New Zoom Screen-Sharing Bug Lets Other Users Access Restricted Apps

A newly discovered glitch in Zoom’s screen sharing feature can
accidentally leak sensitive information to other attendees in a
call, according to the latest findings.

Tracked as CVE-2021-28133^[1], the unpatched security
vulnerability makes it possible to reveal contents of applications
that are not shared, but only briefly, thereby making it harder to
exploit it in the wild.

It’s worth pointing out that the screen sharing^[2]
functionality in Zoom lets users share an entire desktop or phone
screen or limit sharing to one or more specific applications or a
portion of a screen. The issue stems from the fact that a second
application that’s overlayed on top of an already shared
application can reveal its contents for a short period of time.

“When a Zoom user shares a specific application window via the
‘share screen’ functionality, other meeting participants can
briefly see contents of other application windows which were not
explicitly shared,” SySS researchers Michael Strametz and Matthias
Deeg noted^[3]. “The contents of not
shared application windows can, for instance, be seen for a short
period of time by other users when those windows overlay the shared
application window and get into focus.”

The flaw, which was tested on versions 5.4.3 and 5.5.4 across
both Windows and Linux clients, is said to have been disclosed^[4]
to the videoconferencing company on December 2, 2020. The lack of a
fix even after three months could be attributed in part to the
difficulty in exploiting the vulnerability.

But nonetheless, this could have serious consequences depending
on the nature of the inadvertently shared data, the researchers
warned, adding a malicious participant of a Zoom meeting can take
advantage of the weakness by making use of a screen capture tool to
record the meeting and playback the recording to view the private
information.

We have reached out to Zoom for more details on the fix, and we
will update the story if we hear back.