Update node spawn process hardening (elastic#186786)

### Summary
Update node `child_process.spawn` hardening with updated measures.

---------

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

src/setup_node_env/harden/child_process.js

@@ -30,31 +30,34 @@ function patchChildProcess(cp) {
 function patchOptions(hasArgs) {
   return function apply(target, thisArg, args) {
     var pos = 1;
-    if (pos === args.length) {
+    var newArgs = Object.setPrototypeOf([].concat(args), null);
+
+    if (pos === newArgs.length) {
       // fn(arg1)
-      args[pos] = prototypelessSpawnOpts();
-    } else if (pos < args.length) {
-      if (hasArgs && (Array.isArray(args[pos]) || args[pos] == null)) {
+      newArgs[pos] = prototypelessSpawnOpts();
+    } else if (pos < newArgs.length) {
+      if (hasArgs && (Array.isArray(newArgs[pos]) || newArgs[pos] == null)) {
         // fn(arg1, args, ...)
         pos++;
       }
 
-      if (typeof args[pos] === 'object' && args[pos] !== null) {
+      if (typeof newArgs[pos] === 'object' && newArgs[pos] !== null) {
         // fn(arg1, {}, ...)
         // fn(arg1, args, {}, ...)
-        args[pos] = prototypelessSpawnOpts(args[pos]);
-      } else if (args[pos] == null) {
+        newArgs[pos] = prototypelessSpawnOpts(newArgs[pos]);
+      } else if (newArgs[pos] == null) {
         // fn(arg1, null/undefined, ...)
         // fn(arg1, args, null/undefined, ...)
-        args[pos] = prototypelessSpawnOpts();
-      } else if (typeof args[pos] === 'function') {
+        newArgs[pos] = prototypelessSpawnOpts();
+      } else if (typeof newArgs[pos] === 'function') {
         // fn(arg1, callback)
         // fn(arg1, args, callback)
-        args.splice(pos, 0, prototypelessSpawnOpts());
+        // `newArgs` doesn't have prototype and hence `splice` method anymore.
+        Array.prototype.splice.call(newArgs, pos, 0, prototypelessSpawnOpts());
       }
     }
 
-    return target.apply(thisArg, args);
+    return target.apply(thisArg, newArgs);
   };
 }
 

test/harden/_node_script.js

@@ -0,0 +1,9 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+console.log('Hello from _node_script.js!');

test/harden/child_process.js

@@ -307,6 +307,50 @@ for (const name of functions) {
     assertProcess(t, cp.spawn(command, [], { env: { custom: 'custom' } }), { stdout: 'custom' });
   });
 
+  test('spawn(command, options) - prevent object prototype pollution', (t) => {
+    const pathName = path.join(__dirname, '_node_script.js');
+    const options = {};
+    const pollutedObject = {
+      env: {
+        NODE_OPTIONS: `--require ${pathName}`,
+      },
+      shell: process.argv[0],
+    };
+    // eslint-disable-next-line no-proto
+    options.__proto__['2'] = pollutedObject;
+
+    const argsArray = [];
+
+    /**
+     * Declares that 3 assertions should be run.
+     * We don't use the assertProcess function here as we need an extra assertion
+     * for the polluted prototype
+     */
+    t.plan(3);
+
+    t.deepEqual(
+      argsArray[2],
+      pollutedObject,
+      'Prototype should be polluted with the object at index 2'
+    );
+
+    const stdout = '';
+
+    const cmd = cp.spawn(command, argsArray);
+    cmd.stdout.on('data', (data) => {
+      t.equal(data.toString().trim(), stdout);
+    });
+
+    cmd.stderr.on('data', (data) => {
+      t.fail(`Unexpected data on STDERR: "${data}"`);
+    });
+
+    cmd.on('close', (code) => {
+      t.equal(code, 0);
+      t.end();
+    });
+  });
+
   for (const unset of notSet) {
     test(`spawn(command, ${unset})`, (t) => {
       assertProcess(t, cp.spawn(command, unset));