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hunt-mfa-bypass skill

by elementalsouls·elementalsouls/Claude-BugHunter·4.7k stars·MIT

Hunt MFA / 2FA bypass — 7 distinct patterns. (1) MFA not enforced on sensitive endpoints (password change, email change accept without MFA challenge), (2) MFA-step skip via direct navigation to post-login URL, (3) MFA-token replay (same code accepted twice), (4) brute-force the 6-digit OTP without rate limit (10^6 attempts at server speed), (5) race condition on OTP validation, (6) recovery-code dump via /api/me, (7) backup factor downgrade (SMS factor with no rate limit). Plus the chain: cookie theft + password oracle + no step-up = ATO without MFA challenge. Detection: trace auth flow in Burp, find every state transition, check if MFA is middleware-gated vs per-endpoint, check OTP entropy and rate limit on OTP-validate. Validate: attacker session reaching post-MFA state. Use when hunting auth bypass, MFA flows, chaining primitives toward ATO.

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Install the hunt-mfa-bypass skill

A skill is a folder. Copy it into your agent's skills folder and the agent loads it when the task matches its description. 

git clone --depth 1 https://github.com/elementalsouls/Claude-BugHunter.git /tmp/Claude-BugHunter
mkdir -p ~/.claude/skills
cp -r /tmp/Claude-BugHunter/skills/hunt-mfa-bypass ~/.claude/skills/hunt-mfa-bypass
available in every project

In the Claude apps, zip the folder and upload it from the Skills settings. The folder on GitHub

The instructions your agent would load

SKILL.md as published, without the frontmatter. Read it on GitHub

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:

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.

  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.

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.

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