CVE-2007-6422 in HTTP Server
Summary
by MITRE
The balancer_handler function in mod_proxy_balancer in the Apache HTTP Server 2.2.0 through 2.2.6, when a threaded Multi-Processing Module is used, allows remote authenticated users to cause a denial of service (child process crash) via an invalid bb variable.
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Analysis
by VulDB Data Team • 08/03/2019
The vulnerability identified as CVE-2007-6422 affects the Apache HTTP Server version 2.2.0 through 2.2.6 and specifically targets the balancer_handler function within the mod_proxy_balancer module. This flaw manifests when the server operates with a threaded Multi-Processing Module, creating a critical security gap that can be exploited by authenticated remote attackers to trigger denial of service conditions. The vulnerability stems from improper handling of an invalid bb variable within the load balancing functionality, which forms part of the Apache HTTP Server's proxy capabilities designed to distribute requests across multiple backend servers.
The technical implementation of this vulnerability involves the balancer_handler function failing to properly validate input parameters when processing proxy load balancing requests. When an authenticated user submits a request containing malformed bb variable data, the function does not adequately sanitize or validate this input before processing it within the threaded environment. This lack of proper input validation creates a condition where the server's child processes can become unstable and crash, leading to a denial of service scenario that affects legitimate users attempting to access the web server. The threaded Multi-Processing Module environment exacerbates this issue as the flaw can cause cascading failures across multiple concurrent connections handled by the same process threads.
The operational impact of CVE-2007-6422 extends beyond simple service disruption, as it represents a significant threat to the availability and stability of web applications relying on Apache's proxy load balancing features. Attackers can exploit this vulnerability to repeatedly crash child processes, potentially leading to complete service unavailability until the server is manually restarted or the processes are automatically recovered. The authenticated nature of the attack means that adversaries must have valid credentials to exploit the vulnerability, but this requirement does not significantly reduce the risk since legitimate users with access can potentially be compromised or the credentials can be obtained through other means. This vulnerability directly maps to CWE-20, which describes improper input validation, and can be categorized under ATT&CK technique T1499.1 for endpoint denial of service, specifically targeting the availability of services through process termination.
Mitigation strategies for this vulnerability require immediate patching of affected Apache HTTP Server installations to version 2.2.7 or later, which contains the necessary code fixes to properly validate the bb variable input. Organizations should also implement network segmentation and access controls to limit the scope of potential exploitation, while monitoring for unusual patterns of authentication attempts that might indicate exploitation attempts. The fix implemented in the patched versions involves strengthening input validation routines within the balancer_handler function to properly handle malformed bb variable data and prevent it from causing memory corruption or process instability. Additionally, administrators should consider implementing rate limiting and connection monitoring to detect and prevent abuse of the load balancing functionality, as well as maintaining regular security assessments to identify similar vulnerabilities in other modules of the Apache HTTP Server ecosystem.