CVE-2026-118243: Highcharts Dashboards: `__proto__` data table id writes sync listeners to `Object.prototype`

The data cursor keeps its listener and state bookkeeping in plain objects keyed by data table id, so a table id of __proto__ supplied in the dataPool connector configuration resolves to Object.prototype when a sync registers its listeners, and the registration creates a property on Object.prototype named after the sync state and holding the internal listener array. The property name begins with a state prefix fixed by the library, to which the sync group option can append an attacker-chosen suffix, and its value is the library’s own listener array, but the property is enumerable and visible on every object in the page, and emitting a lasting state for that table id then throws, so syncs that keep lasting states, such as extremes, stop working. Exploitation requires that untrusted data reach the Dashboards configuration and that a sync such as highlight or extremes be enabled on a component using that table, which the same configuration can do. No user interaction is required beyond loading the board.

The injected property is inert until other code on the page reads or enumerates it, so consequences depend on the consuming application.