Autonomous Testing Priority

Try workflow bypasses before brute force — they're faster and more likely to succeed.

Pattern 1 — Skip the MFA step entirely (most automatable):

1. Login with valid credentials → receive a "pre-MFA" session state
2. Without completing MFA, directly access a protected resource (/dashboard, /api/me, /account/profile)
3. If the response returns user data → MFA is enforced only in the UI, not server-side = Critical

Pattern 2 — OTP replay (reuse a consumed code):

1. Complete a valid MFA flow to get a working OTP
2. Log out, log in again with the same credentials
3. Submit the same OTP again
4. If accepted → OTP is not invalidated after use

Pattern 3 — Submit obviously wrong OTP, observe response: Try submitting 000000 or 123456. If the response is 200 or returns a session token, OTP validation is broken or client-side only.

Pattern 4 — Partial / incremental validation (prefix oracle): If a guessed full code is rejected, test whether the server validates the OTP prefix-by-prefix instead of all-or-nothing. Submit a short partial code and compare responses:

1. Submit a 1–3 digit value (e.g. otp=1, then otp=12, …) — for a POST verify endpoint the code goes in the request body, not the URL query string, or the server reads an empty value.
2. If a correct prefix gives a DIFFERENT response than a wrong one (a success/flag, a distinct message, or a different length/timing), the validator leaks correctness one chunk at a time.
3. Walk the code digit-by-digit: keep the prefix that "responds correct," append 0–9, repeat. This collapses 10^6 brute force to ~10×N guesses (≤60 for a 6-digit code) — very feasible in a bounded test. This is the go-to when there is no leaked code and no skip/replay path. Some apps award success on any correct prefix outright (so a single correct first digit can win — sweep otp=0,1,…,9 before giving up). CRITICAL — stay in ONE session: re-authenticating (POST /…/login again) regenerates the OTP, throwing away your prefix progress. Do the entire sweep against a single established MFA session; never re-login between guesses.

On full brute force: brute-forcing all 10^6 codes is infeasible in a bounded test — but the prefix oracle above (Pattern 4) usually makes it unnecessary. Only attempt full brute force with evidence of no rate limit AND a small key space.

Proof: A session token or protected resource data in the response without completing MFA confirms the bypass.

19. MFA / 2FA BYPASS

Growing bug class — 7 distinct patterns. Pays High/Critical when it enables ATO without prior session.

Pattern 1: No Rate Limit on OTP

# Test with ffuf — all 1M 6-digit codes
ffuf -u "https://target.com/api/verify-otp" \
  -X POST -H "Content-Type: application/json" \
  -H "Cookie: session=YOUR_SESSION" \
  -d '{"otp":"FUZZ"}' \
  -w <(seq -w 000000 999999) \
  -fc 400,429 -t 5
# -t 5 (slow down) — aggressive rates get 429 or ban

Pattern 2: OTP Not Invalidated After Use

1. Login → receive OTP "123456" → enter it → success
2. Logout → login again with same credentials
3. Try OTP "123456" again
4. If accepted → OTP never invalidated = ATO (attacker sniffs OTP once, reuses forever)

Pattern 3: Response Manipulation

1. Enter wrong OTP → capture response in Burp
2. Change {"success":false} → {"success":true} (or 401 → 200)
3. Forward → if app proceeds → client-side only MFA check

Pattern 4: Skip MFA Step (Workflow Bypass)

# After entering password, app sets a "pre-mfa" cookie → redirects to /mfa
# Test: skip /mfa entirely, access /dashboard directly with pre-mfa cookie
# If app grants access without MFA = auth flow bypass = Critical
curl -s -b "session=PRE_MFA_SESSION" https://target.com/dashboard

Pattern 5: Race on MFA Verification

import asyncio, aiohttp

async def verify(session, otp):
    async with session.post("https://target.com/api/mfa/verify",
                            json={"otp": otp}) as r:
        return r.status, await r.text()

async def race():
    cookies = {"session": "YOUR_SESSION"}
    async with aiohttp.ClientSession(cookies=cookies) as s:
        # Fire ~30 concurrent submissions of the SAME OTP to hit the TOCTOU
        # window before the server marks it used. Two requests are NOT enough —
        # they almost always resolve sequentially as "already-used" (false negative).
        # Best done as a single-packet / 20+ HTTP-2-stream attack (Turbo Intruder).
        results = await asyncio.gather(*[verify(s, "123456") for _ in range(30)])
        # Race confirmed if >1 success (or 1 success among many "already-used").
        for status, body in results:
            print(status, body)
asyncio.run(race())

Pattern 6: Backup Code Brute Force

Backup codes: typically 8 alphanumeric = 36^8 = ~2.8T (too large)
BUT: check if backup codes are only 6-8 digits = 1-10M range = feasible with no rate limit
Also test: can backup codes be reused after exhaustion? Some apps regenerate predictably.

Pattern 7: "Remember This Device" Trust Escalation

1. Complete MFA once on Device A (attacker's browser)
2. Capture the "remember device" cookie
3. Present that cookie from a new IP/browser
4. If MFA skipped = device trust not bound to IP/UA = ATO from any location

MFA Chain Escalation

Rate limit bypass + no lockout = ATO (Critical)
Response manipulation = client-side only check = Critical
Skip MFA step = auth flow bypass = Critical
OTP reuse = persistent session hijack = High

Related Skills & Chains

• hunt-ato — MFA bypass is a primitive; ATO is the destination. Chain primitive: cookie theft (via XSS or session-fixation) + password oracle (login response timing/length diff reveals valid passwords without lockout) + no MFA step-up on password-change endpoint = persistent ATO without ever facing the OTP challenge → password rotated, attacker locks victim out.
• hunt-race-condition — Pattern 5 (OTP race) lives in race-condition territory; load both skills together. Chain primitive: same 6-digit OTP submitted via 20 parallel HTTP/2 streams (single-packet Turbo Intruder attack) before the server marks it used → 1 success + 19 "already-used" → race window confirmed → attacker doesn't need to brute, just guesses once and parallelizes → ATO.
• hunt-auth-bypass — MFA-step-skip is auth-flow bypass at the workflow layer. Chain primitive: pre-MFA cookie issued after password step + direct navigation to /dashboard skipping /mfa route + server only middleware-gates /mfa not /dashboard = full post-auth access from password-only state → MFA never enforced because the route gate was misplaced.
• hunt-misc — Recovery-code dump via /api/me is a misc-class info disclosure that becomes Critical when chained. Chain primitive: /api/me returns full user object including backup_codes array (plaintext, never rotated) → attacker with any read-IDOR or XSS exfils backup codes → uses one backup code → MFA satisfied → ATO without OTP knowledge.
• security-arsenal — Pull the OTP-brute-force payload section (000000-999999 wordlist generator, ffuf rate-limit-evasion patterns with -t 5 -p 0.5-2, distributed-IP rotation via proxychains) and the JWT-token-replay table when "MFA satisfied" claim lives in a JWT claim that can be forged.
• triage-validation — Run the Pre-Severity Gate before claiming Critical on an MFA bypass that only works when the attacker already has the password. Standalone MFA bypass is High; chained-with-password-oracle is Critical; chained-with-cookie-theft-only is Critical. The chain question separates the two.