Ensure Secure File Permissions for Device Identity

src/infra/device-identity.ts

@@ -3,6 +3,63 @@ import fs from "node:fs";
 import path from "node:path";
 import { STATE_DIR } from "../config/paths.js";
 
+// Security: Function to safely set file permissions for sensitive data
+function setSecureFilePermissions(filePath: string): void {
+  try {
+    fs.chmodSync(filePath, 0o600);
+  } catch (error) {
+    const errorMessage = `Failed to set secure permissions (0600) on device identity file: ${error instanceof Error ? error.message : String(error)}`;
+    console.error(`[SECURITY] ${errorMessage}`);
+    throw new Error(`Security violation: ${errorMessage}`);
+  }
+}
+
+// Security: Write file with secure permissions from creation (prevents race conditions)
+function writeSecureFile(filePath: string, data: string): void {
+  // Set restrictive umask before file operations to ensure secure default permissions
+  const oldUmask = process.umask(0o077); // Only owner can access newly created files
+
+  try {
+    // Write file with explicit secure mode
+    fs.writeFileSync(filePath, data, { mode: 0o600 });
+    // Verify permissions were actually set correctly
+    verifySecurePermissions(filePath);
+  } finally {
+    // Always restore original umask
+    process.umask(oldUmask);
+  }
+}
+
+// Security: Verify file has secure permissions
+function verifySecurePermissions(filePath: string): void {
+  try {
+    // Use lstatSync to not follow symlinks (prevents symlink attacks)
+    const stats = fs.lstatSync(filePath);
+
+    // Security check: ensure it's not a symlink
+    if (stats.isSymbolicLink()) {
+      const errorMessage = `Security violation: device identity file is a symbolic link: ${filePath}`;
+      console.error(`[SECURITY] ${errorMessage}`);
+      throw new Error(errorMessage);
+    }
+
+    const permissions = stats.mode & 0o777;
+
+    // File should be readable/writable by owner only (0600)
+    if (permissions !== 0o600) {
+      const errorMessage = `Insecure permissions detected on device identity file: ${permissions.toString(8)} (expected 0600). Fix with: chmod 600 "${filePath}"`;
+      console.error(`[SECURITY] ${errorMessage}`);
+      throw new Error(`Security violation: ${errorMessage}`);
+    }
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code === 'ENOENT') {
+      // File doesn't exist, which is expected in some cases
+      return;
+    }
+    throw error;
+  }
+}
+
 export type DeviceIdentity = {
   deviceId: string;
   publicKeyPem: string;
@@ -64,6 +121,8 @@ function generateIdentity(): DeviceIdentity {
 export function loadOrCreateDeviceIdentity(filePath: string = DEFAULT_FILE): DeviceIdentity {
   try {
     if (fs.existsSync(filePath)) {
+      // Security: Verify existing identity file has secure permissions
+      verifySecurePermissions(filePath);
       const raw = fs.readFileSync(filePath, "utf8");
       const parsed = JSON.parse(raw) as StoredIdentity;
       if (
@@ -78,12 +137,7 @@ export function loadOrCreateDeviceIdentity(filePath: string = DEFAULT_FILE): Dev
             ...parsed,
             deviceId: derivedId,
           };
-          fs.writeFileSync(filePath, `${JSON.stringify(updated, null, 2)}\n`, { mode: 0o600 });
-          try {
-            fs.chmodSync(filePath, 0o600);
-          } catch {
-            // best-effort
-          }
+          writeSecureFile(filePath, `${JSON.stringify(updated, null, 2)}\n`);
           return {
             deviceId: derivedId,
             publicKeyPem: parsed.publicKeyPem,
@@ -110,12 +164,7 @@ export function loadOrCreateDeviceIdentity(filePath: string = DEFAULT_FILE): Dev
     privateKeyPem: identity.privateKeyPem,
     createdAtMs: Date.now(),
   };
-  fs.writeFileSync(filePath, `${JSON.stringify(stored, null, 2)}\n`, { mode: 0o600 });
-  try {
-    fs.chmodSync(filePath, 0o600);
-  } catch {
-    // best-effort
-  }
+  writeSecureFile(filePath, `${JSON.stringify(stored, null, 2)}\n`);
   return identity;
 }