Security

Cosign is a security tool. The whole project's value depends on getting trust boundaries right. This page summarizes them.

Trust boundaries

graph TB
    subgraph Trusted["Trusted (compiled into the binary)"]
        Code[Go source under cmd/, pkg/, internal/]
        Vendor[Vendored modules in go.sum]
    end
    subgraph TUFTrust["Trusted via TUF metadata"]
        TR[trusted_root.json: Fulcio CA, Rekor key, CT key, TSA chain]
    end
    subgraph External["External — verified, not trusted"]
        Fulcio
        Rekor
        TSA
        OIDC[OIDC issuer]
        Reg[OCI registry]
    end
    Code --> TR
    TR --> Fulcio
    TR --> Rekor
    TR --> TSA
    Code --> OIDC
    Code --> Reg

Anything cryptographically tied back to trusted_root.json is trusted; everything else is verified at use time. The TUF metadata itself is updated by cosign initialize (see pkg/cosign/tuf.go).

Where keys and secrets live

• Local cosign keys — cosign.key is encrypted with nacl/secretbox keyed by an scrypt-derived password (pkg/cosign/keys.go::EncryptKey). The password is taken from COSIGN_PASSWORD or an interactive prompt. The pkg/cosign/env/env.go registry marks COSIGN_PASSWORD, COSIGN_PKCS11_PIN, SIGSTORE_ID_TOKEN, GITHUB_TOKEN, GITLAB_TOKEN as Sensitive: true, which excludes them from cosign env output.
• KMS keys — never leave the KMS. Cosign asks the KMS to sign and never sees the private material. Provider URIs: awskms://, gcpkms://, azurekms://, hashivault://.
• Hardware tokens — PIV (pkg/cosign/pivkey/) and PKCS#11 (pkg/cosign/pkcs11key/) hold private keys on the device.
• OIDC tokens — short-lived (typically minutes), exchanged for a 10-minute Fulcio cert and discarded. They're the most sensitive in-flight material; the keyless flow's privacy prompt (internal/ui/prompt.go, "by typing 'y', you attest…") exists to make sure the user knows the email/sub will land in the public Rekor log forever.

Identity-based verification is mandatory

Per the v2.x cleanup, every keyless cosign verify requires both --certificate-identity[-regexp] and --certificate-oidc-issuer[-regexp]. This is enforced in cmd/cosign/cli/verify/common.go and not optional. Without it the cryptographic check would only prove "some certificate signed by Fulcio" rather than "this specific identity signed".

Transparency log requirement

By default every keyless signature is uploaded to Rekor and verifiers require an inclusion proof. --insecure-ignore-tlog and --tlog-upload=false exist for niche workflows but cosign prints Skipping tlog verification is an insecure practice… when they're used (cmd/cosign/cli/verify.go::ignoreTLogMessage). The recently merged commit "bundle create: Prevent IgnoreTlog when bundle contains SET" (Apr 2026) further closes a footgun where a user might silently drop a Signed Entry Timestamp.

Supply chain hygiene of the project itself

Cosign signs its own releases with cosign — see release/ and .goreleaser.yml. CI also runs:

• OpenSSF Scorecard (.github/workflows/scorecard-action.yml) — visible at https://securityscorecards.dev/viewer/?uri=github.com/sigstore/cosign.
• CodeQL static analysis (.github/workflows/codeql-analysis.yml).
• Dependency review (.github/workflows/depsreview.yml).
• Dependabot (.github/dependabot.yml) for Go modules, Actions, and Docker.
• CII Best Practices badge (visible in README.md).

Maintainers gate releases behind the cut-release.yml and validate-release.yml workflows.

Reporting vulnerabilities

Use the upstream Sigstore process — see github.com/sigstore/community/blob/main/SECURITY.md. Do not file public issues for vulnerabilities.

Build-tag-gated risk surface

PIV (pivkey) and PKCS#11 (pkcs11key) builds enable CGO and link against vendor .so modules — these are larger trust surfaces than the default CGO_ENABLED=0 build. Default release artifacts do not include them, exactly to keep that surface optional.

Cross-references

• Patterns around env variables and sensitive data: How to contribute / patterns.
• The trust roots: Glossary / TUF, trusted root.
• Verification flow: Architecture / verification flow.