xmlseclibs 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.
Affected: Packagist: robrichards/xmlseclibs
### Summary An authentication bypass vulnerability exists due to a flaw in the libxml2 canonicalization process, which is used by [xmlseclibs](https://github.com/robrichards/xmlseclibs) during document transformation. This weakness allows an attacker to generate a valid signature once and reuse it indefinitely. In practice, a signature created during a previous interaction - or through a misconfigured authentication flow - can be replayed to bypass authentication checks.
### Details When libxml2’s canonicalization is invoked on an invalid XML input, it may return an empty string rather than a canonicalized node. [xmlseclibs](https://github.com/robrichards/xmlseclibs) then proceeds to compute the DigestValue over this empty string, treating it as if canonicalization succeeded.
https://github.com/robrichards/xmlseclibs/blob/f4131320c6dcd460f1b0c67f16f8bf24ce4b5c3e/src/XMLSecurityDSig.php#L296
### Impact Digest bypass: By crafting input that causes canonicalization to yield an empty string, the attacker can manipulate validation to pass incorrectly.
Signature replay on empty canonical form: If an empty string has been signed once (e.g., in a prior interaction or via a misconfigured flow), that signature can potentially be replayed to bypass authentication.
### Suggested remediation Treat canonicalization failures (exceptions or nil/empty outputs) as fatal and abort validation. Add explicit checks: reject when canonicalize returns nil/empty or raise