CVE-2006-20001 in HTTP Server
Summary
by MITRE • 01/17/2023
A carefully crafted If: request header can cause a memory read, or write of a single zero byte, in a pool (heap) memory location beyond the header value sent. This could cause the process to crash. This issue affects Apache HTTP Server 2.4.54 and earlier.
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Analysis
by VulDB Data Team • 10/08/2025
This vulnerability resides within the Apache HTTP Server software and represents a memory corruption issue that stems from improper handling of HTTP request headers. The flaw specifically manifests when processing the If: request header, which is commonly used for conditional requests in web protocols. The vulnerability falls under the category of buffer over-read or under-read conditions that can lead to memory access violations. According to CWE-125, this represents an out-of-bounds read vulnerability where the application accesses memory beyond the intended buffer boundaries. The issue is particularly dangerous because it operates within the heap memory management system, making it susceptible to exploitation that could result in denial of service or potentially more severe consequences.
The technical implementation of this vulnerability involves the parsing logic within Apache's HTTP server module responsible for processing conditional headers. When a maliciously crafted If: header is sent to the server, the parsing routine fails to properly validate the bounds of the header value before accessing memory locations. This allows an attacker to manipulate the memory access pattern such that a single zero byte operation occurs at a memory location beyond the intended header buffer. The heap memory pool where this occurs is particularly vulnerable because heap-based memory management is more complex and error-prone than stack-based operations. The vulnerability affects all versions of Apache HTTP Server up to and including version 2.4.54, making it a widespread concern for organizations maintaining older server configurations.
The operational impact of this vulnerability extends beyond simple server crashes, as it can be leveraged to create persistent denial of service conditions that degrade system availability. An attacker could repeatedly send malformed If: headers to cause the server process to crash consistently, effectively rendering the web service unavailable to legitimate users. This type of vulnerability aligns with ATT&CK technique T1499.004, which involves network denial of service attacks that target application availability. The memory corruption behavior could potentially be exploited further if the heap memory location accessed contains sensitive data or if the zero byte write operation affects memory layout in ways that could be chained with other exploits. Organizations running affected versions face significant risk of service disruption and potential data exposure through this memory management flaw.
Mitigation strategies for this vulnerability should focus on immediate patch application to upgrade to Apache HTTP Server version 2.4.55 or later, which contains the necessary fixes for the header parsing logic. System administrators should also implement robust input validation mechanisms at the network level, using web application firewalls or intrusion prevention systems to filter out suspicious If: headers before they reach the server. Additional defensive measures include monitoring for unusual patterns of HTTP requests containing malformed conditional headers and implementing rate limiting to prevent abuse of the vulnerability. The fix implemented by Apache developers addresses the core parsing logic by adding proper bounds checking for header values, preventing access to memory locations beyond the allocated buffer boundaries. Organizations should also consider implementing redundant server configurations and automated failover mechanisms to minimize the impact of potential exploitation attempts.