gnutls CN fallback with unsupported SAN type

hi,

GnuTLS accepts hostnames via Common Name fallback when a certificate contains subjectAltName entries with unsupported types (for example URI), even though RFC 6125 requires CN suppression whenever SAN is present. the issue enables identity verification ambiguity in TLS clients that rely on GnuTLS. equivalent certificates with supported SAN types do not trigger this fallback, so the behavior is dependent on SAN subtype.

Severity

MEDIUM

relevant links

root cause

IS_SAN_SUPPORTED marks only DNS and IP SAN types as supported. URI/other SAN types do not set the have_other_addresses flag, so the CN fallback branch is allowed even when SAN is present. this violates RFC 6125 hostname matching rules for SAN-bearing certificates.

impact

enterprise and controlled PKI deployments can mis-validate server identity if an attacker controls a private CA or a compromised CA path can issue a certificate with URI SAN plus a victim hostname CN. clients using GnuTLS may trust the wrong host under specific certificate profiles.

proof of concept 

unzip -q -o poc.zip
bash ./canonical.sh

expected:

[PROOF_MARKER]: gnutls accepts hostname via CN fallback despite URI SAN

unzip -q -o poc.zip
bash ./control.sh

expected:

[NC_MARKER]: certificate with DNS SAN and different CN was correctly rejected by hostname check

expected: with any SAN entry, CN should not decide hostname acceptance actual: unsupported SAN types can trigger CN fallback in GnuTLS a fix accepted when: gnutls suppresses CN fallback whenever a SAN extension is present, regardless of SAN type

attack path

  1. attacker obtains or controls issuance of a certificate with a SAN URI
  2. cert is paired with a victim hostname in CN
  3. GnuTLS client accepts handshake for that hostname

recommended fix

change gnutls_x509_crt_check_hostname2 flow so any SAN presence disables CN fallback unconditionally.

fix accepted when: URI SAN certificates no longer pass hostname check via CN fallback when CN is attacker-controlled

credit: 1seal (https://github.com/1seal)

best, Oleh Konko

poc.zip

PR_DESCRIPTION.md