Medium [CVE-2026-18211] secure-client-uris policy bypass via localhost-prefixed domains

A flaw was found in the secure-client-uris client policy executor within Keycloak core services. This component is responsible for enforcing security requirements on client configurations, such as requiring encrypted connections for redirect URIs.

Due to an improper check that only looks at the start of a web address rather than properly verifying the host, an attacker can bypass these security restrictions by using a specially crafted domain name. This could allow an attacker to intercept sensitive authentication codes over unencrypted connections.

The Red Hat Product Security team has assessed the severity of this vulnerability as Moderate, given that exploitation requires Dynamic Client Registration to be enabled and the attacker to control a specific localhost-prefixed domain.

Successful exploitation allows an attacker to intercept authorization codes over cleartext HTTP by bypassing the secure-client-uris policy. The vulnerability's root cause is improper validation of the redirect URI host component in the secure-client-uris policy executor.

Weakness: CWE-20. 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.