Ironically, even if you had ever installed the Zoom client on your device and simply uninstalled it, a remote attacker can still activate your webcam.
Zoom is one of the most popular cloud-based meeting platforms that provide video, audio, and screen sharing options to users, allowing them to host webinars, teach online courses, conduct online training, or join virtual meetings online.
In a Medium post published today, cybersecurity researcher Jonathan Leitschuh disclosed details of an unpatched critical security vulnerability (CVE-2019-13450) in the Zoom client app for Apple Mac computers, which if combined with a separate flaw, could allow attackers to execute arbitrary code on the targeted systems remotely.
Jonathan responsibly reported the security vulnerability to the affected company over 90 days ago, but the Zoom team failed to offer a proper security patch, putting privacy and security of its over 4 million users at risk.
The vulnerability leverages the click-to-join feature of the popular conferencing software that has been designed to automatically activate Zoom app installed on the system, allowing participants to quickly join a video-meeting through their web browser as soon as they click on an invite link, for example, https://zoom.us/j/492468757.
Jonathan found that to offer this feature the Zoom software runs a local web server on the system—on port 19421—that "insecurely" receives commands through the HTTPS GET paraments and any website in your opened web browser can interact with it.
"Enabling 'Participants: On' when setting up a meeting, I discovered that anyone joining my meeting automatically had their video connected," Jonathan said.
As soon as Mac users with Zoom client installed on their system visits the malicious website, it will forcefully launch the Zoom app and turn on their webcam, exposing them to attackers.
Simply uninstalling the software is not enough to get rid of this problem as Jonathan explained the click-to-join feature also accepts a command that automatically reinstalls Zoom without users' intervention or permission.
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Besides turning on the webcam, the vulnerability can also be abused to DoS attack the targeted Mac computer by simply sending a large number of repeated GET requests to the local server.
"Zoom did end up patching this vulnerability, but all they did was prevent the attacker from turning on the user's video camera," Jonathan said. "They did not disable the ability for an attacker to forcibly join a call anyone visiting a malicious site."
The vulnerability affects the latest version 4.4.4 of Zoom app for Mac.
However, the good news is that users can still fix this issue at their ends. All you need to do is manually disable the setting that allows Zoom to automatically turn your webcam on when joining a meeting.
For this, just go into the Zoom settings window and enable the "Turn off my video when joining a meeting" setting.
You can also run a series of Terminal commands, which you can find at the bottom of Jonathan's post, to uninstall the web server completely.
Zoom Responded to the Researcher's Findings
In a statement published later today, the company has acknowledged the issue but also added that "because the Zoom client user interface runs in the foreground upon launch, it would be readily apparent to the user that they had unintentionally joined a meeting and they could change their video settings or leave immediately."
Besides this, the company said they have "no indication" if the reported issues had been exploited to breach the privacy of any of their users.
Zoom also acknowledged other related concerns over its software and said the local denial of service (DOS) vulnerability reported by the researcher has already been fixed in May 2019, though the company said it did not force its users to update because it is "empirically a low-risk vulnerability."
The company also said it installs a limited-functionality web server when users install Zoom client to offer one-click-to-join meetings feature that could avoid users' extra click before joining every meeting, but it did not comment on why the server remains installed on the local machine even when a user chooses to uninstall the client software.