In SAML you sign a (potentially attacker controlled) subset after normalization. So a lot of saml bugs come down to the attacker adding things that aren't covered by the signature. Sometimes this means appending or prepending stuff, but my favourite is adding comments which can alter the interpretation of the xml document (as it splits text nodes) but doesn't alter the signature.
In SAML you sign a (potentially attacker controlled) subset after normalization. So a lot of saml bugs come down to the attacker adding things that aren't covered by the signature. Sometimes this means appending or prepending stuff, but my favourite is adding comments which can alter the interpretation of the xml document (as it splits text nodes) but doesn't alter the signature.