Getting it into your agent
It runs from inside its repository, so the clone comes first — what it calls does not travel with the file alone.
git clone --depth 1 https://github.com/Miosa-osa/OSAnpx agentmods add skills/miosa-osa/osa/offensive-jwtWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/miosa-osa/osa/offensive-jwt)<a href="https://agentmods.dev/skills/miosa-osa/osa/offensive-jwt"><img src="https://agentmods.dev/badge/skills/miosa-osa/osa/offensive-jwt/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/miosa-osa/osa/offensive-jwt"><img src="https://agentmods.dev/badge/skills/miosa-osa/osa/offensive-jwt.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00097 | $0.03038 |
| Opus 5.5 | $0.00039 | $0.01215 |
| Sonnet 5.5 | $0.00019 | $0.00608 |
| Haiku 4.5 | $0.00010 | $0.00304 |
Grade A, and why
offensive-jwt scanned grade A with 1 finding against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Makes network callslowCapability
Not a fault in itself. Listed so you know the mod talks to something, and to what.
curl -H "Authorization: Bearer <api_key>" https://api.target/resource This is a copy
92% identical to offensive-jwt — 40 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 315 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Overview
Comprehensive JWT attack checklist for offensive security engagements. Follow steps in order; apply each technique to the current target context and track which items have been completed.
Quick Reference: Misconfigurations to Check
- Algorithm set to
none— signature verification bypassed entirely - Algorithm switching between
RSAandHMAC(confusion attack) - Weak or guessable HMAC secret (brute-forceable)
kid,jku,jwk,x5uheader parameters accepted without validation- Expired or tampered tokens accepted by server
- Sensitive data stored unencrypted in payload
Useful tool: JWT Tool
Mechanisms
JWTs (RFC 7519) consist of three Base64URL-encoded parts: header.payload.signature.
Signing algorithms:
| Algorithm | Type | Notes |
|---|---|---|
| HS256/384/512 | Symmetric HMAC | Shared secret; confusion target |
| RS256/384/512 | Asymmetric RSA | Public key can be misused as HMAC secret |
| ES256/384/512 | Asymmetric ECDSA | |
| PS256/384/512 | RSASSA-PSS | |
| EdDSA (Ed25519/Ed448) | Asymmetric | |
| none | Unsigned | Critically insecure |
Additional pitfalls:
- JWS/JWE confusion: server accepts encrypted token (JWE) where signed (JWS) is expected, or fails open on unexpected
typ/cty - JWKS retrieval: SSRF via
jku/x5u, insecure TLS, poisoned key caching,kidcollisions - Token binding (DPoP, mTLS): incorrectly implemented allows replay from other clients
Hunt: Identifying JWT Usage
- Check
Authorization: Bearer <token>headers in all requests - Look for cookies containing JWT structures (
eyJ...) - Examine browser local/session storage
- Decode the token at jwt.io or via BurpSuite JWT extension — inspect claims and header parameters
- Note any
kid,jku,jwk,x5ufields in the header — these are attack surfaces
Vulnerability Map
JWT Vulnerabilities
├── Algorithm Bypass
│ ├── alg:none attack
│ └── RS256→HS256 confusion (public key as HMAC secret)
├── Weak Secret Key → Brute force
├── kid Parameter Injection
│ ├── SQL injection via kid
│ └── Path traversal via kid
├── Header Injection
│ ├── jwk (inline fake key)
│ ├── jku/x5u (remote attacker-controlled JWKS)
│ └── JWKS cache poisoning
└── Missing / Broken Validation
├── No signature check
├── Expired tokens accepted
└── iss/aud/exp not validated
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago First seen · 315 lines · 97 tokens per session scan A b284c9550e89
offensive-jwt is a skill published in the GitHub repository Miosa-osa/OSA (78 stars, last pushed 11d ago), licensed Apache-2.0. It adds 97 tokens to every session and 3,038 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 1 finding (makes network calls). It is 92% identical to offensive-jwt, differing in 40 lines, and is treated as a copy.
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