BrowserStack Runner through 0.9.5 contains a remote code execution vulnerability in the /_log HTTP handler that allows unauthenticated network-adjacent attackers to execute arbitrary code by submitting crafted JSON request bodies to the handler, which passes user-supplied data to vm.runInNewContext() combined with eval(). Attackers can escape the Node.js vm sandbox by leveraging a host-context Function reference through util.format to access the host process via this.constructor.constructor, achieving full remote code execution on the underlying system without any authentication.

Affected: npm: browserstack-runner

### Summary

The HTTP handler `/_log` in `lib/server.js` (lines 491–515) of browserstack-runner passes unauthenticated user-supplied data to `vm.runInNewContext()` combined with `eval()`, enabling a sandbox escape and arbitrary code execution on the host system.

### Details

When `browserstack-runner` starts, it creates an HTTP server on port 8888 (configurable) that listens on all network interfaces (`0.0.0.0`). The `/_log` endpoint accepts POST requests and processes the JSON body as follows:

```javascript // lib/server.js lines 504-510 var context = { input: query.arguments, format: util.format, output: '' }; var tryEvalOrString = 'function (arg) { try { return eval(\'o = \' + arg); } catch (e) { return arg; } }'; vm.runInNewContext('output = format.apply(null, input.map(' + tryEvalOrString + '));', context); ```

The `vm` module is [not a security mechanism](https://nodejs.org/api/vm.html#vm-executing-javascript) per Node.js documentation. The `context` object contains a reference to `util.format` (a host-context Function), enabling sandbox escape via `this.constructor.constructor("return process")()`.

Unlike the `_progress` and `_report` handlers which verify worker UUID authentication, the `_log` handler does not gate on authentication.

### Proof of Concept

```bash # Terminal 1: start the runner echo '<html><body>t</body></html>' > t.html echo '{"username":"X","key":"X","test_path":"t.html","test_framework":"qunit","browsers":[]}' > browserstack.json node bin/runn…