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.

URL: http://shibboleth.net/community/advisories/secadv_20150721.txt
Security: CVE-2015-2684