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client-reverse

Client-side request-signing and anti-bot token reversal for bug bounty — when a request carries a sign/sig/hmac/token/nonce/timestamp/X-Sensor header that Burp Repeater cannot replay, recover the signer just enough to reproduce the request outside the client. Packet-first staging (capture real request → prove replay works → only reverse if replay fails) across the locate→recover→runtime→validation→replay spine. Covers tracing backward from the signature field (writer→builder→entry→source), isolating user-mutable sign inputs (timestamp/nonce/deviceId/body) vs constants (secret key), hooking fetch/XHR in DevTools, JS deobfuscation basics (webpack/wasm/JSVMP), and the bounty payoff: reach the protected API to then hunt IDOR/auth/business-logic. Use when Burp/mitmproxy replay of a signed or anti-bot-gated request fails and you suspect a client-computed field is blocking you.

skills/client-reverse/SKILL.md

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Nov 3, 2026

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Oct 28, 2026
License
MIT

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# CLIENT-SIDE REQUEST-SIGNING / ANTI-BOT TOKEN REVERSAL

You hit a request you cannot replay. Burp Repeater returns `401 invalid signature` or `403 bot detected` even though the browser/app does it fine. There is a `sign`, `sig`, `X-Signature`, `_token`, `nonce`, `X-Acf-Sensor-Data`, or encrypted body the client computes. This skill recovers **just enough** of that signer to reproduce the request **outside** the client.

> **Why a bug bounty hunter cares:** the signature is not the bug. The signature is the **lock on the door**. Behind it is an API the program assumed only their own client would ever reach — so that API is often under-tested for IDOR, BOLA, mass assignment, and business logic. Reversing the signer is the cost of admission; the **payout** comes from what you fuzz once you're inside. Never report "I reversed your sign algorithm" as a finding on its own — that is N/A. Report the IDOR/auth bug you reached **through** it.

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## THE CORE PRINCIPLE: PACKET-FIRST

> Reverse engineering is a **blocker-resolution step, not the default entrypoint.** Capture the real request first. Prove whether it already replays. Only reverse the signer if replay actually fails.

Most hunters waste hours decompiling JS for a "signature" that turns out to be replayable as-is, or gated by a timestamp that's valid for 5 minutes. Run this gate **before** opening DevTools Sources: