The kiosk page interpolates style.fontFamily and style.background into a <style>
block, escaped with escapeHtml. That is the wrong tool twice over: it escapes
& < > " ' but not { } ;, and inside a raw-text <style> element the entities it does
produce are never decoded, so it neither contains the value nor renders it correctly.
A value could therefore close the declaration, close the rule, and append its own —
putting an attacker-chosen rule on every panel showing the page. There is no XSS,
since </style> stays unreachable, but a url() in an injected rule is an outbound
request from every display, which is a beacon and a cross-site tracking channel.
Both values are now checked structurally rather than against a value allowlist,
because background is a free-text field: linear-gradient(), rgb() and url() are all
legitimate and keep working. Only characters that could terminate the declaration or
open a new rule are refused, along with comment syntax (which can swallow the
declarations that follow) and control characters. font-family needs no parentheses,
so it gets a tighter allowlist.
Tests cover both directions — injection refused and falling back to the default, and
ordinary gradients, colours and font stacks passing through untouched.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
