A Cookie-Free Embeddable Support Widget: What Adversarial Review Caught

TL;DR Built an embeddable support widget for a helpdesk product: no cookies — a short-lived bearer token in a header, hashed at rest. Entry is an HMAC-signed assertion from the host page. An adversarial review caught four real holes before launch. Outbound webhooks: sign the exact bytes, dedupe key for idempotency, SSRF guard on destination URLs. The requirement: end users file tickets from pages the product doesn't own. That means an embeddable widget — and embeddable means everything you know about sessions stops working. Why cookie-free The widget lives on customers' domains, so any cookie it sets is a third-party cookie — blocked or partitioned by modern browsers. Fighting that means flaky sessions, so: no cookies at all. The entry exchange mints a short-lived session token the widget sends in a header, and the server caches the session keyed by sha256(token) — a cache dump yields nothing replayable. Sessions last 60 minutes, and expiry shows a real recovery path in the UI instead of dying silently. customer backend widget (on customer page) helpdesk API | signs ref|email|name | | | into HMAC assertion ---> |-- redeem assertion (single use) ->| | |<-- session token (60-min TTL) ---| | |-- X-Widget-Token: ... ---------->| What the adversarial review caught Finding Fix Replay burn keyed by client-chosen nonce Burn by HMAC signature — a leaked assertion can't mint extra sessions `\ ` accepted inside signed fields Origin check failed open when Origin/Referer absent Fall back to the unspoofable Sec-Fetch-Dest header to enforce embedding Widget could request critical severity Clamp effective severity (including the channel default) to the widget's allowlist My favourite is the delimiter one. If you sign ref|email|name and accept | inside a field, two different identity tuples can share one valid signature. Canonicalization bugs, not crypto bugs. Webhooks out: sign the exact bytes Outbound webhooks get composed once at enqueue time and stored; the delivery job re-encod