Abstract: A computer-implemented method of preserving privacy of private user data using third party extensions on a web application platform is disclosed. The method includes receiving private and non-private user data from a user, providing non-private user data to a remote extension component, receiving remote extension data from the remote extension component and storing it on a local extension cache on the web application platform, providing private user data and remote extension data to a local extension component installed on the platform, executing local extension component instructions in a restricted operating system environment on the platform, receiving local extension data from the local extension component, and providing the local extension data to a user. A non-transitory computer-readable medium storing instructions to implement the method of preserving privacy of private user data using third party extensions on a web application platform, and a related system are also disclosed.