Proxy target/handler access leak in Node

Darien Valentine valentinium at
Sun Sep 16 18:59:29 UTC 2018

A few weeks ago I’d commented on an open Node github issue regarding
Proxies and inspection. While the bulk of the comment concerns an opinion
that proxies should not be treated as special case, I included an example
of a mechanism by which the current implementation allows outside code to
access the target and handler objects of a proxy that it does not own.

On reflection I realized this specific issue might be worth drawing more
attention to.

const util = require('util');

const victim = new Proxy({}, {
  SECRET: 'Nothing outside can access this'

let secret;

const invariantViolator = {
  [util.inspect.custom](depth, options) {
    const { stylize } = options;

    options.showProxy = true;

    options.stylize = (value, color) => {
      secret = value;
      options.stylize = stylize;
      return stylize(value, color);

    return victim;


console.log(secret); // 'Nothing outside can access this'

The implication is that even if running Node with no C++ addons, it is
presently possible for proxies to be violated using just the standard lib,
which may be significant from a security perspective. I’m not sure if
that’s the case in practice, but just in case, I figured I should try to
get eyes on it.

Note that even if this particular hole is patched, the "analog hole" (so to
speak) of just analyzing the string output remains.
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