An excluded subtrees iPAddress name constraint with an all-zero mask (0.0.0.0/0 or ::/0) does not restrict iPAddress SANs in certificates issued beneath it. The mask check treated an all-zero mask as matching nothing, when a /0 prefix matches every address of its family, so the exclusion was silently ignored.

CA/Browser Forum Baseline Requirements §7.1.2.5.2 require exactly these exclusions on every technically constrained sub-CA that may not issue for IP addresses. As a result, anyone holding (or having compromised) the key of such a sub-CA can issue a certificate for an arbitrary IP address, and Validator with RFC5280Policy and ServerIdentityPolicy accepts it for that address. A permitted subtrees dNSName entry on the same issuer does not prevent this, because iPAddress SANs are a different name form.

All users of Validator with RFC5280Policy are affected when a chain can contain a name-constrained issuer with an all-zero iPAddress exclusion.

The issue is fixed in x509-validator 0.3.1 (commit da661f8). Users should upgrade to 0.3.1 or later.