CVE-2021-21368 (published 2021-03-12T17:15Z)

msgpack5 is a msgpack v5 implementation for node.js and the browser. In msgpack5 before versions 3.6.1, 4.5.1, and 5.2.1 there is a "Prototype Poisoning" vulnerability. When msgpack5 decodes a map containing a key "__proto__", it assigns the decoded value to __proto__. Object.prototype.__proto__ is an accessor property for the receiver\'s prototype. If the value corresponding to the key __proto__ decodes to an object or null, msgpack5 sets the decoded object\'s prototype to that value. An attacker who can submit crafted MessagePack data to a service can use this to produce values that appear to be of other types; may have unexpected prototype properties and methods (for example length, numeric properties, and push et al if __proto__\'s value decodes to an Array); and/or may throw unexpected exceptions when used (for example if the __proto__ value decodes to a Map or Date). Other unexpected behavior might be produced for other types. There is no effect on the global prototype. This "prototype poisoning" is sort of a very limited inversion of a prototype pollution attack. Only the decoded value\'s prototype is affected, and it can only be set to msgpack5 values (though if the victim makes use of custom codecs, anything could be a msgpack5 value). We have not found a way to escalate this to true prototype pollution (absent other bugs in the consumer\'s code). This has been fixed in msgpack5 version 3.6.1, 4.5.1, and 5.2.1. See the referenced GitHub Security Advisory for an example and more details.

Severity: MEDIUM
CVSS v2 base score: 6.5 (AV:N/AC:L/Au:S/C:P/I:P/A:P)
CVSS v3 base score: 8.8, HIGH (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-1321
cwe_name: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
description: The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.
extended_description: 
				By adding or modifying attributes of an object prototype, it is possible to create attributes that exist on every object, or replace critical attributes with malicious ones. This can be problematic if the product depends on existence or non-existence of certain attributes, or uses pre-defined attributes of object prototype (such as hasOwnProperty, toString or valueOf).
				This weakness is usually exploited by using a special attribute of objects called proto,  constructor or prototype. Such attributes give access to the object prototype. This weakness is often found in code that assigns object attributes based on user input, or merges or clones objects recursively.
			
			

Fix information:
repo_url: https://github.com/mcollina/msgpack5
commit_details: repo_url: https://github.com/mcollina/msgpack5
author: Matteo Collina
author_date: 2021-03-05 11:18:34+01:00
msg: Merge pull request from GHSA-gmjw-49p4-pcfm

Co-authored-by: Simone Busoli <simone.busoli@gmail.com>
repository: repo_url: https://github.com/mcollina/msgpack5
repo_name: mcollina/msgpack5
description: A msgpack v5 implementation for node.js, with extension points / msgpack.org[Node]
repo_language: JavaScript
owner: mcollina
file_changes: filename: README.md
change_type: ModificationType.MODIFY
diff: @@ -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>

programming_language: Markdown
filename: index.js
change_type: ModificationType.MODIFY
diff: @@ -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)

programming_language: JavaScript
filename: decoder.js
change_type: ModificationType.MODIFY
diff: @@ -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]

programming_language: Swift
filename: object-prototype-poisoning.js
change_type: ModificationType.ADD
diff: @@ -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()
+})

programming_language: JavaScript