Prototype poisoning

@@ -19,7 +19,9 @@ function msgpack (options) {
     // if true, skips encoding Dates using the msgpack
     // timestamp ext format (-1)
     disableTimestampEncoding: false,
-    preferMap: false
+    preferMap: false,
+    // options.protoAction: 'error' (default) / 'remove' / 'ignore'
+    protoAction: 'error'
   }
 
   decodingTypes.set(DateCodec.type, DateCodec.decode)

@@ -187,6 +187,17 @@ module.exports = function buildDecode (decodingTypes, options) {
       for (let i = 0; i < 2 * length; i += 2) {
         const key = result[i]
         const val = result[i + 1]
+
+        if (key === '__proto__') {
+          if (options.protoAction === 'error') {
+            throw new SyntaxError('Object contains forbidden prototype property')
+          }
+
+          if (options.protoAction === 'remove') {
+            continue
+          }
+        }
+
         object[key] = val
       }
       return [object, consumedBytes]

@@ -111,6 +111,7 @@ options:
 - `compatibilityMode`, a boolean that enables "compatibility mode" which doesn't use str 8 format. Defaults to false.
 - `disableTimestampEncoding`, a boolean that when set disables the encoding of Dates into the [timestamp extension type](https://github.com/msgpack/msgpack/blob/master/spec.md#timestamp-extension-type). Defaults to false.
 - `preferMap`, a boolean that forces all maps to be decoded to `Map`s rather than plain objects. This ensures that `decode(encode(new Map())) instanceof Map` and that iteration order is preserved. Defaults to false.
+- `protoAction`, a string which can be `error|ignore|remove` that determines what happens when decoding a plain object with a `__proto__` property which would cause prototype poisoning. `error` (default) throws an error, `remove` removes the property, `ignore` (not recommended) allows the property, thereby causing prototype poisoning on the decoded object.
 
 -------------------------------------------------------
 <a name="encode"></a>

@@ -0,0 +1,49 @@
+'use strict'
+
+var test = require('tape').test
+var msgpack = require('../')
+
+test('decode throws when object has forbidden __proto__ property', function (t) {
+  const encoder = msgpack()
+
+  const payload = { hello: 'world' }
+  Object.defineProperty(payload, '__proto__', {
+    value: { polluted: true },
+    enumerable: true
+  })
+
+  const encoded = encoder.encode(payload)
+
+  t.throws(() => encoder.decode(encoded), /Object contains forbidden prototype property/)
+  t.end()
+})
+
+test('decode ignores forbidden __proto__ property if protoAction is "ignore"', function (t) {
+  const encoder = msgpack({ protoAction: 'ignore' })
+
+  const payload = { hello: 'world' }
+  Object.defineProperty(payload, '__proto__', {
+    value: { polluted: true },
+    enumerable: true
+  })
+
+  const decoded = encoder.decode(encoder.encode(payload))
+
+  t.equal(decoded.polluted, true)
+  t.end()
+})
+
+test('decode removes forbidden __proto__ property if protoAction is "remove"', function (t) {
+  const encoder = msgpack({ protoAction: 'remove' })
+
+  const payload = { hello: 'world' }
+  Object.defineProperty(payload, '__proto__', {
+    value: { polluted: true },
+    enumerable: true
+  })
+
+  const decoded = encoder.decode(encoder.encode(payload))
+
+  t.equal(decoded.polluted, undefined)
+  t.end()
+})