Advisory ID: SYSS-2020-044  
Product: Zoom  
Manufacturer: Zoom Video Communications, Inc.  
Affected Version(s): 5.4.3 (54779.1115)  
5.5.4 (13142.0301)  
Tested Version(s): 5.4.3 (54779.1115)  
5.5.4 (13142.0301)  
Vulnerability Type: Exposure of Resource to Wrong Sphere (CWE-668)  
Risk Level: Medium  
Solution Status: Open  
Manufacturer Notification: 2020-12-02  
Solution Date: -  
Public Disclosure: 2021-03-18  
CVE Reference: CVE-2021-28133  
Authors of Advisory: Michael Strametz, Matthias Deeg  
Zoom is a video conferencing and messaging software with support for  
many different devices.  
Some of the supported features as described by the manufacturer are  
(see [1]):  
* Unparalleled usability  
Enable quick adoption with meeting capabilities that make it easy to  
start, join, and collaborate across any device.  
* Join anywhere, on any device  
Zoom Meetings syncs with your calendar system and delivers streamlined  
enterprise-grade video conferencing from desktop and mobile.  
* Powerful meeting security  
Robust security settings ensure disruption-free meetings. Encryption,  
role-based security, Passcode protection, Waiting Rooms and more.  
Due to a security issue concerning the "share screen" functionality,  
screen contents of applications which are not explicitly shared by the  
screen-sharing user can be seen by other meeting participants.  
Vulnerability Details:  
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.  
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.  
Depending on the unintentionally shared data, this short exposure of  
screen contents may be a more or less severe security issue.  
A participant of a Zoom meeting recording a meeting using a screen  
recorder software may afterwards have access to sensitive data of  
other users which is accessible in a few frames of the recorded video.  
Proof of Concept (PoC):  
SySS could successfully demonstrate the described attack concerning  
screen recordings of Zoom meetings with unintentionally shared screen  
contents both using the current Windows and Linux Zoom client software.  
In this attack scenario, the two users Alice and Mallory are in the  
same Zoom meeting and Alice shares her web browser window via the "share  
screen" functionality.  
Mallory records her whole desktop screen using a screen recorder  
software, for instance SimpleScreenRecorder [3].  
Between showing different things in her shared web browser window, Alice  
uses another application whose application window happens to overlay  
the shared web browser window.  
The contents of this other application window, which is explicitly not  
shared with Mallory, can sometimes briefly be seen by Mallory.  
When watching the created screen recording, Mallory can pause the video  
at will and thus see the unintentionally shared application window  
contents from Alice.  
A SySS proof of concept video illustrating this security issue is  
available on our SySS Pentest TV YouTube channel [5].  
SySS GmbH is not aware of a fix for the described security issue.  
Please contact the software manufacturer for further information.  
Disclosure Timeline:  
2020-12-02: Vulnerability reported to manufacturer  
2020-12-02: Manufacturer acknowledges receipt of security advisory  
2020-12-02: Manufacturer asks for more information  
2020-12-03: SySS provides more information concerning the security issue  
2020-12-03: Manufacturer confirms reproducing the security issue in both  
the Windows and the Linux client and asks further questions  
2020-12-04: SySS answers open questions  
2020-12-04: Manufacturer responds and will look into the reported  
security issue  
2021-01-21: SySS asks for status update  
2021-02-01: SySS asks for status update  
2021-03-18: Public release of security advisory  
[1] Product Website for Zoom  
[2] SySS Security Advisory SYSS-2020-044  
[3] SySS GmbH, SySS Responsible Disclosure Policy  
[4] GitHub Website of SimpleScreenRecorder  
[5] SySS Proof of Concept Video: Zoom Unintended Screen Sharing Issue  
This security vulnerability was found by Michael Strametz of SySS Cyber  
Security GmbH (Austria) and Matthias Deeg of SySS GmbH.  
Public Key:  
Key Fingerprint: AD50 E8B8 4E6E 5E00 F45F CE35 744F A11A 2EAC 214D  
E-Mail: matthias.deeg (at)  
Public Key:  
Key fingerprint = D1F0 A035 F06C E675 CDB9 0514 D9A4 BF6A 34AD 4DAB  
The information provided in this security advisory is provided "as is"  
and without warranty of any kind. Details of this security advisory may  
be updated in order to provide as accurate information as possible. The  
latest version of this security advisory is available on the SySS website.  
Creative Commons - Attribution (by) - Version 3.0