CVE-2011-4415 in HTTP Server
Summary
by MITRE
The ap_pregsub function in server/util.c in the Apache HTTP Server 2.0.x through 2.0.64 and 2.2.x through 2.2.21, when the mod_setenvif module is enabled, does not restrict the size of values of environment variables, which allows local users to cause a denial of service (memory consumption or NULL pointer dereference) via a .htaccess file with a crafted SetEnvIf directive, in conjunction with a crafted HTTP request header, related to (1) the "len +=" statement and (2) the apr_pcalloc function call, a different vulnerability than CVE-2011-3607.
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Analysis
by VulDB Data Team • 08/09/2024
The vulnerability identified as CVE-2011-4415 represents a critical denial of service flaw within the Apache HTTP Server software family affecting versions through 2.0.64 and 2.2.21. This issue specifically targets the ap_pregsub function located in server/util.c and becomes exploitable when the mod_setenvif module is enabled. The vulnerability stems from insufficient input validation and memory management within the environment variable handling mechanism, creating a pathway for malicious actors to consume excessive system resources or trigger null pointer dereferences that can crash the web server process.
The technical exploitation of this vulnerability occurs through a carefully crafted .htaccess file that contains a malicious SetEnvIf directive, combined with a specially constructed HTTP request header. The flaw manifests in two distinct operational points within the vulnerable code path: first through the "len +=" statement that fails to properly validate or limit the size of environment variable values, and second through the apr_pcalloc function call that allocates memory without adequate bounds checking. This combination allows attackers to manipulate the memory allocation process in a way that either exhausts available memory resources or forces the server into a state where it attempts to dereference a null pointer, resulting in service interruption.
From an operational perspective, this vulnerability presents a significant risk to Apache server administrators as it requires minimal privileges to exploit, allowing local users to potentially disrupt web server operations without requiring elevated system access. The impact extends beyond simple service interruption to potentially consuming all available system memory or causing unpredictable server behavior through NULL pointer dereference conditions. The vulnerability's classification aligns with CWE-770, which addresses allocation of resources without limits or with inadequate limits, and relates to ATT&CK technique T1499.004 for network denial of service attacks. The memory consumption aspect of this vulnerability specifically maps to CWE-122, which deals with heap-based buffer overflow conditions, while the NULL pointer dereference component corresponds to CWE-476, addressing null pointer dereference scenarios.
The exploitation of CVE-2011-4415 demonstrates the importance of proper input validation and resource management in server-side applications, particularly those handling user-supplied data through configuration files. The vulnerability's persistence across multiple Apache versions indicates a fundamental flaw in the memory handling logic that was not adequately addressed until later releases. Organizations running affected Apache versions should implement immediate mitigations including disabling the mod_setenvif module when not required, implementing proper input validation for .htaccess files, and applying the relevant security patches that address the memory allocation bounds checking issues. The vulnerability also highlights the need for comprehensive security testing of configuration file parsing mechanisms and the potential for seemingly benign configuration directives to create significant operational risks when combined with specific execution paths in web server software.