When verifying a certificate chain which contains a certificate containing
multiple email address constraints (composed of the full email address) which
share common local portions (the portion of the address before the '@'
character) but different domain portions (the portion of the address after the
'@' character), these constraints will not be properly applied, and only the
last constraint will be considered.

This can allow certificates in the chain containing email addresses which are
either not permitted or excluded by the relevant constraints to be returned by
calls to Certificate.Verify. Since the name constraint checks happen after chain
building is complete, this only applies to certificate chains which chain to
trusted roots (root certificates either in VerifyOptions.Roots or in the system
root certificate pool), requiring a trusted CA to issue certificates containing
either not permitted or excluded email addresses.

This issue only affects Go 1.26.

Thanks to Jakub Ciolek for reporting this issue.

This is CVE-2026-27137 and Go issue #77952.

This is a PRIVATE issue for CVE-2026-27137, tracked in http://b/483702190.