{"id":"CVE-2021-21368","aliases":["EUVD-2021-0658","GHSA-gmjw-49p4-pcfm"],"status":"published","title":"Prototype poisoning","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.","severity":{"version":"3.1","score":6.7,"vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.01649,"percentile":0.76822,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-1321","CWE-915"],"published":"2021-03-12T17:00:19Z","modified":"2026-10-20T10:54:29Z","fixes":[{"ecosystem":"npm","package":"msgpack5","fixed_in":["3.6.1","4.5.1","5.2.1"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"msgpack5","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.6.1"}]}]},{"source":"ghsa","ecosystem":"npm","package":"msgpack5","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"4.0.0"},{"fixed":"4.5.1"}]}]},{"source":"ghsa","ecosystem":"npm","package":"msgpack5","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"5.0.0"},{"fixed":"5.2.1"}]}]},{"source":"osv","ecosystem":"npm","package":"msgpack5","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"3.6.1"}]}]},{"source":"osv","ecosystem":"npm","package":"msgpack5","ranges":[{"type":"SEMVER","events":[{"introduced":"4.0.0"},{"fixed":"4.5.1"}]}]},{"source":"osv","ecosystem":"npm","package":"msgpack5","ranges":[{"type":"SEMVER","events":[{"introduced":"5.0.0"},{"fixed":"5.2.1"}]}]}],"products":[{"vendor":"mcollina","product":"msgpack5"},{"vendor":"msgpack5_project","product":"msgpack5"}],"references":[{"url":"https://github.com/mcollina/msgpack5/security/advisories/GHSA-gmjw-49p4-pcfm","tags":["x_refsource_CONFIRM","x_transferred","Exploit","Third Party Advisory","evidence","web"]},{"url":"https://www.npmjs.com/package/msgpack5","tags":["x_refsource_MISC","x_transferred","Product","Third Party Advisory","advisory","web"]},{"url":"https://github.com/mcollina/msgpack5/releases/tag/v3.6.1","tags":["x_refsource_MISC","x_transferred","Release Notes","Third Party Advisory","advisory","web"]},{"url":"https://github.com/mcollina/msgpack5/releases/tag/v4.5.1","tags":["x_refsource_MISC","x_transferred","Release Notes","Third Party Advisory","advisory","web"]},{"url":"https://github.com/mcollina/msgpack5/releases/tag/v5.2.1","tags":["x_refsource_MISC","x_transferred","Release Notes","Third Party Advisory","advisory","web"]},{"url":"https://github.com/mcollina/msgpack5/commit/d4e6cb956ae51c8bb2828e71c7c1107c340cf1e8","tags":["x_refsource_MISC","x_transferred","Patch","Third Party Advisory","fix","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2021-21368","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-gmjw-49p4-pcfm"}],"sources":{"cve":[{"id":"CVE-2021-21368","title":"Prototype poisoning","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.","status":"published","published":"2021-03-12T17:00:19Z","modified":"2024-08-03T18:09:15.915Z","cvss":[{"version":"3.1","score":6.7,"vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H","level":"medium","assessor":"cna"}],"cwes":["CWE-915"],"updated_at":"2026-11-02T14:32:59.802172Z"}],"euvd":[{"id":"EUVD-2021-0658","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.","status":"published","published":"2021-03-12T17:00:19Z","modified":"2024-08-03T18:09:15Z","cvss":[{"version":"3.1","score":6.7,"vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H","level":"medium"}],"updated_at":"2026-11-03T03:25:06.913643Z"}],"ghsa":[{"id":"GHSA-gmjw-49p4-pcfm","title":"Prototype poisoning","description":"### Impact\n\nThe issue is as follows: when `msgpack5` decodes a map containing a \nkey `\"__proto__\"`, it assigns the decoded value to `__proto__`. As you \nare no doubt aware, `Object.prototype.__proto__` is an accessor \nproperty for the receiver's prototype. If the value corresponding to \nthe key `__proto__` decodes to an object or `null`, `msgpack5` sets \nthe decoded object's prototype to that value. \n\nAn attacker who can submit crafted MessagePack data to a service can \nuse this to produce values that appear to be of other types; may have \nunexpected prototype properties and methods (for example `length`, \nnumeric properties, and `push` et al if `__proto__`'s value decodes to \nan `Array`); and/or may throw unexpected exceptions when used (for \nexample if the `__proto__` value decodes to a `Map` or `Date`). Other \nunexpected behavior might be produced for other types. \n\nThere is no effect on the global prototype.\n\nAn example: \n\n```js \nconst msgpack5 = require('msgpack5')(); \n\nconst payload = {}; \nObject.defineProperty(payload, '__proto__', { \nvalue: new Map().set(1, 2), \nenumerable: true \n}); \n\nconst encoded = msgpack5.encode(payload); \nconsole.log(encoded); // \u003cBuffer 81 a9 5f 5f 70 72 6f 74 6f 5f 5f 81 01 02\u003e \n\nconst decoded = msgpack5.decode(encoded); \n\n// decoded's prototype has been overwritten \nconsole.log(Object.getPrototypeOf(decoded)); // Map(1) { 1 =\u003e 2 } \nconsole.log(decoded.get); // [Function: get] \n\n// decoded appears to most common typechecks to be a Map \nconsole.log(decoded instanceof Map); // true \nconsole.log(decoded.toString()); // [object Map] \nconsole.log(Object.prototype.toString.call(decoded)); // [object Map] \nconsole.log(decoded.constructor.name); // Map \nconsole.log(Object.getPrototypeOf(decoded).constructor.name); // Map \n\n// decoded is not, however, a Map \nconsole.log(Object.getPrototypeOf(decoded) === Map.prototype); // false \n\n// using decoded as though it were a Map throws \ntry { \ndecoded.get(1); \n} catch (error) { \nconsole.log(error); // TypeError: Method Map.prototype.get called \n// on incompatible receiver #\u003cMap\u003e \n} \ntry { \ndecoded.size; \n} catch (error) { \nconsole.log(error); // TypeError: Method get Map.prototype.size \n// called on incompatible receiver #\u003cMap\u003e \n} \n\n// re-encoding the decoded value throws \ntry { \nmsgpack5.encode(decoded); \n} catch (error) { \nconsole.log(error); // TypeError: Method Map.prototype.entries \n// called on incompatible receiver #\u003cMap\u003e \n} \n``` \n\nThis \"prototype poisoning\" is sort of a very limited inversion of a \nprototype pollution attack. Only the decoded value's prototype is \naffected, and it can only be set to `msgpack5` values (though if the \nvictim makes use of custom codecs, anything could be a `msgpack5` \nvalue). We have not found a way to escalate this to true prototype \npollution (absent other bugs in the consumer's code). \n\n### Patches\n\nVersions v5.2.1, v4.5.1, v3.6.1 include the fix.\n\n### Workarounds\n\nAlways validate incoming data after parsing before doing any processing.\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n* Open an issue in [example link to repo](http://example.com)\n* Email us at [example email address](mailto:example@example.com)","status":"published","published":"2021-03-12T22:44:17Z","modified":"2023-01-29T05:07:30Z","cvss":[{"version":"3.1","score":6.7,"vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H","level":"medium"}],"cwes":["CWE-915","CWE-1321"],"updated_at":"2026-11-02T14:36:26.667347Z"}],"nvd":[{"id":"CVE-2021-21368","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.","status":"published","published":"2021-03-12T17:15:12.863Z","modified":"2026-06-17T03:35:24.303Z","cvss":[{"version":"3.1","score":6.7,"vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H","level":"medium","assessor":"secondary"},{"version":"3.1","score":8.8,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","level":"high","assessor":"primary"},{"version":"2.0","score":6.5,"vector":"AV:N/AC:L/Au:S/C:P/I:P/A:P","level":"medium","assessor":"primary"}],"cwes":["CWE-915","CWE-1321"],"updated_at":"2026-11-02T14:06:50.805771Z"}],"osv":[{"id":"CVE-2021-21368","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.","status":"published","published":"2021-03-12T17:15:12.863Z","modified":"2026-10-20T10:54:29.148358958Z","cvss":[{"version":"3.1","score":8.8,"vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H","level":"high"}],"updated_at":"2026-11-02T15:44:18.004149Z"},{"id":"GHSA-gmjw-49p4-pcfm","title":"Prototype poisoning","description":"### Impact\n\nThe issue is as follows: when `msgpack5` decodes a map containing a \nkey `\"__proto__\"`, it assigns the decoded value to `__proto__`. As you \nare no doubt aware, `Object.prototype.__proto__` is an accessor \nproperty for the receiver's prototype. If the value corresponding to \nthe key `__proto__` decodes to an object or `null`, `msgpack5` sets \nthe decoded object's prototype to that value. \n\nAn attacker who can submit crafted MessagePack data to a service can \nuse this to produce values that appear to be of other types; may have \nunexpected prototype properties and methods (for example `length`, \nnumeric properties, and `push` et al if `__proto__`'s value decodes to \nan `Array`); and/or may throw unexpected exceptions when used (for \nexample if the `__proto__` value decodes to a `Map` or `Date`). Other \nunexpected behavior might be produced for other types. \n\nThere is no effect on the global prototype.\n\nAn example: \n\n```js \nconst msgpack5 = require('msgpack5')(); \n\nconst payload = {}; \nObject.defineProperty(payload, '__proto__', { \nvalue: new Map().set(1, 2), \nenumerable: true \n}); \n\nconst encoded = msgpack5.encode(payload); \nconsole.log(encoded); // \u003cBuffer 81 a9 5f 5f 70 72 6f 74 6f 5f 5f 81 01 02\u003e \n\nconst decoded = msgpack5.decode(encoded); \n\n// decoded's prototype has been overwritten \nconsole.log(Object.getPrototypeOf(decoded)); // Map(1) { 1 =\u003e 2 } \nconsole.log(decoded.get); // [Function: get] \n\n// decoded appears to most common typechecks to be a Map \nconsole.log(decoded instanceof Map); // true \nconsole.log(decoded.toString()); // [object Map] \nconsole.log(Object.prototype.toString.call(decoded)); // [object Map] \nconsole.log(decoded.constructor.name); // Map \nconsole.log(Object.getPrototypeOf(decoded).constructor.name); // Map \n\n// decoded is not, however, a Map \nconsole.log(Object.getPrototypeOf(decoded) === Map.prototype); // false \n\n// using decoded as though it were a Map throws \ntry { \ndecoded.get(1); \n} catch (error) { \nconsole.log(error); // TypeError: Method Map.prototype.get called \n// on incompatible receiver #\u003cMap\u003e \n} \ntry { \ndecoded.size; \n} catch (error) { \nconsole.log(error); // TypeError: Method get Map.prototype.size \n// called on incompatible receiver #\u003cMap\u003e \n} \n\n// re-encoding the decoded value throws \ntry { \nmsgpack5.encode(decoded); \n} catch (error) { \nconsole.log(error); // TypeError: Method Map.prototype.entries \n// called on incompatible receiver #\u003cMap\u003e \n} \n``` \n\nThis \"prototype poisoning\" is sort of a very limited inversion of a \nprototype pollution attack. Only the decoded value's prototype is \naffected, and it can only be set to `msgpack5` values (though if the \nvictim makes use of custom codecs, anything could be a `msgpack5` \nvalue). We have not found a way to escalate this to true prototype \npollution (absent other bugs in the consumer's code). \n\n### Patches\n\nVersions v5.2.1, v4.5.1, v3.6.1 include the fix.\n\n### Workarounds\n\nAlways validate incoming data after parsing before doing any processing.\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n* Open an issue in [example link to repo](http://example.com)\n* Email us at [example email address](mailto:example@example.com)","status":"published","published":"2021-03-12T22:44:17Z","modified":"2026-09-10T03:49:01.975317177Z","cvss":[{"version":"3.1","score":6.7,"vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:H/A:H","level":"medium"}],"cwes":["CWE-1321","CWE-915"],"updated_at":"2026-11-02T15:46:47.822182Z"}]}}