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)

# Concept: JWT Algorithm Confusion ```

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

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

Phase 3: Modifying the Payload (Privilege Escalation)

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

Phase 4: Signing the Forged JWT

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

Decision Point 🔀

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 — Exploit chaining methodology and high-payout chain patterns
• _shared/references/elite-report-writing.md — HackerOne-optimized report writing, CWE quick reference
• _shared/references/real-world-bounties.md — Verified disclosed bounties by vulnerability class

References

• PortSwigger: JWT algorithm confusion
• Auth0: Critical vulnerabilities in JSON Web Token libraries