Low [CVE-2026-18209] OIDC redirect_uri fragment bypass in HTTP parameter pollution check

A flaw was found in the keycloak-services component of Keycloak, which handles OpenID Connect (OIDC) authentication flows. The issue occurs because the security check designed to prevent HTTP parameter pollution only inspects the query portion of a redirect URL and ignores the fragment portion.

When a client is configured with a wildcard redirect URI, an attacker can use this to inject duplicate security parameters into the login response. If a client application is not configured correctly, it might trust the attacker's injected data instead of the real security information from Keycloak, leading to session fixation or account confusion.

The Red Hat Product Security team has assessed the severity of this vulnerability as Low, given that it requires a specific client configuration and a non-conformant relying party to be successful. The vulnerability's root cause is an incomplete validation check in RedirectUtils that fails to inspect the URI fragment for forbidden parameters.

Weakness: CWE-1288. Affected Red Hat products: Red Hat Build of Keycloak.

Red Hat lists Red Hat Data Grid 8; Red Hat JBoss Enterprise Application Platform Expansion Pack; Red Hat Single Sign-On 7 as not affected. Red Hat does not currently list a fixing RHSA for this CVE.