gh attestation verify: match the OIDC subject (sub) claim #13928

Problem

gh attestation verify can pin the source repo (--repo/--owner), the signer workflow (--cert-identity/--signer-workflow), and — since #9602 / #10308 — source ref/digest and signer digest. The one identity axis still not matchable is the OIDC subject (sub) claim, e.g.:

repo:my-org@<ORG_ID>/my-repo@<REPO_ID>:environment:release-stable

Two reasons this is worth a first-class matcher rather than only --format json:

1. It's a security boundary, not metadata. In a large org, repo-write access is not release authority: anyone who can push a workflow can produce attestations that still satisfy --repo and --signer-workflow. Only the :environment: segment of sub proves the run passed a GitHub Environment gate (required reviewers, prevent self-review) — the boundary a verifier actually needs to enforce.

2. It isn't in the --format json output. The parsed signature.certificate exposes sourceRepository*, buildSigner*, runnerEnvironment, and subjectAlternativeName (the signer workflow URI, e.g. .../deployment.yml@refs/heads/trunk) — but no field for the raw sub (Fulcio extension 1.3.6.1.4.1.57264.1.24, "Token Subject") or the deployment environment (.1.23). The sub is only reachable by DER-decoding attestation.bundle.verificationMaterial.certificate.rawBytes with openssl, which a policy engine can't do from JSON. (Verified on gh v2.96.0.)

This mirrors GitHub's own guidance: cloud OIDC trust policies match on sub, and its immutable numeric-ID form is the recommended rename-resistant identity.

Proposal

1. Surface Token Subject (.1.24), and ideally Deployment Environment (.1.23), under signature.certificate in --format json, so --jq and policy engines can match without DER parsing.

2. Add matcher flags paired with the existing --cert-oidc-issuer:

   • --cert-oidc-subject <value>
   • --cert-oidc-subject-regex <pattern>

   e.g. --cert-oidc-subject-regex ':environment:release-stable$'. Matching the full sub also binds the source repo by immutable ID in a single value.

Workaround today

gh attestation verify my-artifact --repo my-org/my-repo --format json \
  | jq -r '.[0].attestation.bundle.verificationMaterial.certificate.rawBytes' \
  | base64 -d | openssl x509 -inform DER -noout -text \
  | grep -A1 '57264\.1\.24' | tail -1 \
  | grep -Fq ':environment:release-stable'

Sources

• Fulcio oid-info.md
• Immutable subject claims changelog
• GitHub OIDC reference