@@ -36,11 +36,19 @@ exports.compile = function (messages, target) {
         if (code === 'root' ||
             Template.isTemplate(message)) {
 
+            // A flat code named __proto__ triggers the legacy accessor on plain-object
+            // assignment, replacing target's own prototype instead of creating an own property
+
+            assert(code !== '__proto__', 'Cannot use __proto__ as a message code');
+
             target[code] = message;
             continue;
         }
 
         if (typeof message === 'string') {
+
+            assert(code !== '__proto__', 'Cannot use __proto__ as a message code');
+
             target[code] = new Template(message);
             continue;
         }
@@ -59,6 +67,8 @@ exports.compile = function (messages, target) {
         for (const key of Object.keys(message)) {
             const localized = message[key];
 
+            assert(key !== '__proto__', 'Cannot use __proto__ as a message code');
+
             if (key === 'root' ||
                 Template.isTemplate(localized)) {
 
@@ -146,11 +156,18 @@ exports.merge = function (base, extended) {
         if (code === 'root' ||
             Template.isTemplate(message)) {
 
+            // Same as in compile(), a flat code named __proto__ replaces target's own prototype
+
+            assert(code !== '__proto__', 'Cannot use __proto__ as a message code');
+
             target[code] = message;
             continue;
         }
 
         if (typeof message === 'string') {
+
+            assert(code !== '__proto__', 'Cannot use __proto__ as a message code');
+
             target[code] = new Template(message);
             continue;
         }
@@ -169,6 +186,8 @@ exports.merge = function (base, extended) {
         for (const key of Object.keys(message)) {
             const localized = message[key];
 
+            assert(key !== '__proto__', 'Cannot use __proto__ as a message code');
+
             if (key === 'root' ||
                 Template.isTemplate(localized)) {
 
 
@@ -0,0 +1,44 @@
+'use strict';
+
+const Code = require('@hapi/code');
+const Lab = require('@hapi/lab');
+
+const Joi = require('..');
+const Messages = require('../lib/messages');
+
+const { describe, it } = exports.lab = Lab.script();
+const expect = Code.expect;
+
+// JSON.parse produces a genuine own "__proto__" property (unlike the { __proto__: ... }
+// object literal shorthand, which the parser special-cases as prototype-setting syntax
+// and never creates an own key), matching how an attacker delivers this in practice
+// (a JSON request body or config file fed into .messages()/.prefs()).
+const attackerJson = (message) => JSON.parse(`{"__proto__": ${JSON.stringify(message)}}`);
+
+
+describe('messages() proto guard', () => {
+
+    it('does not let a top-level __proto__ code hijack the compiled messages object prototype', () => {
+
+        const before = Messages.compile({ 'number.min': 'too small' });
+        expect(Object.getPrototypeOf(before)).to.equal(Object.prototype);
+
+        expect(() => Messages.compile(attackerJson('pwned'))).to.throw();
+    });
+
+    it('rejects __proto__ as a language-scoped error code', () => {
+
+        expect(() => Messages.compile({ english: attackerJson('pwned') })).to.throw();
+    });
+
+    it('rejects __proto__ via merge()', () => {
+
+        expect(() => Messages.merge({ 'number.min': 'too small' }, attackerJson('pwned'))).to.throw();
+    });
+
+    it('rejects __proto__ through the public .messages()/.prefs() schema API', () => {
+
+        expect(() => Joi.any().messages(attackerJson('pwned'))).to.throw();
+        expect(() => Joi.any().prefs({ messages: attackerJson('pwned') })).to.throw();
+    });
+});