The shibboleth_login_form function in shibboleth.php in the Shibboleth plugin before 1.8 for WordPress is prone to an XSS vulnerability due to improper use of add_query_arg().
https://wpvulndb.com/vulnerabilities/8901
https://github.com/michaelryanmcneill/shibboleth/commit/1d65ad6786282d23ba1865f56e2fd19188e7c26a