Shibboleth consortium reports:
Shibboleth SP software crashes on well-formed but invalid XML. The Service Provider software contains a code path with an uncaught exception that can be triggered by an unauthenticated attacker by supplying well-formed but schema-invalid XML in the form of SAML metadata or SAML protocol messages. The result is a crash and so causes a denial of service. You must rebuild opensaml and shibboleth with xmltooling-1.5.5 or later. The easiest way to do so is to update the whole chain including shibboleth-2.5.5 an opensaml2.5.5.