Individual skill
web-client-side-signing-bypass
Reverse client-side request signing / obfuscated JS so you can replay and fuzz a protected API outside the browser. Load when requests carry a computed guard you must reproduce: an X-Signature / X-Sign / sign / hmac / _s / token header or body field, a nonce+timestamp, a "signature invalid" 401, encrypted request bodies, or minified/webpack/WASM/JSVMP signer code.
skills/web/web-client-side-signing-bypass/SKILL.md
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# Reversing client-side request signing
## When it applies
An endpoint you want to fuzz (for `web-idor`, `api-bola`, `api-mass-assignment`, or business
logic) rejects tampered requests with a `signature invalid` / `401` because the client computes a
guard value in JavaScript — an HMAC over the body, a nonce+timestamp, or an encrypted payload.
You need to reproduce *one* valid signed request outside the browser so you can mutate the parts
that matter and re-sign them.
## Why it works
The signature is **client-side integrity theater**: the secret and the algorithm ship to the
browser, so anything the browser can compute, you can compute. The signing weakness itself is
almost always N/A — the payout is the access-control or logic bug on the endpoint it was
"protecting". Reproduce the signer, and that endpoint becomes as fuzzable as an unsigned one.