Vega is a visualization grammar, a declarative format for creating, saving, and sharing interactive visualization designs. The Vega `scale` expression function has the ability to call arbitrary functions with a single controlled argument. The scale expression function passes a user supplied argument group to getScale, which is then used as if it were an internal context. The context.scales[name].value is accessed from group and called as a function back in scale. This can be exploited to escape the Vega expression sandbox in order to execute arbitrary JavaScript. This issue has been fixed in version 5.13.1.