A flaw was found in the secure-client-uris client policy executor in Keycloak. The executor permits non-HTTPS redirect URIs when allow-http-on-localhost is enabled by performing a simple string prefix check for http://localhost or http://127.0.0.1 instead of parsing the URI to validate the host component.
An attacker can exploit this by registering a client via Dynamic Client Registration using a redirect URI that starts with the allowed prefix but points to an attacker-controlled domain (e.g., http://localhost.evil.test). If the policy is scoped to authorization_code requests, it fails to block the registration or the subsequent use of the malicious URI. When a victim authenticates through the attacker's client, the authorization code is transmitted over cleartext HTTP to the attacker's server. This allows the attacker to intercept the authorization code and potentially gain unauthorized access to the victim's session.