The WhatsApp breach was discovered by researcher Gal Weizman and involves a technique called Cross-Site Scripting (XSS). Through it, a malicious person could send texts and manipulated links to a user, in a phishing scam, and plant a malicious file on the victim's PC, which would give access to his files, links and texts. In the test, the PerimeterX specialist used his knowledge to access local files on a computer.
In response to the TechRadar Pro website, a WhatsApp spokesman said the company works with digital security researchers from around the world to detect these bugs before the criminals. The company also reported that the problem was fixed in December 2019.
For the end user, there is nothing that should be done at the moment, just continue with good practices of not clicking on suspicious links, even if it came from a known person. Be wary and always ask what it is and the origin of the URL or video received.