CVE-2012-0021 in HTTP Server
Summary
by MITRE
The log_cookie function in mod_log_config.c in the mod_log_config module in the Apache HTTP Server 2.2.17 through 2.2.21, when a threaded MPM is used, does not properly handle a %{}C format string, which allows remote attackers to cause a denial of service (daemon crash) via a cookie that lacks both a name and a value.
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Analysis
by VulDB Data Team • 03/20/2021
The vulnerability identified as CVE-2012-0021 represents a critical denial of service flaw within the Apache HTTP Server's mod_log_config module. This issue specifically affects Apache versions ranging from 2.2.17 through 2.2.21 and manifests when the server operates with a threaded Multi-Processing Module configuration. The vulnerability stems from improper handling of the %{}C format string within the log_cookie function, which is responsible for processing cookie data in server logs. When a maliciously crafted cookie is presented that contains neither a name nor a value, the function fails to properly validate or sanitize this input before attempting to process it, creating a condition that leads to daemon instability.
The technical exploitation of this vulnerability occurs through a carefully crafted HTTP request containing a malformed cookie that lacks both name and value components. This specific format string handling issue creates a buffer overflow condition or memory corruption scenario within the threaded server environment where multiple threads are processing concurrent requests. The flaw exists because the log_cookie function does not adequately check for the presence of cookie components before attempting to parse and log them, resulting in unpredictable memory access patterns that can cause the Apache daemon to crash and restart. This behavior directly violates the principle of robust input validation and demonstrates a classic example of improper handling of edge case inputs in logging mechanisms.
The operational impact of this vulnerability extends beyond simple service disruption as it provides attackers with a reliable method to repeatedly crash the Apache HTTP Server daemon, leading to sustained denial of service conditions that can severely impact web applications and services. The threaded MPM configuration makes this vulnerability particularly dangerous because multiple threads can be simultaneously affected by the same malformed cookie input, potentially causing cascading failures within the server's thread management system. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique for Network Denial of Service and represents a privilege escalation vector through service disruption. The vulnerability also aligns with CWE-121, which addresses stack-based buffer overflow conditions, and CWE-20, which covers improper input validation scenarios.
Mitigation strategies for CVE-2012-0021 require immediate patching of affected Apache installations to versions 2.2.22 or later where the vulnerability has been resolved through proper input validation of cookie format strings. Organizations should also implement additional defensive measures such as configuring custom error handling for malformed cookies, implementing rate limiting on cookie-based requests, and deploying web application firewalls that can detect and block suspicious cookie patterns. The fix implemented by Apache developers involved adding proper validation checks within the log_cookie function to ensure that cookie data contains at least minimal structure before attempting to process it, thereby preventing the crash condition that occurred when both name and value were absent. Security teams should also consider monitoring for unusual patterns in server crash logs and implementing automated response mechanisms that can detect and isolate services affected by similar denial of service conditions.