CVE-2021-21368

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.

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.

### 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: ```js 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](http://example.com) * Email us at [example email address](mailto:example@example.com)

### 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: ```js 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](http://example.com) * Email us at [example email address](mailto:example@example.com)

msgpack5 es una implementación de msgpack v5 para node.js y el navegador. En msgpack5 versiones anteriores a 3.6.1, 4.5.1 y 5.2.1, se presenta una vulnerabilidad de "Prototype Poisoning". Cuando msgpack5 decodifica un mapa que contiene una clave "__proto__", asigna el valor decodificado a __proto__. Object.prototype.__proto__ es una propiedad accesoria para el prototipo del receptor. Si el valor correspondiente a la clave __proto__ se decodifica para un objeto o null, msgpack5 establece el prototipo del objeto decodificado a ese valor. Un atacante que pueda enviar datos de MessagePack manipulados a un servicio puede usar esto para producir valores que parezcan ser de otros tipos; pueden tener propiedades y métodos de prototipo inesperados (por ejemplo, longitud, propiedades numéricas, y presione et al si el valor de __proto __ se decodifica en una matriz); y/o pueden lanzar excepciones inesperadas cuando se usan (por ejemplo, si el valor de __proto__ decodifica a un Map o Date). Pueden producirse otros comportamientos inesperados para otros tipos. No se presenta ningún efecto en el prototipo global. Este "Prototype Poisoning" es una especie de inversión muy limitada de un ataque de contaminación de prototipo. Sólo afecta al prototipo del valor decodificado, y sólo puede establecerse en valores msgpack5 (aunque si la víctima hace uso de códecs personalizados, cualquier cosa podría ser un valor msgpack5). No hemos encontrado una manera de escalar esto a una verdadera contaminación del prototipo (en ausencia de otros errores en el código del consumidor). Esto ha sido corregido en las versiones 3.6.1, 4.5.1 y 5.2.1 de msgpack5. Consulte el GitHub Security Advisory para visualizar un ejemplo y más detalles