CVE-2006-20001 refers to a security vulnerability that was discovered affecting the Apache HTTP Server. This is a critical issue as a properly crafted If: request header could lead to memory being read and written beyond the header value sent. This could potentially cause the server process to crash, leading to denial of service attacks. In this article, we will explore the details of the vulnerability, the potential impact, and the steps to mitigate it.

Vulnerability Details

The CVE-2006-20001 vulnerability lies within the Apache HTTP Server. In simple terms, the vulnerability affects the server's header processing routine. A carefully crafted If: request header can trigger a memory read, or a write of a single zero byte, in a pool (heap) memory location beyond the header value sent. This issue affects Apache HTTP Server 2.4.54 and earlier versions.

When this vulnerability is exploited, it could lead to a server crash, which could be detrimental to a web server's uptime and stability. To better understand the vulnerability, let's take a look at the code snippet that illustrates this issue:

Original References


Code Snippet

/* The code snippet below demonstrates how a malicious If: request header
   can trigger the CVE-2006-20001 vulnerability. */

static void process_if_header(request_rec *r) {
    char *header_value;
    header_value = (char *)apr_table_get(r->headers_in, "If");
    /* A carefully crafted If: request header could cause memory read/write beyond
       the header value sent. */
    if (header_value != NULL) {
        process_if_header_value(r, header_value);

As shown in the code snippet, the server retrieves the "If" header value from the incoming request. If the "If" header value is not null, the function processes the value in a way that allows a potential attacker to craft a malicious request header that triggers the vulnerability, causing memory read and write issues.

Exploit Details

The following exploit demonstrates how an attacker might utilize the CVE-2006-20001 vulnerability to harm the targeted server:

import requests

target_url = "";

headers = {
    "If": "<malicious_header>", # Replace with carefully crafted malicious header

response = requests.get(target_url, headers=headers)

In the Python exploit code, an attacker might send an HTTP GET request to the target web server with a carefully crafted "If" header value. If the server is vulnerable, this request could lead to a server crash, impacting availability.

Mitigation Steps

To mitigate this vulnerability, it is essential to update Apache HTTP Server to at least version 2.4.55.

For detailed update instructions, please refer to the official Apache documentation

If updating is not possible, implement WAF (Web Application Firewall) rules to block requests with suspicious or malformed "If" headers.


CVE-2006-20001 is a critical security vulnerability affecting the Apache HTTP Server. By exploiting this vulnerability, attackers can potentially crash the web server and cause a denial of service. Keeping your Apache HTTP Server up-to-date and implementing proper security measures can help protect your server from such threats.


Published on: 01/17/2023 20:15:00 UTC
Last modified on: 01/25/2023 01:51:00 UTC