A flaw was found in mod_auth_mellon where it does not sanitize logout URLs properly. This issue could be used by an attacker to facilitate phishing attacks by tricking users into visiting a trusted web application URL that redirects to an external and potentially malicious server. The highest threat from this liability is to confidentiality and integrity.
https://github.com/latchset/mod_auth_mellon/commit/42a11261b9dad2e48d70bdff7c53dd57a12db6f5