FreeScout is a free, self-hosted help desk and shared mailbox. Versions of FreeScout prior to 1.8.139 contain a Prototype Pollution vulnerability in the /public/js/main.js source file. The Prototype Pollution arises because the getQueryParam Function recursively merges an object containing user-controllable properties into an existing object (For URL Query Parameters Parsing), without first sanitizing the keys. This can allow an attacker to inject a property with a key __proto__, along with arbitrarily nested properties. The merge operation assigns the nested properties to the params objects prototype instead of the target object itself. As a result, the attacker can pollute the prototype with properties containing harmful values, which are then inherited by user-defined objects and subsequently used by the application dangerously. The vulnerability lets an attacker control properties of objects that would otherwise be inaccessible. If the application subsequently handles an attacker-controlled property in an unsafe way, this can potentially be chained with other vulnerabilities like DOM-based XSS, Open Redirection, Cookie Manipulation, Link Manipulation, HTML Injection, etc. Version 1.8.139 contains a patch for the issue.
The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.
By adding or modifying attributes of an object prototype, it is possible to create attributes that exist on every object, or replace critical attributes with malicious ones. This can be problematic if the product depends on existence or non-existence of certain attributes, or uses pre-defined attributes of object prototype (such as hasOwnProperty, toString or valueOf). This weakness is usually exploited by using a special attribute of objects called proto, constructor or prototype. Such attributes give access to the object prototype. This weakness is often found in code that assigns object attributes based on user input, or merges or clones objects recursively.