CVE-2016-2161 in macOS
Summary
by MITRE
In Apache HTTP Server versions 2.4.0 to 2.4.23, malicious input to mod_auth_digest can cause the server to crash, and each instance continues to crash even for subsequently valid requests.
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Analysis
by VulDB Data Team • 01/21/2021
The vulnerability identified as CVE-2016-2161 affects Apache HTTP Server versions 2.4.0 through 2.4.23 and specifically targets the mod_auth_digest module responsible for handling digest authentication. This flaw represents a denial of service condition that can be triggered by sending malformed or malicious input to the authentication module, causing the web server to become unresponsive and crash. The vulnerability is particularly concerning because once triggered, the server remains in a crashed state and continues to fail even when processing legitimate requests, effectively rendering the service unavailable to users.
The technical root cause of this vulnerability lies in insufficient input validation within the mod_auth_digest module. When the server processes authentication requests containing malformed digest challenges or credentials, the parsing logic fails to properly handle edge cases or invalid input sequences. This lack of proper boundary checking and input sanitization leads to memory corruption or unexpected program flow that results in the server process terminating abnormally. The vulnerability manifests as a crash condition that occurs during the authentication processing phase, making it difficult to distinguish between legitimate authentication failures and attack-induced crashes.
From an operational perspective, this vulnerability creates significant impact for organizations relying on Apache HTTP Server with digest authentication enabled. The persistent nature of the crash means that administrators cannot simply restart the service to resolve the issue, as subsequent valid requests will continue to trigger the same failure condition. This makes the vulnerability particularly dangerous in production environments where continuous availability is critical. The attack vector is relatively simple, requiring only that an attacker send carefully crafted authentication requests to the server, making it accessible to threat actors with basic knowledge of web server vulnerabilities.
The vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and represents a classic example of improper input validation leading to service disruption. From the MITRE ATT&CK framework perspective, this vulnerability could be leveraged as part of a broader attack strategy targeting the availability component of the CIA triad, potentially serving as an initial foothold for more sophisticated attacks or as a means to disrupt business operations. Organizations using Apache HTTP Server with digest authentication should prioritize patching this vulnerability, as it represents a straightforward denial of service opportunity that can severely impact service availability and user access to web applications.
Mitigation strategies should include immediate deployment of Apache HTTP Server version 2.4.24 or later, which contains the necessary patches to address the input validation issues in mod_auth_digest. Additionally, administrators should consider implementing rate limiting or authentication request throttling mechanisms to reduce the impact of potential exploitation attempts. Network-level protections such as intrusion detection systems can help identify suspicious authentication patterns that may indicate exploitation attempts. Organizations should also review their authentication configurations to ensure that digest authentication is only enabled where absolutely necessary, as alternative authentication methods may provide better security posture while avoiding this specific vulnerability.