A critical vulnerability, identified as CVE-2023-6291, has been found in the open-source identity and access management solution Keycloak, which may have serious security implications for its users across various platforms and applications. This post aims to outline the details of this flaw, how it can be exploited by attackers, and the potential risks associated with it. Furthermore, we will also provide links to the original sources and references to facilitate a better understanding of the issue.

The Vulnerability: CVE-2023-6291

The flaw in question was discovered in the redirect_uri (uniform resource identifier) validation logic implemented within the Keycloak system. Essentially, the redirect_uri is a crucial component that defines where the user is redirected after they have been authenticated by the server. This specific vulnerability, however, allows a malicious user to bypass the explicitly allowed hosts and use a different URI instead.

Exploiting the Vulnerability

A successful attack exploiting CVE-2023-6291 could lead to an attacker acquiring the user's access token, enabling them to then impersonate other users and potentially gain unauthorized access to sensitive resources within the system. The process can be summarized as follows:

1. The attacker constructs a malicious redirect_uri that bypasses the allowed list of URIs within the system.
2. The victim is then tricked into clicking on this malicious link, often through social engineering tactics.

The following example demonstrates how the vulnerability can be exploited in practice

// Create a malicious redirect_uri
String redirect_uri = "https://evil.com/?#"; + allowed_redirect_uri;

// Craft a URL with the malicious redirect_uri
String url = keycloak_base_url + "/auth/realms/" + realm_name + "/protocol/openid-connect/auth" +
 "?response_type=code" +
 "&client_id=" + client_id +
 "&scope=openid" +
 "&redirect_uri=" + URLEncoder.encode(redirect_uri, "UTF-8");

// Trick the victim into clicking the URL

In this example, the attacker combines an allowed redirect_uri with their malicious domain, which can then be included in a phishing email or message sent to the unsuspecting victim.

The original discovery and reporting of the CVE-2023-6291 vulnerability can be found at

- Official Keycloak Github Issue
- Official Keycloak Advisory

Additional relevant references and further information can be located at

- CVE Details
- National Vulnerability Database (NVD)

Mitigation and Protection

In order to effectively mitigate the risks associated with the CVE-2023-6291 vulnerability, it is recommended that users of the affected Keycloak system:

Ensure that a strict list of allowed redirect_uri values is maintained and regularly updated.

- Encourage users to be wary of suspicious links and adequately train staff members on recognizing potential social engineering attempts.


The discovery of CVE-2023-6291 highlights the importance of constant vigilance in maintaining, updating, and securing systems such as Keycloak. By being proactive in addressing vulnerabilities, organizations can minimize the risk of compromising sensitive data and protect their users from potential threats. Remember to always stay informed about recent developments and be prepared to act accordingly to ensure the safety and security of your digital assets.


Published on: 01/26/2024 15:15:08 UTC
Last modified on: 02/04/2024 20:15:46 UTC