Unvalidated URL Execution Enables Arbitrary URI Scheme Invocation

Magic: The Gathering Arena (Windows/Steam client; 2026.59.30.12801.127931.6 and certain later 2026.60.x builds) passes a server-supplied URL from a home-screen carousel GoToExternalUrl action directly to the Windows shell via Application.OpenURL/ShellExecuteW without validating the URI scheme or domain. A hypothetical attacker able to control the carousel content delivered to clients can cause arbitrary registered URI-scheme handlers to be invoked on client hosts with no user interaction. For example, one might expect that the carousel content only has https: URIs, not ms-calculator: URIs.