CVE-2026-118247: Highcharts Dashboards / Grid Pro: `DataTable.setCell` and column writers treat `__proto__` as an existing column.

When a single cell value is written to a data table, the column id is used as a key on the table’s column collection without filtering, so a column id of __proto__ resolves to Object.prototype, is taken for an existing column, and the write lands at the row index on Object.prototype; a column id of constructor writes onto the Object constructor in the same way. In Dashboards releases that still supported column aliases, the id is first resolved through the alias map, so a literal __proto__ id is redirected harmlessly there, while an alias whose target is __proto__, which the connector’s data table options accept, reaches the same write. An attacker who controls the column id can thereby set integer-keyed properties on Object.prototype, one per row index, holding the written value; named properties cannot be reached through these paths, because the row index is a number. The integer keys still affect every object in the page: reading an out-of-range index on any array or plain object returns the injected value, and for...in over any object yields the injected keys, so unrelated code can take branches it should not using attacker-supplied data, and rendering can fail as a side effect. In Dashboards the write is reached through the Highcharts component’s connector write-back, which is enabled by default through allowConnectorUpdate and takes the column id from the component’s columnAssignment, or in the earliest releases from the series name, when a point is updated; in Grid Pro it is reached through cell editing in earlier releases and remains reachable through the public DataTable API. Exploitation requires that untrusted data reach the board or Grid configuration and that a point be updated or a cell edited, or that the application write a cell with an untrusted column id.

Consequences beyond disruption depend on the consuming application.