Csrf Cross Site Request Forgery

# SKILL: CSRF — Cross-Site Request Forgery — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: Expert CSRF techniques. Covers modern bypass vectors (SameSite gaps, custom header flaws, tokenless bypass patterns), JSON CSRF, multipart CSRF, chaining with XSS. Base models often present only basic CSRF without covering SameSite edge cases and common broken token implementations.

## 0. RELATED ROUTING

Also load:

- [cors cross origin misconfiguration](../cors-cross-origin-misconfiguration/SKILL.md) when JSON endpoints become readable cross-origin
- [oauth oidc misconfiguration](../oauth-oidc-misconfiguration/SKILL.md) when login, account linking, or callback binding relies on OAuth state

---

## 1. CORE CONCEPT

CSRF exploits a victim's active session to perform state-changing requests **from the attacker's origin**.

**Required conditions**:
1. Victim is authenticated (active session cookie)
2. Server identifies session via cookie only (no secondary check)
3. Attacker can predict/construct the valid request
4. Cookie is sent cross-origin (SameSite=None or legacy behavior)

---

## 2. FINDING CSRF TARGETS

**High-value state-changing endpoints**:
```
- Password change         ← account takeover
- Email change            ← account takeover
- Add admin / change role ← privilege escalation
- Bank/payment transfer   ← financial impact
- OAuth app authorization ← hijack oauth flow
- Account deletion
- Two-factor auth disable  
- SSH key / API key addition
- Webhook configuration
- Profile/contact info update
```

---

## 3. TOKEN BYPASS TECHNIQUES

### No Token Present
Simplest case — form simply lacks CSRF token. Check if POST /change-email has any token. If not → trivially exploitable.

### Token Not Validated (most common finding!)
Token exists in request but is never verified server-side:
```
Remove the _csrf_token parameter entirely → does request still succeed?
→ YES → trivial bypass
```

### Token Tied to Session but Not to User
```
Step 1: Log in as UserA → obtain valid CSRF token
Step 2: Log in as UserB in other browser → obtain UserB CSRF token  
Step 3: Use UserB's CSRF token in UserA's session (attacker controls UserB)
→ If server validates token exists but doesn't check if it belongs to the session → bypass
```

### Token in Cookie Only
When server sets CSRF token as cookie and expects it back in a header/form:
```
Set-Cookie: csrf=ATTACKER_CONTROLLED
→ If cookie can be set by subdomain (cookie tossing): set cookie to known value
→ Submit form with known token in header + known token in cookie = bypass
```

### Static or Predictable Token
```
→ Same token across all users/sessions
→ Token = base64(username) or md5(session_id) → reversible
→ Token = timestamp → predictable
```

### Double Submit Cookie Pattern (broken if subdomain trusted)
```
If attacker can write cookies for .target.com from subdomain XSS or cookie tossing:
→ Set csrf_cookie=CONTROLLED on .target.com
→ Submit request with X-CSRF-Token: CONTROLLED
→ Server checks header == cookie → match → bypass
```

---

## 4. SAMESITE BYPASS SCENARIOS

**SameSite=Lax** (modern browser default): cookies sent for top-level GET navigation, NOT for cross-site iframe/form POST.

**Bypass SameSite=Lax via GET method**:
```html
<!-- If server accepts GET for state-changing endpoint: -->
<img src="https://target.com/account/delete?confirm=yes">
<script>document.location = 'https://target.com/transfer?to=attacker&amount=1000';</script>
```

**Bypass via subdomain XSS (SameSite Lax/Strict)**:
```javascript
// XSS on sub.target.com → same-site origin → SameSite cookies sent!
// Use XSS as staging point for CSRF
window.location = 'https://target.com/account/modify?evil=true';
```

**SameSite=None** (legacy or explicit): cookies sent everywhere → classic CSRF ap