Chrome DevTools for agents: daemon.pid write follows symlinks in /tmp fallback runtime directory

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

const pidFilePath = getPidFilePath(sessionId);
fs.mkdirSync(path.dirname(pidFilePath), {
  recursive: true,
});
fs.writeFileSync(pidFilePath, process.pid.toString());

src/daemon/utils.ts:49-68

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 . If an attacker creates first and places a symlink at , the victim's daemon process follows that link when writing the PID.