The web server that the Zoom Client installs on Macs can be abused to execute code remotely, security researchers have discovered. 

Zoom is a popular service that offers “enterprise video conferencing with real-time messaging and content sharing,” with support for both desktop and mobile. It offers end-to-end encryption and allows users to join meetings with a single click.

Earlier this week, security researcher Jonathan Leitschuh revealed that an attacker could force a Zoom user into joining a meeting by simply tricking them into clicking on a link. Thus, if the user hadn’t set an option to turn video off when joining a meeting, the attacker could access their video feed. 

The researcher also revealed that, upon installation, the Zoom Client sets up a web server onto the user’s machine, which is not removed when the application is uninstalled. The web server would allow an attacker to reinstall the Zoom application onto the user’s machine by simply sending them a link.

On Wednesday, Zoom issued an update to remove the web server from the user’s machines and it has included a new option in the application to allow users to remove the server at uninstall. 

Apple too stepped in and issued an update to completely remove the web server from all Macs, even if the user did not update the Zoom application or deleted it before Zoom’s own patch.

On Thursday, information emerged on a remote code execution vulnerability residing in the Zoom web server. Tracked as CVE-2019-13567, the vulnerability can be triggered via malicious links, if the Zoom Client is unreachable. 

“If the ZoomOpener daemon (aka the hidden web server) is running, but the Zoom Client is not installed or can't be opened, an attacker can remotely execute code with a maliciously crafted launch URL,” a National Vulnerability Database advisory reads. 

Proof of the vulnerability being exploited has been already published online, but the updates that Zoom and Apple already published were meant to remove the vulnerable web server and keep users safe. 

With Zoom popular among enterprises, administrators should make sure that the Zoom client is updated on all Macs in their environments, or that the web server has been removed from those where the application was uninstalled.

Related: Apple Steps in: Removes Zoom Web Server From All Macs

Related: Vulnerability Gives Attackers Remote Access to Zoom Users’ Cameras