CVE-2012-4557 in HTTP Server
Summary
by MITRE
The mod_proxy_ajp module in the Apache HTTP Server 2.2.12 through 2.2.21 places a worker node into an error state upon detection of a long request-processing time, which allows remote attackers to cause a denial of service (worker consumption) via an expensive request.
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Analysis
by VulDB Data Team • 12/20/2021
The vulnerability described in CVE-2012-4557 affects the mod_proxy_ajp module within Apache HTTP Server versions 2.2.12 through 2.2.21, representing a significant security flaw that enables remote attackers to execute denial of service attacks against affected systems. This vulnerability specifically targets the application layer proxy functionality that handles requests between Apache and backend application servers using the AJP protocol. The module's design includes a mechanism to monitor request processing times and automatically place worker nodes into an error state when requests exceed predetermined time thresholds, creating a cascading effect that can exhaust available worker resources and render the service unavailable to legitimate users.
The technical flaw stems from the mod_proxy_ajp module's implementation of timeout handling and worker state management, where excessive request processing times trigger automatic worker node failure detection. This behavior is particularly dangerous because it allows attackers to craft requests that consume significant processing time without actually executing malicious code. The vulnerability operates by exploiting the module's inability to distinguish between legitimate long-running requests and maliciously crafted requests designed to exhaust worker resources. When the module detects a request taking longer than expected, it transitions the worker node to an error state, effectively removing it from the available worker pool and preventing subsequent requests from being processed by that worker. This creates a resource exhaustion scenario where the available worker pool gradually diminishes until no workers remain to handle legitimate requests.
The operational impact of this vulnerability extends beyond simple service disruption, as it enables attackers to consume system resources in a manner that can affect overall server performance and availability. Attackers can leverage this vulnerability by submitting requests that are designed to take longer than the configured timeout thresholds, causing the system to continuously remove workers from the pool and forcing the server to either reject new requests or exhaust all available workers. This type of denial of service attack can be particularly devastating in high-traffic environments where worker resources are already constrained, as it can effectively bring the entire proxy service to a halt. The vulnerability is classified under CWE-400, which addresses unspecified denial of service conditions in software, and aligns with ATT&CK technique T1499.004, which covers network denial of service attacks through resource exhaustion.
Mitigation strategies for this vulnerability should include immediate patching of affected Apache HTTP Server installations to versions that address the worker state management issue in mod_proxy_ajp. Administrators should also implement request timeout configurations that prevent excessive processing times from triggering worker failures, while monitoring for unusual patterns in request processing that might indicate exploitation attempts. Additionally, network-level protections such as rate limiting and request filtering can help reduce the impact of such attacks by preventing malicious requests from reaching the vulnerable module. The vulnerability demonstrates the importance of proper timeout handling and resource management in proxy modules, as inadequate implementation can create attack vectors that allow remote adversaries to consume system resources and disrupt service availability. Organizations should also consider implementing intrusion detection systems that can identify and alert on suspicious request patterns that might indicate exploitation attempts targeting this specific vulnerability.