CVE-2022-46175: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

JSON5 is an extension to the popular JSON file format that aims to be easier to write and maintain by hand (e.g. for config files). The parse method of the JSON5 library before and including version 2.2.1 does not restrict parsing of keys named __proto__, allowing specially crafted strings to pollute the prototype of the resulting object. This vulnerability pollutes the prototype of the object returned by JSON5.parse and not the global Object prototype, which is the commonly understood definition of Prototype Pollution. However, polluting the prototype of a single object can have significant security impact for an application if the object is later used in trusted operations. This vulnerability could allow an attacker to set arbitrary and unexpected keys on the object returned from JSON5.parse. The actual impact will depend on how applications utilize the returned object and how they filter unwanted keys, but could include denial of service, cross-site scripting, elevation of privilege, and in extreme cases, remote code execution. JSON5.parse should restrict parsing of __proto__ keys when parsing JSON strings to objects. As a point of reference, the JSON.parse method included in JavaScript ignores __proto__ keys. Simply changing JSON5.parse to JSON.parse in the examples above mitigates this vulnerability. This vulnerability is patched in json5 version 2.2.2 and later.