Imported from yoanbernabeu/supabase-pentest-skills (skills/extraction/supabase-extract-jwt/SKILL.md) via skills.sh. Install upstream with npx skills add yoanbernabeu/supabase-pentest-skills --skill supabase-extract-jwt. Copyright stays with the author.

Supabase JWT Extraction

🔴 CRITICAL: PROGRESSIVE FILE UPDATES REQUIRED

You MUST write to context files AS YOU GO, not just at the end.

Write to .sb-pentest-context.json IMMEDIATELY after each discovery

Log to .sb-pentest-audit.log BEFORE and AFTER each action

DO NOT wait until the skill completes to update files

If the skill crashes or is interrupted, all prior findings must already be saved

This is not optional. Failure to write progressively is a critical error.

This skill extracts and analyzes JSON Web Tokens (JWTs) related to Supabase from client-side code.

When to Use This Skill

To find all JWT tokens exposed in client code
To analyze token claims and expiration
To detect hardcoded user tokens (security issue)
To understand the authentication flow

Prerequisites

Target application accessible
Supabase detection completed (auto-invokes if needed)

Types of JWTs in Supabase

Type	Purpose	Client Exposure
Anon Key	API authentication	✅ Expected
Service Role Key	Admin access	❌ Never
Access Token	User session	⚠️ Dynamic only
Refresh Token	Token renewal	⚠️ Dynamic only

Detection Patterns

1. API Keys (Static)

// Supabase API keys are JWTs
const SUPABASE_KEY = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...'

2. Hardcoded User Tokens (Problem)

// ❌ Should never be hardcoded
const userToken = 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwiZW1haWwiOiJ1c2VyQGV4YW1wbGUuY29tIn0...'

3. Storage Key Patterns

// Code referencing where JWTs are stored
localStorage.getItem('supabase.auth.token')
localStorage.getItem('sb-abc123-auth-token')
sessionStorage.getItem('supabase_session')

Usage

Basic Extraction

Extract JWTs from https://myapp.example.com

With Claim Analysis

Extract and analyze all JWTs from https://myapp.example.com

Output Format

═══════════════════════════════════════════════════════════
 JWT EXTRACTION RESULTS
═══════════════════════════════════════════════════════════

 Found: 3 JWTs

 ─────────────────────────────────────────────────────────
 JWT #1: Supabase Anon Key
 ─────────────────────────────────────────────────────────
 Type: API Key (anon)
 Status: ✅ Expected in client code

 Header:
 ├── alg: HS256
 └── typ: JWT

 Payload:
 ├── iss: supabase
 ├── ref: abc123def
 ├── role: anon
 ├── iat: 2021-12-20T00:00:00Z
 └── exp: 2031-12-20T00:00:00Z

 Location: /static/js/main.js:1247

 ─────────────────────────────────────────────────────────
 JWT #2: Hardcoded User Token ⚠️
 ─────────────────────────────────────────────────────────
 Type: User Access Token
 Status: ⚠️ P1 - Should not be hardcoded

 Header:
 ├── alg: HS256
 └── typ: JWT

 Payload:
 ├── sub: 12345678-1234-1234-1234-123456789012
 ├── email: developer@company.com
 ├── role: authenticated
 ├── iat: 2025-01-15T10:00:00Z
 └── exp: 2025-01-15T11:00:00Z (EXPIRED)

 Location: /static/js/debug.js:45

 Risk: This token may belong to a real user account.
       Even if expired, it reveals user information.

 ─────────────────────────────────────────────────────────
 JWT #3: Storage Reference
 ─────────────────────────────────────────────────────────
 Type: Storage Key Pattern
 Status: ℹ️ Informational

 Pattern: localStorage.getItem('sb-abc123def-auth-token')
 Location: /static/js/auth.js:89

 Note: This is the expected storage key for user sessions.
       Actual token value is set at runtime.

═══════════════════════════════════════════════════════════

JWT Claim Analysis

The skill identifies key claims:

Standard Claims

Claim	Description	Security Impact
`sub`	User ID	Identifies specific user
`email`	User email	PII exposure if hardcoded
`role`	Permission level	`service_role` is critical
`exp`	Expiration	Expired tokens less risky
`iat`	Issued at	Indicates when created

Supabase-Specific Claims

Claim	Description
`ref`	Project reference
`iss`	Should be "supabase"
`aal`	Authenticator assurance level
`amr`	Authentication methods used

Security Findings

P0 - Critical

🔴 Service role key exposed (role: service_role)
   → Immediate key rotation required

P1 - High

🟠 User token hardcoded with PII (email, sub visible)
   → Remove from code, may need to notify user

P2 - Medium

🟡 Expired test token in code
   → Clean up, potential information disclosure

Context Output

Saved to .sb-pentest-context.json:

{
  "jwts": {
    "found": 3,
    "api_keys": [
      {
        "type": "anon",
        "project_ref": "abc123def",
        "location": "/static/js/main.js:1247"
      }
    ],
    "user_tokens": [
      {
        "type": "access_token",
        "hardcoded": true,
        "severity": "P1",
        "claims": {
          "sub": "12345678-1234-1234-1234-123456789012",
          "email": "developer@company.com",
          "expired": true
        },
        "location": "/static/js/debug.js:45"
      }
    ],
    "storage_patterns": [
      {
        "pattern": "sb-abc123def-auth-token",
        "storage": "localStorage",
        "location": "/static/js/auth.js:89"
      }
    ]
  }
}

Common Issues

❌ Problem: JWT appears truncated ✅ Solution: May span multiple lines. The skill attempts to reassemble.

❌ Problem: JWT won't decode ✅ Solution: May be encrypted (JWE) or custom format. Noted as undecodable.

❌ Problem: Many false positives ✅ Solution: Base64 strings that look like JWTs. Skill validates structure.

Remediation for Hardcoded Tokens

Before (Wrong)

// ❌ Never hardcode user tokens
const adminToken = 'eyJhbGciOiJIUzI1NiI...'
fetch('/api/admin', {
  headers: { Authorization: `Bearer ${adminToken}` }
})

After (Correct)

// ✅ Get token from Supabase session
const { data: { session } } = await supabase.auth.getSession()
fetch('/api/admin', {
  headers: { Authorization: `Bearer ${session.access_token}` }
})

MANDATORY: Progressive Context File Updates

⚠️ This skill MUST update tracking files PROGRESSIVELY during execution, NOT just at the end.

Critical Rule: Write As You Go

DO NOT batch all writes at the end. Instead:

Before starting any action → Log the action to .sb-pentest-audit.log
After each discovery → Immediately update .sb-pentest-context.json
After each significant step → Log completion to .sb-pentest-audit.log

This ensures that if the skill is interrupted, crashes, or times out, all findings up to that point are preserved.

Required Actions (Progressive)

Update .sb-pentest-context.json with extracted data:

{
  "jwts": {
    "found": 3,
    "api_keys": [ ... ],
    "user_tokens": [ ... ],
    "storage_patterns": [ ... ]
  }
}

Log to .sb-pentest-audit.log:

[TIMESTAMP] [supabase-extract-jwt] [START] Beginning JWT extraction
[TIMESTAMP] [supabase-extract-jwt] [SUCCESS] Found 3 JWTs
[TIMESTAMP] [supabase-extract-jwt] [CONTEXT_UPDATED] .sb-pentest-context.json updated

If files don't exist, create them before writing.

FAILURE TO UPDATE CONTEXT FILES IS NOT ACCEPTABLE.

MANDATORY: Evidence Collection

📁 Evidence Directory: .sb-pentest-evidence/02-extraction/

Evidence Files to Create

File	Content
`extracted-jwts.json`	All JWTs found with analysis

Evidence Format

{
  "evidence_id": "EXT-JWT-001",
  "timestamp": "2025-01-31T10:08:00Z",
  "category": "extraction",
  "type": "jwt_extraction",

  "jwts_found": [
    {
      "type": "anon_key",
      "severity": "info",
      "location": "/static/js/main.js:1247",
      "decoded_payload": {
        "iss": "supabase",
        "ref": "abc123def",
        "role": "anon"
      }
    },
    {
      "type": "hardcoded_user_token",
      "severity": "P1",
      "location": "/static/js/debug.js:45",
      "decoded_payload": {
        "sub": "[REDACTED]",
        "email": "[REDACTED]@example.com",
        "role": "authenticated",
        "exp": "2025-01-15T11:00:00Z"
      },
      "expired": true,
      "issue": "Hardcoded user token with PII"
    }
  ],

  "storage_patterns_found": [
    {
      "pattern": "localStorage.getItem('sb-abc123def-auth-token')",
      "location": "/static/js/auth.js:89"
    }
  ]
}

Related Skills

supabase-extract-anon-key — Specifically extracts the anon key
supabase-extract-service-key — Checks for service key (critical)
supabase-audit-auth-config — Analyzes auth configuration

supabase-extract-jwt

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