The (1) HttpResource and (2) FileBackedHttpResource implementations in Shibboleth Identity Provider (IdP) before 2.4.1 and OpenSAML Java 2.6.2 do not verify that the server hostname matches a domain name in the subject's Common Name (CN) or subjectAltName field of the X.509 certificate, which allows man-in-the-middle attackers to spoof SSL servers via an arbitrary valid certificate.