Scheme validation bypass in Phoenix.LiveView.Utils leads to XSS via <.link>

Cross-site scripting vulnerability in phoenixframework phoenix_live_view allows an attacker to bypass URL scheme validation and execute JavaScript in a victim's browser session.

The Phoenix.LiveView.Utils.valid_destination!/2 and Phoenix.LiveView.Utils.valid_live_navigation_destination!/2 functions in lib/phoenix_live_view/utils.ex rely on an internal uri_scheme/1 helper that only detects a scheme when the input's first byte is an ASCII letter. Inputs beginning with an ASCII control character or space fall through to a nil-returning clause, causing the URL to be treated as a safe relative path.

Standard browsers implement the WHATWG URL parser, which strips leading C0 control and space characters before parsing. As a result, an input such as " javascript:alert(1)" is passed unchanged into <.link href={...}> and, when clicked, is parsed by the browser as a javascript: URL that executes attacker-controlled script in the victim's session.

Applications that render user-supplied URLs (for example profile links, redirect targets, or external references) via <.link href={...}> are affected.

This issue affects phoenix_live_view: from 1.2.2 before 1.2.7.

Strip leading ASCII control and space bytes (code points 0x00 through 0x20) from any untrusted URL before passing it to <.link href={...}>, or reject such inputs outright. This aligns application-level validation with the WHATWG URL parser used by browsers and prevents the scheme-detection bypass.