Exploiting Jwt Algorithm Confusion Attack

---
name: exploiting-jwt-algorithm-confusion-attack
description: Use when exploits JWT algorithm confusion vulnerabilities where the server's
  token verification library accepts the algorithm specified in the JWT header rather
  than enforcing a fixed algorithm. The tester manipulates the alg header to switch
  from RS256 to HS256 (using the RSA public key as the HMAC secret), sets alg to none
  to bypass signature verification, or exploits kid/jku/x5u header injection to supply
  attacker-controlled keys. Use when working with exploiting jwt algorithm confusion
  attack.
domain: cybersecurity
tags:
- api-security
- jwt
- algorithm-confusion
- token-forgery
- cryptographic-attack
subdomain: api-security
version: 1.0.0
author: oyi77
license: Apache-2.0
nist_csf:
- PR.PS-01
- ID.RA-01
- PR.DS-10
- DE.CM-01
category: cybersecurity
---

# Exploiting Jwt Algorithm Confusion Attack

## Overview

Cybersecurity skill for exploiting jwt algorithm confusion attack. Follows industry best practices and security standards.

## When to Use
**Trigger phrases:**
- "exploiting jwt algorithm confusion attack"
- "Exploits JWT algorithm confusion vulnerabilities where the server's token verifi"


- Testing APIs that use RS256 (asymmetric) JWT tokens for authentication to check for algorithm downgrade to HS256
- Assessing JWT implementations for alg:none bypass where the server skips signature verification
- Evaluating JWT libraries for key confusion vulnerabilities where the public key is used as HMAC secret
- Testing kid (Key ID), jku (JWK Set URL), and x5u (X.509 URL) header parameters for injection
- Validating that the API server enforces a specific algorithm and does not trust the JWT header

**Do not use** without written authorization. JWT exploitation can lead to authentication bypass and account takeover.


## When NOT to Use

- When you lack proper authorization for testing
- For production systems without change management
- When the task requires legal or compliance expertise beyond technical scope


## Prerequisites

- Written authorization specifying the target API and JWT-based authentication in scope
- A valid JWT token from the target API (obtained through legitimate authentication)
- The server's RSA public key (obtainable from JWKS endpoint, TLS certificate, or public key endpoint)
- Python 3.10+ with `PyJWT`, `cryptography`, and `requests` libraries
- jwt_tool for automated JWT attack testing
- Burp Suite with JWT Editor extension


> **Legal Notice:** This skill is for authorized security testing and educational purposes only. Unauthorized use against systems you do not own or have written permission to test is illegal and may violate computer fraud laws.

## Workflow

```python
# Example: IOC detection
import re

IOC_PATTERNS = {
    "ip": r"\b(?:\d{1,3}\.){3}\d{1,3}\b",
    "domain": r"\b[a-z0-9-]+\.[a-z]{2,}\b",
    "hash_md5": r"\b[a-f0-9]{32}\b",
    "hash_sha256": r"\b[a-f0-9]{64}\b",
}

def extract_iocs(text: str) -> dict:
    return {k: re.findall(v, text) for k, v in IOC_PATTERNS.items()}
```

1. **Reconnaissance** — Gather information about the target related to jwt algorithm confusion attack. Identify attack surface.
2. **Vulnerability Identification** — Enumerate potential jwt algorithm confusion attack weaknesses using automated and manual techniques.
3. **Exploit Development/Selection** — Choose or develop exploits targeting identified jwt algorithm confusion attack vulnerabilities.
4. **Execution** — Execute the jwt algorithm confusion attack test in a controlled manner with proper authorization.
5. **Post-Exploitation** — Document the impact and extent of successful exploitation.
6. **Reporting** — Write detailed findings with reproduction steps, impact assessment, and remediation guidance.

## Tools

- **Vulnerability Scanner** — Automated weakness identification
- **Exploitation Framework** — Controlled exploitation testing
- **Reporting Tool** — Findings documentation and tracking


## Process

1. **Reconnaissance** — Gather target information, identify attack surface, enumerate services
1. **Analysis/Exploitation** — Execute the technique, analyze results, document findings
1. **Reporting** — Document IOCs, write findings, provide remediation recommendations

## Verification

- [ ] All jwt algorithm confusion attack procedures executed completely and documented
- [ ] Findings validated against multiple data sources
- [ ] False positives identified and filtered
- [ ] Results documented with evidence and timestamps
- [ ] Recommendations provided with risk-based prioritization

## Anti-Rationalization Table

| Rationalization | Reality |
|---|---|
| "We are too small to be targeted" | Automated attacks target everyone. Size does not matter. |
| "Security slows us down" | A breach slows you down 100x more. Build security in from the start. |
| "We will fix it after launch" | Vulnerabilities in production are exploited within hours. Fix before deploy. |