Low [CVE-2026-18217] SAML HTTP-Redirect binding response preserves query string leading to parameter pollution

A flaw was found in the SAML protocol implementation of Keycloak, an open-source identity and access management solution. The issue occurs when Keycloak handles SAML authentication requests using the HTTP-Redirect binding.

If a client is configured with a wildcard redirect URL, an attacker can craft a request that includes malicious parameters. When a user authenticates, Keycloak appends its legitimate response to the attacker's parameters.

This can cause some service providers to process the attacker's data instead of the real login information, potentially leading to a user being logged into the wrong account.

The Red Hat Product Security team has assessed the severity of this vulnerability as Moderate, given that exploitation requires a specific client configuration (wildcard redirect URIs) and a downstream service provider that incorrectly handles duplicate HTTP parameters.

Successful exploitation allows an attacker to perform login CSRF or session swapping on the affected service provider. The vulnerability's root cause is the improper handling of the query string in the SAML HTTP-Redirect binding response, which fails to strip or validate pre-existing SAML parameters.

Weakness: CWE-20. Affected Red Hat products: Red Hat Build of Keycloak.