CVE-2009-1890 in HTTP Server
Summary
by MITRE
The stream_reqbody_cl function in mod_proxy_http.c in the mod_proxy module in the Apache HTTP Server before 2.3.3, when a reverse proxy is configured, does not properly handle an amount of streamed data that exceeds the Content-Length value, which allows remote attackers to cause a denial of service (CPU consumption) via crafted requests.
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Analysis
by VulDB Data Team • 09/06/2019
The vulnerability described in CVE-2009-1890 represents a critical denial of service weakness within the Apache HTTP Server's mod_proxy module, specifically affecting versions prior to 2.3.3. This flaw exists in the stream_reqbody_cl function within the mod_proxy_http.c file, which handles reverse proxy operations. The issue manifests when the server processes requests where the actual amount of streamed data exceeds the Content-Length header value specified in the HTTP request. This discrepancy creates a scenario where the proxy server enters an infinite loop or excessive CPU processing while attempting to handle the malformed data stream, ultimately leading to resource exhaustion and service disruption.
The technical implementation of this vulnerability stems from improper validation and handling of HTTP request bodies within the reverse proxy context. When Apache acts as a reverse proxy, it must process incoming requests and forward them to backend servers while potentially modifying or streaming the request body. The stream_reqbody_cl function fails to properly validate that the amount of data received matches the Content-Length header, creating a condition where attackers can craft requests with a Content-Length value that is smaller than the actual data being transmitted. This mismatch causes the proxy to continuously process or wait for additional data that will never arrive, leading to excessive CPU consumption and potential system instability.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire proxy infrastructure. Attackers can exploit this weakness by sending carefully crafted HTTP requests with manipulated Content-Length headers, causing the Apache server to consume excessive CPU resources in an attempt to process the malformed data stream. This type of denial of service attack can be particularly devastating in high-traffic environments where proxy servers handle thousands of requests per second, as the resource exhaustion can affect the availability of legitimate services. The vulnerability affects the core functionality of Apache's reverse proxy capabilities, making it a significant concern for organizations relying on Apache as their primary web proxy solution.
Organizations should prioritize immediate patching of affected Apache installations to mitigate this vulnerability, as the exploit requires no authentication and can be executed remotely. The recommended mitigation involves upgrading to Apache HTTP Server version 2.3.3 or later, where the fix addresses the improper handling of Content-Length values during streamed data processing. Additionally, administrators should implement monitoring solutions to detect unusual CPU consumption patterns that may indicate exploitation attempts. Security teams should also consider implementing rate limiting and request validation mechanisms at the network level to provide additional defense in depth. This vulnerability aligns with CWE-129, which addresses improper validation of length fields, and maps to ATT&CK technique T1499.004 for network denial of service attacks. Organizations should also review their proxy configurations to ensure proper handling of Content-Length headers and implement comprehensive logging to detect potential exploitation attempts.