authentication modules, particularly those handling sensitive data flows. Security practitioners should consider this vulnerability as representative of common weaknesses in web application security where input validation and error recovery mechanisms fail to account for malformed data conditions. The incident underscores the necessity for comprehensive testing of authentication modules under various stress and malformed input conditions to prevent similar vulnerabilities from being exploited in production environments.