mod_auth_mellon before 0.13.1 is vulnerable to a Cross-Site Session Transfer attack, where a user with access to one web site running on a server can copy their session cookie to a different web site on the same server to get access to that site.
https://sympa.uninett.no/lists/uninett.no/arc/modmellon/2017-03/msg00008.html
https://github.com/UNINETT/mod_auth_mellon/releases/tag/v0.13.1