CVE-2025-66578xmlseclibs is a library written in PHP for working with XML Encryption and Signatures. Versions 3.1.3 contain an authentication bypass vulnerability due to a flaw in the libxml2 canonicalization process during document transformation. When libxml2’s canonicalization is invoked on an invalid XML input, it may return an empty string rather than a canonicalized node. xmlseclibs then proceeds to compute the DigestValue over this empty string, treating it as if canonicalization succeeded. This issue is fixed in version 3.1.4. Workarounds include treating canonicalization failures (exceptions or nil/empty outputs) as fatal and aborting validation, and/or adding explicit checks to reject when canonicalize returns nil/empty or raises errors.