The i-tag UUID used for webxdc coordination is attacker-controlled.
Using it directly as the iframe.diy subdomain lets a malicious event
author pick a subdomain that collides with another app's origin,
gaining access to its localStorage/IndexedDB.
Introduce a persistent random seed in localStorage (ditto:seed) and
derive the subdomain as base36(HMAC-SHA256(seed, prefix|identifier)).
The prefix (e.g. "webxdc") domain-separates different use-cases.
The subdomain is stable per device+app but unpredictable to event
authors.
Alex Gleason
be65c659b2