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securitycritical PriorityID: sigstore-keyless-oidc-attestation-verification-standard

Sigstore Keyless OIDC Attestation & Ephemeral Certificate Verification Guardrail

Enforces strict admission-time cryptographic verification of Sigstore bundles for container images and build artifacts. Mandates exact OIDC Subject Alternative Name (SAN) regex matching, Fulcio OIDC issuer validation, and Rekor transparency log inclusion proof verification to guarantee signatures occurred during the ephemeral certificate validity window.

Prohibited Anti-Pattern: Accepting Any Fulcio-Signed Artifact Without Strict OIDC Identity and Rekor Inclusion Verification

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---
description: Sigstore Keyless OIDC Attestation & Ephemeral Certificate Verification Guardrail - Enforces strict admission-time cryptographic verification of Sigstore bundles for container images and build artifacts. Mandates exact OIDC Subject Alternative Name (SAN) regex matching, Fulcio OIDC issuer validation, and Rekor transparency log inclusion proof verification to guarantee signatures occurred during the ephemeral certificate validity window.
globs: src/admission/**/*.{ts,js}, src/security/**/*.{ts,js}, deploy/**/*.yaml, policies/**/*.{yaml,rego}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/sigstore-keyless-oidc-attestation-verification-standard"
---

# Sigstore Keyless OIDC Attestation & Ephemeral Certificate Verification Guardrail

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Accepting Any Fulcio-Signed Artifact Without Strict OIDC Identity and Rekor Inclusion Verification

## System Constraints
You are a Principal Software Architect enforcing Sigstore keyless attestation verification standards. You must ensure that deploy-time admission checks validate cryptographically signed provenance against ephemeral Fulcio X.509 certificates and Rekor transparency logs. Reject any code that accepts arbitrary Fulcio-issued certificates without explicit OIDC issuer and subject matching, or that skips Rekor log inclusion proofs.

## Strict Guidelines
- Engineers MUST ALWAYS validate the certificate Subject Alternative Name (SAN) and Issuer against an explicit, immutable allowlist of trusted OIDC identities (e.g., specific GitHub Actions workflows or GitLab CI runners).
- Engineers MUST NOT rely solely on public key signature validity; ephemeral Fulcio certificates expire within minutes and MUST be corroborated by a verified Rekor transparency log entry timestamp.
- Admission verification code MUST verify the Rekor inclusion proof (Signed Entry Timestamp / SET) cryptographically against the Rekor public key before trusting the signature time window.
- Services MUST verify in-toto SLSA attestation predicates against pre-configured policy invariants (e.g., repository source, commit SHA, build runner identity) before allowing deployment.
- Components NEVER fall back to unauthenticated or key-based signatures when keyless OIDC verification fails; verification failures must hard-reject the admission request.

## Bad vs Good Implementations
### Sigstore Bundle Verification in Admission Webhook

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: The bad example verifies that the signature was generated by *a* Fulcio certificate, but omits the OIDC identity and issuer constraints. An attacker using their own public GitHub Actions workflow can sign a malicious image with Fulcio and successfully bypass this check.
```typescript
import { verify } from '@sigstore/verify';

export async function validateImageAdmission(bundle: any, artifactDigest: Buffer): Promise<boolean> {
  // ANTI-PATTERN: Verifying bundle without checking specific OIDC identity
  // Anyone who obtained a valid Fulcio cert (any public GitHub repo) can pass this check!
  try {
    await verify(bundle, artifactDigest, {
      // Omitting certificateIssuer and certificateIdentity
      // Skipping Rekor tlog online or offline SET validation
    });
    return true;
  } catch (err) {
    return false;
  }
}
```

#### Production Standard (Correct)
> **Standard**: The good example binds verification to an explicit OIDC Issuer (GitHub Actions) and an immutable repository/workflow URI regex. It enforces Rekor transparency log inclusion (SET) and Certificate Transparency checks to ensure the ephemeral certificate was valid when signed.
```typescript
import { verify, CertificateIdentity } from '@sigstore/verify';

const TRUSTED_ISSUER = 'https://token.actions.githubusercontent.com';
const TRUSTED_SAN_REGEX = /^https:\/\/github\.com\/my-org\/my-secure-repo\/\.github\/workflows\/release\.yml@refs\/heads\/main$/;

export async function validateImageAdmission(bundle: any, artifactDigest: Buffer): Promise<boolean> {
  // PRODUCTION INVARIANT: Enforce strict OIDC issuer, subject identity regex, and Rekor inclusion verification
  const expectedIdentities: CertificateIdentity[] = [{
    issuer: TRUSTED_ISSUER,
    subjectAlternativeName: {
      type: 'uri',
      value: TRUSTED_SAN_REGEX
    }
  }];

  try {
    await verify(bundle, artifactDigest, {
      certificateIdentities: expectedIdentities,
      tlogOptions: {
        threshold: 1, // Require verified inclusion in at least 1 Rekor transparency log
        performOnlineVerification: true
      },
      ctlogOptions: {
        threshold: 1 // Enforce Certificate Transparency log presence
      }
    });
    return true;
  } catch (error) {
    // Hard reject on any attestation or signature failure
    return false;
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Sigstore Policy Verifier MCP
Validates container provenance attestations and checks Rekor log proofs against cluster admission constraints.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Cosign CLI
Standard container signing, verification, and storage tool for OIDC keyless operations with Fulcio and Rekor.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: Sigstore Keyless OIDC Attestation & Ephemeral Certificate Verification Guardrail

Sigstore Keyless OIDC Attestation & Ephemeral Certificate Verification Guardrail defines an industrial-grade behavioral standard for security engineering. Enforces strict admission-time cryptographic verification of Sigstore bundles for container images and build artifacts. Mandates exact OIDC Subject Alternative Name (SAN) regex matching, Fulcio OIDC issuer validation, and Rekor transparency log inclusion proof verification to guarantee signatures occurred during the ephemeral certificate validity window. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Accepting Any Fulcio-Signed Artifact Without Strict OIDC Identity and Rekor Inclusion Verification Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftAccepting Any Fulcio-Signed Artifact Without Strict OIDC Identity and Rekor Inclusion VerificationCritical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS validate the certificate Subject Alternative Name (SAN) and Issuer against an explicit, immutable allowlist of trusted OIDC identities (e.g., specific GitHub Actions workflows or GitLab CI runners).

Engineers MUST ALWAYS validate the certificate Subject Alternative Name (SAN) and Issuer against an explicit, immutable allowlist of trusted OIDC identities (e.g., specific GitHub Actions workflows or GitLab CI runners).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST NOT rely solely on public key signature validity

Engineers MUST NOT rely solely on public key signature validity; ephemeral Fulcio certificates expire within minutes and MUST be corroborated by a verified Rekor transparency log entry timestamp.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Admission verification code MUST verify the Rekor inclusion proof (Signed Entry Timestamp / SET) cryptographically against the Rekor public key before trusting the signature time window.

Admission verification code MUST verify the Rekor inclusion proof (Signed Entry Timestamp / SET) cryptographically against the Rekor public key before trusting the signature time window.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Services MUST verify in-toto SLSA attestation predicates against pre-configured policy invariants (e.g., repository source, commit SHA, build runner identity) before allowing deployment.

Services MUST verify in-toto SLSA attestation predicates against pre-configured policy invariants (e.g., repository source, commit SHA, build runner identity) before allowing deployment.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Components NEVER fall back to unauthenticated or key-based signatures when keyless OIDC verification fails

Components NEVER fall back to unauthenticated or key-based signatures when keyless OIDC verification fails; verification failures must hard-reject the admission request.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is Sigstore Keyless OIDC Attestation & Ephemeral Certificate Verification Guardrail critical for AI coding assistants?

Enforces strict admission-time cryptographic verification of Sigstore bundles for container images and build artifacts. Mandates exact OIDC Subject Alternative Name (SAN) regex matching, Fulcio OIDC issuer validation, and Rekor transparency log inclusion proof verification to guarantee signatures occurred during the ephemeral certificate validity window. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Accepting Any Fulcio-Signed Artifact Without Strict OIDC Identity and Rekor Inclusion Verification) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

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