CVE-2021-21368 Prototype poisoning
Attackers can craft MessagePack data to poison object prototypes, causing type confusion, unexpected exceptions, or potential code execution in applications using msgpack5.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2021-21368?
CVE-2021-21368 is a medium-severity unsafe deserialization vulnerability in mcollina msgpack5, published on 12 Mar 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Official description
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.
Which versions are affected by CVE-2021-21368?
6 open source packages ship an affected version, including msgpack5. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
msgpack5 npm | All versions | 3.6.1 |
msgpack5 npm | All versions | 3.6.1 |
msgpack5 npm | From 4.0.0 | 4.5.1 |
msgpack5 npm | From 4.0.0 | 4.5.1 |
msgpack5 npm | From 5.0.0 | 5.2.1 |
msgpack5 npm | From 5.0.0 | 5.2.1 |
Products include Msgpack5 Project Msgpack5.
How do I fix CVE-2021-21368?
Upgrade msgpack5 to a fixed version for your release line: 3.6.1, 4.5.1 or 5.2.1.
Upgrade, and only deserialise data from trusted sources or with an allow-list of types. More about Unsafe deserialization.
Is CVE-2021-21368 being exploited?
There is no known exploitation of CVE-2021-21368 in the wild and no public exploit code.
Sources
Also known as GHSA-gmjw-49p4-pcfm.