In August 2023, a critical issue was discovered at the heart of one of the world’s most popular web servers: Apache HTTP Server. Known as CVE-2023-38709, this vulnerability impacts all Apache HTTP Server versions through 2.4.58. At the root, it’s a classic case: the server doesn’t properly validate inputs from the backend, letting attackers manipulate HTTP responses in dangerous ways. Below, we break down what’s going on, review how it can be exploited, see a code example, and offer tips for staying safe.

1. What Is CVE-2023-38709?

This vulnerability was discovered in the core of Apache HTTP Server and is about how the server handles responses from backend servers or "content generators" (like CGI scripts or proxied services). Specifically, if a backend returns malicious data containing special characters (CRLF, i.e., carriage return \r and line feed \n), Apache may allow those to pass into the user's browser as extra HTTP headers or even a full second response. This is called HTTP Response Splitting.

In simple terms

- Attackers can make Apache send extra HTTP headers or even new HTTP responses by taking control of backend data.
- Unchecked input from backends (like CGI output, FastCGI, or proxied apps) can break the separation between one HTTP response and the next.

2. Why Is Response Splitting Dangerous?

- Set Cookies for Other Domains: Attackers can set malicious cookies in the victim’s browser (session fixation, account takeover, etc.).

Web Cache Poisoning: Attackers can poison shared caches with false content.

- Cross-Site Scripting (XSS): Often, splitting the response can trigger XSS or other client-side attacks.

3. How Does the Exploit Work?

Attackers look for places where they can control backend output that Apache will include in the HTTP response. If that output contains their own specially crafted input, like CRLF (\r\n), Apache may blindly copy it to the HTTP response.

Suppose you have a CGI script that prints an HTTP header value based on a user’s input

#!/usr/bin/perl
print "Content-type: text/html\n";
print "X-MyHeader: " . $ENV{'QUERY_STRING'} . "\n\n";
print "<html>Hello, world!</html>\n";

If a user visits

http://example.com/cgi-bin/vulnerable.cgi?foo=bar

Normally, this returns a safe header

X-MyHeader: foo=bar

But if an attacker visits

http://example.com/cgi-bin/vulnerable.cgi?attacker=bad%d%aContent-Length:%200%d%a

The backend sends

X-MyHeader: attacker=bad
Content-Length:

Apache (before 2.4.59) may not properly check for those CRLFs and sends two headers! This can be abused for:

Let’s make it concrete. Suppose Apache is reverse proxying to a backend using mod_proxy

ProxyPass /api/ http://127...1:808/

If that backend writes a header controlled by the client

# Python Flask backend example
from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/set-header')
def hello():
    resp = make_response("Hello, world!")
    unhashed = request.args.get('foo', '')
    # HEADER NOT SANITIZED!
    resp.headers['X-Thing'] = unhashed
    return resp

Now, the attacker hits

http://victim.com/api/set-header?foo=hi%d%aSet-Cookie:%20sessionid=evil

Raw HTTP sent to browser

X-Thing: hi
Set-Cookie: sessionid=evil
Content-Type: text/html

The Official Patch

Apache fixed this in 2.4.59, released Oct 2023 (release notes). They improved how the server filters out unexpected newline characters from backend headers and content.

Vulnerable versions: All up to and including 2.4.58
Fixed in: 2.4.59

Upgrade to 2.4.59 or later: This is the best solution.

- Sanitize Backend Output: Escape or remove CR/LF from any header data created from user input.

Use Trusted Backends: Only connect Apache to backends you control.

- ProxyHeaderCheck directive: In some Apache configurations, controls can be set, but patching is best.

6. References for deep dive

- Apache HTTP Server Security Advisory
- National Vulnerability Database: CVE-2023-38709
- Release Notes for Apache 2.4.59

7. Final Words

CVE-2023-38709 is a classic web server flaw that enables attackers to "break out" of the normal response flow and inject their own HTTP data. If you run any Apache HTTP Server before 2.4.59—especially if it proxies or uses dynamic scripts—update immediately and review your backend code for unsafe user input handling. This bug is easy to exploit but also easy to prevent, so act now to keep your users safe.


*Stay secure, patch regularly, and always sanitize user input!*

Timeline

Published on: 04/04/2024 20:15:08 UTC
Last modified on: 11/05/2024 20:35:10 UTC