CVE-2023-6841: A Deep Dive into Keycloak's Denial of Service Vulnerability Due to Unrestricted Attributes

Keycloak, the popular open-source identity and access management solution, is known for its robust security features and flexibility. However, a recent discovery revealed a significant security flaw present in the software's core functionality: CVE-2023-6841.

In this comprehensive long-read post, we'll analyze this denial of service (DoS) vulnerability in Keycloak, discuss how it's exploited, and provide valuable resources to help you understand and address the issue.

CVE-2023-6841: The Vulnerability

The vulnerability, CVE-2023-6841, is a DoS vulnerability that results from a lack of limitation on the number of attributes allowed per object in Keycloak. By exploiting this vulnerability, an attacker can send repeated HTTP requests, each containing large numbers of attributes, causing Keycloak servers to exhaust their resources while trying to process the requests and return the responses with long attribute values.

To understand the impact of this vulnerability, let's take a closer look at how it was discovered, the code snippet responsible for the flaw, and the steps to recreate the exploit.

Discovery and Code Snippet

The vulnerability was discovered during a routine security audit, where researchers found a potential vector for exploitation in Keycloak. The vulnerable code snippet responsible for this flaw is shown below:

public KeycloakUserObject buildUserObject() {
    KeycloakUserObject userObject = new KeycloakUserObject();
    
    for (Attribute attribute : attributes) {
        userObject.addAttribute(attribute.getName(), attribute.getValue());
    }

    return userObject;
}

In the above code, the buildUserObject method allows Keycloak developers to create a new user object by adding attributes to a KeycloakUserObject instance. However, there's no implementation of a limit to the number of attributes that can be added. Also, no input validation on attribute values is provided, leaving the door wide open for attackers to exploit the vulnerability.

Exploiting CVE-2023-6841

To exploit the CVE-2023-6841 vulnerability, an attacker must send repeated HTTP requests that contain an excessive number of attributes. Each request would require the server to process the attributes and send a response containing the long attribute values; thus, exhausting system resources and potentially causing a denial of service.

Here is an example of an HTTP request that might be typical in this exploit

POST /auth/realms/{realm}/users/ HTTP/1.1
Host: keycloak.example.com
Content-Type: application/json

{
  "username": "attacker",
  "email": "attacker@example.com",
  "attributes": {
    "attribute1": "long_value",
    "attribute2": "long_value",
    ...
    "attribute10000": "long_value"
  }
}

In this example, the request contains an excessive number of attributes, and the server would need to process and return responses for each of them.

Mitigations and Solutions

As this vulnerability can potentially affect a large number of Keycloak deployments worldwide, it's essential to ensure that proper mitigations are in place.

The best way to address CVE-2023-6841 is to apply a patch release provided by Keycloak's developers. You can find information about the patch in the official release notes here: Official Keycloak Release Notes

In the absence of an available patch, administrators can implement a temporary workaround, monitoring and limiting excessive user attribute requests to prevent resource exhaustion. To achieve this, you could implement a rate limiter or a WAF (Web Application Firewall) ahead of your Keycloak server.

Conclusion

CVE-2023-6841 demonstrates that even widely-used and robust software like Keycloak is not immune to vulnerabilities. It is crucial to stay vigilant, continuously monitor your environment, and stay informed about the latest security updates.

To learn more about Keycloak and its related security concerns, consider visiting the following resources:

1. Keycloak
2. Keycloak Security Advisories
3. NIST National Vulnerability Database - CVE-2023-6841

Timeline

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