SignXML is an implementation of the W3C XML Signature standard in Python. When verifying signatures with X509 certificate validation turned off and HMAC shared secret set ("signxml.XMLVerifier.verify(require_x509=False, hmac_key=..."), versions of SignXML prior to 4.0.4 are vulnerable to a potential algorithm confusion attack. Unless the user explicitly limits the expected signature algorithms using the "signxml.XMLVerifier.verify(expect_config=...)" setting, an attacker may supply a signature unexpectedly signed with a key other than the provided HMAC key, using a different (asymmetric key) signature algorithm. Starting with SignXML 4.0.4, specifying "hmac_key" causes the set of accepted signature algorithms to be restricted to HMAC only, if not already restricted by the user.