competition-jwt-claim-confusion

Imported from zhaoxuya520/reverse-skill (CTF-Sandbox-Orchestrator/competition-jwt-claim-confusion/SKILL.md). Install upstream with npx skills add zhaoxuya520/reverse-skill --skill competition-jwt-claim-confusion. Copyright stays with the author.

Competition JWT Claim Confusion

Use this skill only as a downstream specialization after $ctf-sandbox-orchestrator is already active and has established sandbox assumptions, node ownership, and evidence priorities. If that has not happened yet, return to $ctf-sandbox-orchestrator first.

Use this skill when the decisive bug is not just "there is a JWT," but how headers, claims, and key selection turn into accepted identity.

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Quick Start

1. Split the token path into parse, key lookup, signature or decryption, claim validation, and final acceptance.
2. Record header fields, claims, key source, issuer, audience, and role mapping before mutating anything.
3. Separate possession of a token from the exact service that accepts it.
4. Keep parser behavior, trust policy, and resulting app session or privilege in one chain.
5. Reproduce the smallest token-to-acceptance flow that proves the decisive confusion.

Workflow

1. Map Header And Key Selection

• Record header fields such as alg, kid, typ, cty, jku, or embedded key material when present.
• Note where keys come from: static config, JWKS, local file, cache, or dynamic lookup.
• Keep token parser, key selection path, and validation mode tied together.

2. Prove Claim-To-Privilege Acceptance

• Show how subject, audience, issuer, tenant, scope, role, or custom claims become app session, route access, or backend privilege.
• Record expiration, not-before, clock skew, issuer matching, audience matching, and claim normalization behavior.
• Distinguish token parse success from actual authorization success.

3. Reduce To The Decisive JWT Path

• Compress the result to the smallest sequence: token supplied -> parser or key path taken -> claim accepted -> resulting capability.
• Keep one canonical accepted token path and one mutated token path if confusion or bypass depends on a delta.
• If the task broadens into a larger OAuth redirect chain, hand back to the tighter OAuth skill.

Read This Reference

• Load references/jwt-claim-confusion.md for the header checklist, claim checklist, and evidence packaging.

What To Preserve

• Raw headers, claims, key source, JWKS or local key path, and the accepting service
• The exact validation or normalization step that turns the token into accepted identity
• One minimal replayable token-to-acceptance sequence