GHSA-cvrm-5hp6-h523

### Summary The logout endpoint accepts a `url` query parameter to redirect to. casserver treats that url as trusted, and either (depending on configuration) redirects the browser there, or shows a "you've been logged out" page with a link to continue to that url. There are a number of other things broken with logout in 7 (cas v3 uses a different query parameters, etc) ### Details https://github.com/simplesamlphp/simplesamlphp-module-casserver/blob/21418f7efbea8c4f078fd4a7d1b9eacf94dd4941/src/Controller/LogoutController.php#L104 Previous module checked the url against the valid service urls. ### PoC The docker instructions from the README.md run an image with a vulnerable config. Accessing https://localhost/cas/logout?url=https://google.com will redirect to Google ### Impact Impacted configs have ```php 'enable_logout' => true, ``` and are most impacted if they also have ``` 'skip_logout_page' -> true, ```