Chrome DevTools for agents: daemon.pid write follows symlinks in /tmp fallback runtime directory
Date: 2026-06
Affected: npm/chrome-devtools-mcp
Attack vector: path-traversal
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-53765
CWE: CWE-59
CVSS: 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_RUNT


Mitigations:

Architectural layers (MAESTRO):