It’s a bad vulnerability, made worse by the fact that it remains even if you uninstall the Zoom app:
This vulnerability allows any website to forcibly join a user to a Zoom call, with their video camera activated, without the user’s permission.
On top of this, this vulnerability would have allowed any webpage to DOS (Denial of Service) a Mac by repeatedly joining a user to an invalid call.
Additionally, if you’ve ever installed the Zoom client and then uninstalled it, you still have a localhost web server on your machine that will happily re-install the Zoom client for you, without requiring any user interaction on your behalf besides visiting a webpage. This re-install ‘feature’ continues to work to this day.
Zoom didn’t take the vulnerability seriously:
This vulnerability was originally responsibly disclosed on March 26, 2019. This initial report included a proposed description of a ‘quick fix’ Zoom could have implemented by simply changing their server logic. It took Zoom 10 days to confirm the vulnerability. The first actual meeting about how the vulnerability would be patched occurred on June 11th, 2019, only 18 days before the end of the 90-day public disclosure deadline. During this meeting, the details of the vulnerability were confirmed and Zoom’s planned solution was discussed. However, I was very easily able to spot and describe bypasses in their planned fix. At this point, Zoom was left with 18 days to resolve the vulnerability. On June 24th after 90 days of waiting, the last day before the public disclosure deadline, I discovered that Zoom had only implemented the ‘quick fix’ solution originally suggested.
This is why we disclose vulnerabilities. Now, finally, Zoom is taking this seriously and fixing it for real.
EDITED TO ADD (8/8): Apple silently released a macOS update that removes the Zoom webserver if the app is not present.