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CVE-2026-18568                                       CPAN Security Group
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         CVE ID:  CVE-2026-18568
   Distribution:  XML-Sig
       Versions:  from 0.29 before 0.72

       MetaCPAN:  https://metacpan.org/dist/XML-Sig
       VCS Repo:  https://github.com/perl-net-saml2/perl-XML-Sig

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

Description
-----------
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.

Problem types
-------------
- CWE-347 Improper Verification of Cryptographic Signature

Solutions
---------
Upgrade to XML::Sig 0.72 or later, which returns false when no
reference digest was checked and matched.

References
----------
https://github.com/perl-net-saml2/perl-XML-Sig/commit/ef22cfed1ac0f29b316d17eb79cf6480e03ae16a.patch
https://metacpan.org/release/TIMLEGGE/XML-Sig-0.72/changes
https://www.cve.org/CVERecord?id=CVE-2025-40934

Timeline
--------
- 2026-08-03: Version 0.72 released with fix.