CVE-2023-6291: The redirect_uri validation logic allows for bypassing explicitly allowed hosts that would otherwise be restricted

An issue was found in the redirect_uri validation logic that allows for a bypass of otherwise explicitly allowed hosts.

The problem arises in the verifyRedirectUri method, which attempts to enforce rules on user-controllable input, but essentially causes a desynchronization in how Keycloak and browsers interpret URLs. Keycloak, for example, receives "www%2ekeycloak%2eorg%2fapp%2f:y@example.com" and thinks the authority to be keycloak.org when it is actually example.com. This happens because the validation logic is performed on a URL decoded version, which no longer represents the original input.

Acknowledgements

Karel Knibbe