almost every application is vulnerable to attacks. Pick an application (e.g., browser, email client), explain the strongest threat for it and any existing solutions to prevent it. My Application is WhatsApp

Here is an example about google chrome (this is a classmate so please do not duplicate, but it should be as simple as this):

Google chrome is considered a web browser that can be used on a mobile device or a computer. Using the internet in general makes the open up the user to a lot of threats within the web. While using Google chrome there are many vulnerabilities that users are exposed to. The vulnerabilities are as follows:

use after free vulnerability in auto fill

use after free vulnerability in drag and drop (CVE-2021-21107)

use after free vulnerability in media (CVE-2021-21108)

use after free vulnerability in payments (CVE-2021-21109)

use after free vulnerability in safe browsing (CVE-2021-21110, CVE-2021-21115)

An insufficient policy enforcement vulnerability in WebUI (CVE-2021-21111)

use after free in Blink (CVE-2021-21112)

A heap buffer overflow vulnerability in Skia (CVE-2021-21113)

insufficient data validation vulnerability in networking (CVE-2020-16043)

use after free vulnerability in audio (CVE-2021-21114)

out of bounds write vulnerability in V8 (CVE-2020-15995)

A heap buffer overflow vulnerability in audio (CVE-2021-21116)

If an attacker succeeds through these severe vulnerabilities, they could execute arbitrary code within the browser. Depending on the application and what privileges are associated the attacker can get access to that data and affect it. Updating the system regularly could help with the vulnerabilities within the system. Run all software as a user who has no administrative privileges to prevent a successful attack. Inform all users to not to visit and un-trusted websites or links that are provided by unknown sources.