The purpose of this thesis is to prevent websites located in public internet from accessing user's internal network through web browser. Acquired knowdledge about modern browser's security mechanism - same-origin policy and options of implementing the web browser extensions using WebExtensions, was used in the solution. Proposed solution is based on WebRequest API, which intercepts and modifies HTTP requests, and extends functionality of existing browser extension JavaScript Restrictor with the ability to detect and prevent the browser to be abused as a proxy for scanning and accessing user's internal network. The implemented solution was tested and accepted as a part of JavaScript Restrictor. The main benefit of this thesis is the protection from possible abusement of a web browser as a proxy, which is not present in existing extensions.