CVE-2009-3896 in nginx
Summary
by MITRE
src/http/ngx_http_parse.c in nginx (aka Engine X) 0.1.0 through 0.4.14, 0.5.x before 0.5.38, 0.6.x before 0.6.39, 0.7.x before 0.7.62, and 0.8.x before 0.8.14 allows remote attackers to cause a denial of service (NULL pointer dereference and worker process crash) via a long URI.
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Analysis
by VulDB Data Team • 08/28/2021
The vulnerability identified as CVE-2009-3896 represents a critical denial of service flaw within the nginx web server software. This issue affects multiple versions of the popular open source web server, specifically ranging from version 0.1.0 through 0.4.14, 0.5.x versions prior to 0.5.38, 0.6.x versions before 0.6.39, 0.7.x versions before 0.7.62, and 0.8.x versions before 0.8.14. The flaw exists within the HTTP parsing component of nginx, specifically in the src/http/ngx_http_parse.c file, which handles the processing of HTTP requests and URI parsing operations. This vulnerability allows remote attackers to exploit the web server by crafting malicious HTTP requests containing excessively long URIs, leading to system instability and service disruption.
The technical nature of this vulnerability stems from a NULL pointer dereference condition that occurs during URI parsing operations. When nginx encounters a particularly long URI in an HTTP request, the parsing logic fails to properly validate the input length and subsequently attempts to access a NULL pointer, resulting in an immediate crash of the worker process handling that request. This NULL pointer dereference represents a classic software flaw categorized under CWE-476, which specifically addresses NULL pointer dereference conditions that can lead to application crashes and potential system instability. The vulnerability exploits the lack of proper bounds checking in the URI parsing algorithm, allowing attackers to bypass normal input validation mechanisms and trigger the memory access violation through carefully crafted malformed requests.
The operational impact of CVE-2009-3896 extends beyond simple service disruption, as it can be leveraged to create sustained denial of service conditions against nginx web servers. Attackers can repeatedly send malformed requests with excessively long URIs to crash worker processes, forcing nginx to restart or become unresponsive, which directly impacts web server availability and can affect numerous websites and applications hosted on affected systems. This vulnerability particularly impacts organizations relying on nginx for web hosting, reverse proxy services, or load balancing operations, as the attack can be executed remotely without requiring authentication or special privileges. The vulnerability also aligns with ATT&CK technique T1499.004, which describes network denial of service attacks that target application availability through exploitation of software flaws in web servers and network infrastructure components.
Mitigation strategies for this vulnerability require immediate patching of affected nginx installations to versions that address the URI parsing flaw. System administrators should upgrade to the latest stable releases of nginx that contain the necessary fixes, typically versions 0.5.38, 0.6.39, 0.7.62, and 0.8.14 respectively for each affected release line. Additionally, implementing rate limiting and request length restrictions at the network level can provide temporary protection while patches are deployed. Organizations should also consider deploying intrusion detection systems that can identify and block suspicious HTTP requests with unusually long URIs. The vulnerability demonstrates the importance of proper input validation and bounds checking in network services, particularly in critical infrastructure components like web servers that handle untrusted input from external sources. Security monitoring should include detection of worker process crashes and restart patterns that may indicate exploitation attempts, as these behaviors directly correlate with the successful exploitation of this denial of service vulnerability.