XML::Sig versions from 0.29 before 0.72 for Perl allow signature verification bypass because verify returns true when every signature was skipped before any cryptographic check. verify in lib/XML/Sig.pm counts the "//dsig:Signature" elements into "$numsigs" and iterates over them, but two paths reach "next" before any digest or key check runs: a "SignedInfo/Reference/@URI" that resolves to no element while "$numsigs" is greater than 1, and, when "id_attr" is set, a reference that does not match the requested ID. The loop records nothing about what it checked, so when every signature takes one of those paths control reaches the unconditional "return 1" that ends verify. Two "Signature" elements whose Reference URI names an ID that no element carries is enough, as is one such element combined with "id_attr". Any caller that passes untrusted XML to verify can receive a true return for a document in which no digest and no signature value was checked; a "cert" or "cert_text" trust anchor does not change this, because no key check runs. Versions up to 0.28 use an XML::XPath based verify that has no such skip and are not affected.