cosign keyless "getting Fulcio ... 403" / OIDC signer error in CI

By Kaveh Alemi·Latchkey

Keyless cosign exchanges a workflow OIDC token for a short-lived Fulcio certificate. A 403 or "getting signer" error means the OIDC token was missing or rejected, usually because id-token: write was not granted.

What this error means

cosign fails with "error getting signer: getting key from Fulcio" and an HTTP 403, or "fetching ambient OIDC token" errors, during a keyless sign or attest.

Error: signing [image]: getting signer: getting key from Fulcio:
POST https://fulcio.sigstore.dev/api/v2/signingCert: 403

Common causes

Missing id-token: write permission

Without id-token: write, GitHub does not issue an OIDC token, so cosign has nothing to present to Fulcio and the certificate request is rejected.

The wrong OIDC provider or issuer for the environment

Running outside the expected provider (or a self-hosted context without ambient OIDC) means cosign cannot detect a token to exchange.

How to fix it

Grant id-token: write to the job

1. Add id-token: write to the workflow or job permissions.
2. Ensure cosign runs in a context that has ambient OIDC (GitHub-hosted or an OIDC-enabled runner).
3. Re-run so cosign mints a token and Fulcio issues the certificate.

permissions:
  id-token: write
  packages: write

Confirm connectivity to Fulcio

On restricted networks, allow outbound access to fulcio.sigstore.dev (and rekor.sigstore.dev) so the certificate exchange can complete.

cosign sign --yes ghcr.io/acme/app@sha256:...   # keyless, needs Fulcio reachable

How to prevent it

• Always set id-token: write for keyless cosign jobs.
• Allow outbound access to Fulcio and Rekor from the runner network.
• Test keyless signing on a throwaway digest before gating releases on it.

Frequently asked questions

Related guides

References

• GitHub Actions documentation