An adversary, through a previously installed malicious application, injects code into the context of a web page displayed by a WebView component. Through the injected code, an adversary is able to manipulate the DOM tree and cookies of the page, expose sensitive information, and can launch attacks against the web application from within the web page.
The only known mitigation to this type of attack is to keep the malicious application off the system. There is nothing that can be done to the target application to protect itself from a malicious application that has been installed and executed.