Flarum's Logout Route allows open redirects

Vulnerability Description

Flarum is open source discussion platform software. Prior to version 1.8.5, the Flarum `/logout` route includes a redirect parameter that allows any third party to redirect users from a (trusted) domain of the Flarum installation to redirect to any link. For logged-in users, the logout must be confirmed. Guests are immediately redirected. This could be used by spammers to redirect to a web address using a trusted domain of a running Flarum installation. The vulnerability has been fixed and published as flarum/core v1.8.5. As a workaround, some extensions modifying the logout route can remedy this issue if their implementation is safe.

References

• https://github.com/flarum/framework/security/advisories/GHSA-733r-8xcp-w9mr
• https://github.com/flarum/flarum-core/commit/ee8b3b4ad1413a2b0971fdd9e40f812d2a3a9d3a
• https://github.com/flarum/framework/commit/7d70328471cf3091d92d95c382d277aec7996176