Keycloak Cross-site Scripting (XSS) via assertion consumer service URL in SAML POST-binding flow

Keycloak allows arbitrary URLs as SAML Assertion Consumer Service POST Binding URL (ACS), including JavaScript URIs (javascript:).

Allowing JavaScript URIs in combination with HTML forms leads to JavaScript evaluation in the context of the embedding origin on form submission.

Acknowledgements:

Special thanks to Lauritz Holtmann for reporting this issue and helping us improve our project.