CVE-2026-42790: nameConstraints DNS bypass via subject CommonName fallback in public_key hostname verification

Improper Certificate Validation vulnerability in Erlang OTP publickey (pubkeycert and publickey modules) allows a DNS nameConstraints bypass via subject CommonName fallback in TLS hostname verification.

Two flaws combine to allow a subordinate CA whose DNS nameConstraints are restricted (e.g. permitted;DNS:allowed.example.com) to issue a leaf certificate that an OTP TLS client accepts as a valid identity for an out-of-scope hostname (e.g. victim.example.com):

First, pubkeycert:validatenames/6 in lib/publickey/src/pubkeycert.erl only checks SAN DNS entries against nameConstraints. Per RFC 5280, a permitted DNS subtree only restricts certificates that contain a DNS-typed name. A leaf with no subjectAltName therefore trivially satisfies any permitted;DNS:... constraint regardless of its subject commonName.

Second, publickey:pkixverifyhostname/3 in lib/publickey/src/publickey.erl falls back to the subject commonName when no subjectAltName is present, extracting id-at-commonName attributes as presented IDs and matching them against the reference hostname. The strict pkixverifyhostnamematchfun(https) matcher does not suppress this fallback.

The result is that path validation accepts a CN-only leaf under a DNS-constrained intermediate (no SAN means the nameConstraints are not triggered), and hostname verification then accepts it via the CN fallback. The bypass is reachable from stock ssl:connect with verifypeer, a trusted CA, SNI, and the canonical strict https hostname matcher.

This issue affects OTP from OTP 19.3 before OTP 29.0.1, OTP 28.5.0.1, OTP 27.3.4.12 and OTP 26.2.5.21, corresponding to publickey from 1.4 before 1.21.1, 1.20.3.1, 1.17.1.3 and 1.15.1.7.

— MITRE

nameConstraints DNS bypass via subject CommonName fallback in publickey hostname verification

— Microsoft