CVE-2002-1850 in HTTP Server
Summary
by MITRE
mod_cgi in Apache 2.0.39 and 2.0.40 allows local users and possibly remote attackers to cause a denial of service (hang and memory consumption) by causing a CGI script to send a large amount of data to stderr, which results in a read/write deadlock between httpd and the CGI script.
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Analysis
by VulDB Data Team • 09/01/2025
The vulnerability described in CVE-2002-1850 affects the mod_cgi module in Apache web server versions 2.0.39 and 2.0.40, representing a critical denial of service weakness that can be exploited by both local and remote attackers. This issue stems from how the web server handles CGI script output redirection, specifically when scripts write substantial amounts of data to stderr. The flaw creates a scenario where the Apache httpd process becomes trapped in a read/write deadlock condition, effectively consuming system resources and rendering the web service unavailable to legitimate users. The vulnerability operates through a specific race condition in the inter-process communication between the web server and the CGI script execution environment, where the server's attempt to read from the script's stderr stream conflicts with the script's own attempt to write to that same stream, resulting in a complete system hang.
The technical mechanism behind this vulnerability involves the improper handling of file descriptor management within the Apache CGI execution framework. When a CGI script outputs large volumes of data to stderr, the mod_cgi module fails to properly manage the buffering and stream synchronization between the parent httpd process and the child CGI process. This creates a deadlock condition where the httpd process waits indefinitely for data from stderr while the CGI script simultaneously attempts to write to that same stream, causing both processes to become unresponsive. The issue is classified under CWE-121 as a buffer overflow condition, though more specifically represents a resource exhaustion and deadlock scenario. The vulnerability can be exploited by sending malicious data to CGI scripts that have been designed to produce excessive stderr output, effectively consuming available memory and CPU resources until the system becomes unresponsive.
From an operational impact perspective, this vulnerability presents a severe threat to web server availability and can be leveraged to perform denial of service attacks against Apache installations. The attack requires minimal privileges and can be executed remotely if the affected web server allows CGI execution, making it particularly dangerous for public-facing web applications. The resource consumption aspect of this vulnerability means that attackers can cause sustained system degradation without requiring complex exploitation techniques, simply by triggering CGI scripts to generate excessive stderr output. The deadlock condition created by this flaw can persist for extended periods, potentially causing cascading failures in web applications that depend on the Apache server for functionality. This vulnerability directly impacts the availability component of the CIA triad and can be classified under ATT&CK technique T1499.004 for resource exhaustion and T1566.001 for social engineering through denial of service attacks.
Mitigation strategies for CVE-2002-1850 should focus on immediate patching of affected Apache installations to versions that contain the corrected mod_cgi implementation. Organizations should implement strict CGI script execution policies that limit stderr output and monitor for unusual resource consumption patterns. The recommended approach includes upgrading to Apache 2.0.41 or later versions where this vulnerability has been addressed through improved file descriptor management and stream handling. Additionally, administrators should consider implementing resource limits on CGI script execution, such as memory and time constraints, to prevent malicious scripts from consuming excessive system resources. Network-level protections can include implementing rate limiting and monitoring for unusual stderr traffic patterns, while application-level controls should enforce proper input validation and output management within CGI scripts. The vulnerability demonstrates the importance of proper inter-process communication handling in web server modules and highlights the need for robust resource management practices in server-side applications, particularly those involving external process execution and stream redirection.