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.