Phy Jwt Auth AuditCAUTION

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The instruction file, verbatim from the audited commit — this is the text the model reads, and the surface the audit's instruction layer examines. Quoted here so you can judge it without cloning anything.

---
name: phy-jwt-auth-audit
description: JWT and OAuth/OIDC security auditor. Decodes any JWT token (without verification) to inspect alg/exp/iss/aud/scope claims, detects the "alg:none" bypass vulnerability, expired or no-expiry tokens, overly broad OAuth scopes, JWT stored in localStorage (XSS theft risk), JWT in URL parameters (log leakage), missing issuer/audience validation in source code, hardcoded tokens in .env files, and weak HMAC secrets. Also scans source files for insecure token handling patterns: Bearer token logged, token compared with ==, auth bypass via role:admin in payload. Generates a severity-ranked report with exact code locations and fixes. Zero external API — pure local analysis. Triggers on "JWT audit", "token security", "auth security", "alg none", "OAuth scopes", "bearer token", "token expiry", "/jwt-audit".
license: Apache-2.0
metadata:
  author: PHY041
  version: "1.0.0"
  tags:
    - security
    - jwt
    - oauth
    - authentication
    - token-security
    - static-analysis
    - developer-tools
    - owasp
    - api-security
    - compliance
---

# JWT & Auth Auditor

A developer pastes a JWT into a debug log. The logger ships it to Datadog. An attacker finds it in the logs 6 months later. The token never expires.

This skill decodes JWTs without verifying them (which is the point — you need to inspect them even when you don't have the secret), checks their claims against security best practices, scans your codebase for insecure token handling, and finds the OAuth scopes that give more access than necessary.

**Zero external API — all analysis runs locally. Works with any JWT/OAuth provider.**

---

## Trigger Phrases

- "JWT audit", "token security check"
- "auth security", "authentication review"
- "alg:none vulnerability", "JWT algorithm"
- "OAuth scopes", "bearer token check"
- "token expiry", "no exp claim"
- "JWT in localStorage", "token in URL"
- "/jwt-audit"

---

## How to Provide Input

```bash
# Option 1: Decode and audit a specific JWT token
/jwt-audit eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...

# Option 2: Audit source code for insecure token handling
/jwt-audit --scan src/

# Option 3: Audit .env files for hardcoded tokens
/jwt-audit --env-scan

# Option 4: Audit a specific auth file
/jwt-audit src/middleware/auth.ts

# Option 5: Full audit (token decode + code scan + env scan)
/jwt-audit eyJhbG... --scan . --env-scan

# Option 6: Check OAuth scopes in API calls
/jwt-audit --check-scopes

# Option 7: CI mode (exit 1 on critical findings)
/jwt-audit --scan src/ --ci --max-critical 0
```

---

## Step 1: Decode and Analyze JWT Claims

```python
import base64
import json
import time
from dataclasses import dataclass, field
from typing import Any, Optional


@dataclass
class JwtFinding:
    severity: str       # CRITICAL / HIGH / MEDIUM / LOW / INFO
    claim: str          # which claim is affected
    issue: str
    detail: str
    fix: str


def decode_jwt_unsafe(token: str) -> tuple[dict, dict, str]:
    """
    Decode a JWT token WITHOUT verifying the signature.
    Returns (header, payload, signature_b64).
    Safe for inspection purposes — never use for auth decisions.
    """
    parts = token.strip().split('.')
    if len(parts) != 3:
        raise ValueError(f"Invalid JWT format: expected 3 parts, got {len(parts)}")

    def b64_decode(s: str) -> dict:
        # JWT uses URL-safe base64 without padding
        padding = 4 - len(s) % 4
        if padding != 4:
            s += '=' * padding
        raw = base64.urlsafe_b64decode(s)
        return json.loads(raw)

    header = b64_decode(parts[0])
    payload = b64_decode(parts[1])
    signature = parts[2]

    return header, payload, signature


def analyze_jwt_claims(token: str) -> list[JwtFinding]:
    """Full security analysis of a JWT token's claims and header."""
    findings = []

    try:
        header, payload, sig = decode_jwt_unsafe(token)
    except Exception as e:
        return [JwtFinding(
            severity='CRITICAL', claim='format',
            issue='Invalid JWT format', detail=str(e),
            fix='Ensure token is a valid JWT (3 base64url parts separated by dots)'
        )]

    now = int(time.time())

    # ── Algorithm checks ─────────────────────────────────────────────────────

    alg = header.get('alg', '')

    if alg.lower() == 'none':
        findings.append(JwtFinding(
            severity='CRITICAL', claim='alg',
            issue='Algorithm "none" — signature verification disabled',
            detail=(
                'alg:none means the JWT has no signature. Any payload can be crafted '
                'and will be accepted by a vulnerable server. This is CVE-2015-9235.'
            ),
            fix=(
                'Server must reject tokens with alg:none. '
                'In jsonwebtoken: jwt.verify(token, secret, { algorithms: ["HS256"] })'
            ),
        ))

    elif alg.startswith('HS') and len(sig) < 32:
        findings.append(JwtFinding(
            severity='HIGH', claim='alg',
            issue=f'Algorithm {alg} with suspiciously short signature',
            detail='Short signature may indicate a weak or guessable HMAC secret.',
            fix='Use a minimum 256-bit (32-byte) random secret for HMAC-SHA256',
        ))

    elif alg == 'RS256' or alg == 'ES256':
        findings.append(JwtFinding(
            severity='INFO', claim='alg',
            issue=f'Algorithm: {alg} (asymmetric — good)',
            detail='RSA/ECDSA signature — cannot be forged without the private key.',
            fix='Ensure public key is loaded from a trusted source, not from the JWT header itself.',
        ))

    # Algorithm confusion: HS256 when server expects RS256
    if alg == 'HS256':
        findings.append(JwtFinding(
            severity='MEDIUM', claim='alg',
            issue='HS256 — verify server rejects RS256→HS256 algorithm confusion',
            detail=(
                'If the server also supports RS256, an attacker may forge 

CAUTIONgrade B · trust 89/100 Install with care. The audit found things worth knowing before you trust its output.

What the source does

Filesystem

none-observed

Network

none-observed

Shell

none-observed

Dependencies

pinned

Secrets in source

none-found

Findings (2)

'fix': 'Load from environment: process.env.JWT_SECRET or os.getenv("JWT_SECRET")',

<key>k5Ey9KFlkqpj+SDkUw+5ED9lTA3En/qUi0zdrydUCH3kMWTE3Eh65NXnFCaxlY2omY2JHnlEoK7Li7oOEvM7eG5VPdcO/sFlMfoCRdnLYdepJ+uLzYwOWR8W4yQVve/clxVFTVRL4DFleKInGdpAxIbHZT2yi4ADAMENls1N1XSLojRuqXePXDeAT/4Mv4TTx0s

Gates applied: no_behavioural_pass.