CVE-2003-0189 in HTTP Server
Summary
by MITRE
The authentication module for Apache 2.0.40 through 2.0.45 on Unix does not properly handle threads safely when using the crypt_r or crypt functions, which allows remote attackers to cause a denial of service (failed Basic authentication with valid usernames and passwords) when a threaded MPM is used.
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Analysis
by VulDB Data Team • 08/24/2025
The vulnerability described in CVE-2003-0189 represents a critical thread safety issue within the Apache HTTP Server authentication module affecting versions 2.0.40 through 2.0.45 on Unix operating systems. This flaw specifically manifests when the server employs threaded Multi-Processing Modules MPMs such as the worker or prefork MPMs that utilize the crypt_r or crypt functions for password verification. The issue stems from improper handling of concurrent thread access during the authentication process, creating a scenario where multiple threads attempting simultaneous authentication operations can interfere with each other's execution. The vulnerability operates at the core of Apache's security infrastructure, directly impacting the server's ability to properly authenticate users through Basic authentication mechanisms.
The technical root cause of this vulnerability lies in the improper synchronization of cryptographic functions within a multi-threaded environment. When the crypt_r function is called concurrently by multiple threads, the internal state of the cryptographic library becomes corrupted or overwritten, leading to authentication failures even when valid credentials are provided. This occurs because the crypt_r function, while designed to be thread-safe in theory, does not properly handle the shared state management when accessed through Apache's threaded MPM architecture. The flaw essentially creates a race condition where thread execution contexts interfere with each other during the password verification process, causing the authentication module to return incorrect results or fail entirely. This behavior aligns with CWE-362, which specifically addresses race conditions in concurrent programming environments.
The operational impact of CVE-2003-0189 extends beyond simple service disruption to create a more insidious form of denial of service that can be exploited remotely. Attackers can leverage this vulnerability by sending multiple concurrent authentication requests to a targeted Apache server running with threaded MPMs, causing legitimate users with valid credentials to be denied access to protected resources. The service disruption is particularly damaging because it affects the fundamental authentication mechanism that protects web applications and sensitive data. In practical scenarios, this vulnerability could be exploited to render web applications unusable for legitimate users, effectively creating a denial of service condition that undermines the availability of web services. The vulnerability's remote exploitability means that attackers do not need physical access to the server and can target Apache installations over the network.
Mitigation strategies for this vulnerability require immediate action to address the underlying thread safety issues. The most effective approach involves upgrading to Apache HTTP Server versions 2.0.46 or later, where the threading issues have been resolved through proper synchronization mechanisms. System administrators should also consider implementing additional security measures such as limiting concurrent connections, implementing rate limiting for authentication requests, and using alternative MPM configurations that do not rely on the vulnerable threaded authentication paths. Organizations should conduct thorough vulnerability assessments to identify affected systems and implement proper monitoring to detect exploitation attempts. The mitigation process should also include reviewing and updating security policies to ensure that threaded MPM configurations are properly configured with appropriate locking mechanisms. This vulnerability demonstrates the critical importance of proper thread safety implementation in server-side applications and aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion or corruption.