Keycloak is an open-source identity and access management tool, used to secure applications with login, single sign-on, and more. But even trustworthy tools can have bugs, and sometimes the result is a security nightmare. CVE-2023-0657 is a flaw in Keycloak that let authenticated attackers trade specialized "logout tokens" for powerful "access tokens," effectively letting them hijack data or break the app’s trust boundaries. If you run Keycloak in any sensitive environment, keep reading—this bug had real teeth.

What is CVE-2023-0657?

CVE-2023-0657 describes a weakness where Keycloak failed to properly enforce *token types* during the signature validation process. In short, when Keycloak validated JWT (JSON Web Tokens), it didn’t make sure a token claiming to be a logout token was really only usable for logging out. Instead, an attacker could replay that token, trade it in, and potentially receive an *access token*—gaining privileges they shouldn't have.

References

- NVD CVE-2023-0657 Detail Page
- Keycloak Security Advisory

The expiry, audience, and permissions

But with CVE-2023-0657, Keycloak trusted any *valid signature*, no matter the token type. It failed to double-check that a logout token was only for logging out, not for gaining access elsewhere.

Buggy Logic Example

Here’s a simplified, vulnerable code snippet (NOT real Keycloak code, but shows the core issue):

// Vulnerable pseudo-code
DecodedJWT token = JWT.decode(incomingToken);
if (token.isSignatureValid()) {
    // Oops! Assuming any valid JWT is an access token.
    processAsAccessToken(token);
} else {
    reject(token);
}

Notice: No check for token type! So someone could present a logout token, and as long as it passes signature checking, it’s accepted as an access token.

Real-World Impact

Imagine users logging out of a sensitive app. They get a valid "logout" token (meant to signal session termination). If an attacker intercepts this, or even replays their own, they could switch it for an access token—giving themselves extra abilities, and possibly seeing or messing with data outside their permissions.

Systems with token type–based security boundaries

Exploitation Requirements:

Exploiting the Flaw: How Attackers Can Abuse It

1. Get a Logout Token: Since logout tokens are presented during normal logout flows, an attacker who can watch network traffic (over insecure channels or with local access), or who can control their own application client, can obtain one.

Send the Logout Token Where Access Token is Expected:

Because of the improper token type enforcement, the attacker submits the logout token as if it were an access token.

Gain Unauthorized Access:

Keycloak, missing the type check, lets the request through, treating this logout token as a permission-granting access token.

Here’s a theoretical example using curl

# Assume LOGOUT_TOKEN has been obtained by the attacker
curl -H "Authorization: Bearer $LOGOUT_TOKEN" https://victim-app/api/user/data

If the vulnerable Keycloak accepts this, the attacker gets access!

The Fix

Keycloak fixed the bug by strictly enforcing token types during validation. Now, the app checks not just for a valid JWT, but also that the token is the *correct* type for the requested operation.

Fix References

- Keycloak GH PR for CVE-2023-0657

Patched Code (Simplified)

DecodedJWT token = JWT.decode(incomingToken);
if (token.isSignatureValid() && token.getType().equals("access")) {
    processAsAccessToken(token);
} else {
    reject(token);
}

Update Keycloak ASAP: Upgrade to version 19..3 or higher, or apply the vendor patch.

- Review Custom Token Handling: If you have custom APIs checking tokens, make sure to verify token types as well as signatures!
- Monitor for Strange Logins: Watch for unusual tokens or users accessing routes they shouldn’t reach.

Summary

CVE-2023-0657 was a classic case of “assuming all signed tokens are equal.” By failing to enforce strict token type checks, Keycloak left a loophole for attackers to trade logout tokens for access tokens, risking data leaks and broken access controls.

Further Reading

- Keycloak official security advisories
- Keycloak Token Exchange Docs

If you run Keycloak, patch it NOW!

Feel free to ask questions or share your experience with this (or similar) security bugs in the comments.

Timeline

Published on: 11/17/2024 11:15:05 UTC
Last modified on: 11/18/2024 17:11:17 UTC