cosign

• Control: provenance
• Tool: Sigstore cosign, https://github.com/sigstore/cosign
• Status: ✅ implemented
• Target: a component's images: (ImageTarget)
• License / terms: Apache-2.0 (permissive). Run via exec. No account, no key and no terms to accept: verification reads what the registry and the public Sigstore instances already publish.

What it does#

Runs cosign verify against each image, with the issuer and identity the provenance controller resolved for it, and turns the outcome into at most one finding, plus what it observed either way.

cosign verify --output json \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com \
  --certificate-identity https://github.com/acme/ci/.github/workflows/build.yml@refs/tags/v3 \
  ghcr.io/acme/payments@sha256:9c8f…

The verdict is cosign's. Draugr decides what to expect and passes it in; it never reads a signature back with a permissive pattern and then compares the subject itself. A pattern loose enough to match anything accepts a signature from anybody, and putting the guarantee behind a string comparison in Draugr would be the same weakness one layer further in.

Signatures and attestations answer differently#

An artifact can carry a bare cosign signature or an attestation, and cosign reports on the two in different ways. A GitHub artifact attestation pushed to the registry with push-to-registry: true is the second kind, which makes it the common case rather than the exotic one.

For an attestation, cosign exits 1 and explains, naming the identity it found in the same sentence. So a refusal carrying both no matching attestations and failed to verify certificate identity is an unexpected signer, and the identity is read out of that message rather than by asking a second time. Every other reason for exit 1 stays an error: reading them all as a mismatch would turn a registry nobody could reach into a finding about somebody's artifact.

An attestation's identity is also absent from the payload a successful read prints, so discovery asks for it separately, by naming an identity no certificate can carry and reading the one cosign names instead. Without that, the images most likely to carry provenance are the ones discovery says nothing about.

What the exit code means#

For a signature, cosign distinguishes its failures with exit codes it defines, and this scanner reads only those. Observed against cosign 3.1.1:

The last row is the one that matters. cosign returns 1 for everything it has no specific code for, which includes a registry it could not reach and a transparency log that did not answer. Reading that as "unsigned" would turn an unreachable registry into a finding somebody triages, and reading it as a pass would turn it into nothing at all.

On code 12 the image is read back a second time with a permissive identity pattern, to say who did sign it. That read never produces a pass; it only supplies a name for a finding that has already been made.

Rules#

An image no signer covers produces no finding under the default unmatched: observe. What was found is recorded in the control's account of the run instead: the identity on each signed image, and a count of the ones carrying none.

A note per image would be the obvious alternative and is the wrong shape. Most of what a project runs is published by somebody else, so an inventory of fourteen images becomes fourteen rows about nothing anybody can fix. And a Sigstore identity runs to about ninety-five characters where the console gives a finding's message ninety-six, so the one part worth copying is the part that would be cut.

Saga options#

None yet. The expectation belongs to the control, so it is declared once under config.controls.provenance.signers and applies to every scanner serving it. See the provenance controller.

Digests and tags#

An image declared with a digest: is verified against that digest. One declared with only a tag is verified against whatever the tag resolves to at scan time, and the report says how many images were checked that way under Measured against.

Verifying a tag establishes less than verifying a digest, because a tag can be repointed after the check. It establishes a great deal more than not looking, and refusing would withhold the control from exactly the descriptors most likely to carry an unexpected signer. What it may not do is go unsaid.

Notes#

• Integration mode: exec. cosign must be on PATH; draugr tools install cosign provisions the pinned build.
• --insecure-ignore-tlog is never passed. Without the transparency log there is nothing to establish that the signing certificate was valid when it signed rather than valid now, and a certificate that lives ten minutes is the whole design. cosign 2.x had a vulnerability (GO-2026-4529) in exactly that combination.
• A signature establishes where an artifact came from. It does not establish that the artifact is safe, and this scanner's findings never relax another control's.

Data#

Sigstore trust root, from tuf-repo-cdn.sigstore.dev. cosign keeps it under ~/.sigstore, and Draugr warms it once per run with cosign initialize so a component with fourteen images does not open fourteen concurrent requests for one file.

A runner that cannot reach it verifies against a copy instead: point config.controls.provenance.trustRoot at a trusted-root JSON file, which becomes cosign --trusted-root. Without either, verification reports an error rather than a pass.

What is sent#

The digest of each image checked, to the registry holding it, and to the Sigstore transparency log where a signature carries no inclusion proof of its own. Most signatures made in the last few years carry one, and cosign then verifies the log entry offline and contacts nothing.

No source, no manifest and no credential leaves the machine. The registry already knows which digests you pull; the transparency log is public, and an entry looked up there is an entry anybody can already read.

Links#

• cosign: https://github.com/sigstore/cosign
• Verifying signatures: https://docs.sigstore.dev/cosign/verifying/verify/
• OIDC identities in Fulcio certificates: https://github.com/sigstore/fulcio/blob/main/docs/oidc.md