CVE-2021-28133: Zoom Information Disclosure Vulnerability

CVE-2021-28133 Overview

CVE-2021-28133 is an information disclosure vulnerability affecting Zoom versions through 5.5.4. The flaw exists in Zoom's Screen Sharing functionality, where meeting participants can briefly view contents of application windows that were explicitly not shared by another participant. When a user shares a specific application window, other windows that overlay the shared window or come into focus can be momentarily exposed to all meeting attendees.

This vulnerability poses a significant privacy risk in enterprise and personal environments where sensitive information may be displayed on secondary windows during a Zoom meeting. An attacker could leverage a separate screen-recording application to capture and analyze these brief exposures of private content.

Critical Impact

Meeting participants may unintentionally expose sensitive data from non-shared application windows, potentially revealing confidential documents, credentials, or private communications to unauthorized viewers.

Affected Products

• Zoom through version 5.5.4
• Zoom desktop client for Windows, macOS, and Linux
• All Zoom meeting participants with screen sharing capability

Discovery Timeline

• 2021-03-18 - CVE-2021-28133 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-28133

Vulnerability Analysis

This information exposure vulnerability (CWE-200) occurs due to improper handling of window rendering during screen sharing sessions. When Zoom captures and transmits a specific application window, the client fails to properly mask or exclude content from other overlapping windows before sending the video stream to meeting participants.

The vulnerability requires user interaction in the sense that the victim must be actively sharing their screen. However, no special privileges are required by the attacker—any meeting participant can observe the leaked information. The exposure typically occurs during window focus changes or when other applications momentarily overlap the shared window area.

The severity of the information leak depends entirely on what data happens to be displayed in the non-shared windows at the moment of exposure. This could range from innocuous content to highly sensitive business information, personal data, authentication credentials, or confidential communications.

Root Cause

The root cause lies in Zoom's screen capture mechanism not properly implementing window exclusion filters. When capturing the shared window's region, the application briefly includes pixel data from other windows that occupy the same screen coordinates, particularly during window state transitions such as gaining or losing focus, minimizing, or resizing.

The screen capture pipeline lacks adequate synchronization between the window focus state and the frame capture timing, resulting in race conditions where frames containing unintended content are transmitted before the exclusion filter can be applied.

Attack Vector

This vulnerability is exploited through a network-based attack vector where an attacker participates in a Zoom meeting with screen sharing enabled. The attack is entirely passive from the attacker's perspective—they simply need to observe the shared screen stream and wait for unintended content to appear.

The attack methodology involves:

1. Joining a Zoom meeting where another participant is sharing their screen
2. Running a screen recording application locally to capture the entire meeting
3. Waiting for the victim to interact with other windows while sharing
4. Reviewing recorded footage to extract briefly exposed sensitive information

An attacker can increase their chances of capturing sensitive data by prolonging the meeting or engaging the victim in activities that require them to reference other applications while continuing to share their screen.

Detection Methods for CVE-2021-28133

Indicators of Compromise

• Unusual screen recording processes running during Zoom meetings
• Meeting participants requesting extended screen sharing sessions without clear purpose
• Post-meeting reports of sensitive data exposure
• Unexpected access to confidential information by meeting attendees

Detection Strategies

• Monitor for screen recording applications launched concurrently with Zoom meetings
• Implement Data Loss Prevention (DLP) tools that can detect sensitive content in video streams
• Review meeting recordings for evidence of unintended content exposure
• Analyze Zoom client logs for anomalous screen sharing behavior patterns

Monitoring Recommendations

• Enable endpoint detection and response (EDR) solutions to track screen capture activity
• Implement user behavior analytics to identify suspicious meeting patterns
• Configure alerts for sensitive application windows being opened during active screen sharing sessions
• Deploy SentinelOne Singularity to monitor for potential data exfiltration attempts during video conferencing

How to Mitigate CVE-2021-28133

Immediate Actions Required

• Update Zoom client to the latest available version beyond 5.5.4
• Train users to close or minimize all sensitive applications before initiating screen sharing
• Consider using virtual desktops or separate user profiles for meetings requiring screen sharing
• Disable automatic window focus changes during screen sharing sessions where possible

Patch Information

Zoom has acknowledged this vulnerability and users should consult the Zoom Security Bulletin for the latest patch information and updated client versions. Organizations should implement a policy requiring all Zoom clients to be updated to the most recent version available.

For additional technical details, refer to the SYSS Security Advisory and the Packet Storm Security Report.

Workarounds

• Use Zoom's "Share Screen" with "Desktop" option only when absolutely necessary, and close all sensitive applications first
• Share individual application windows only after ensuring no sensitive data exists in overlapping windows
• Enable a separate virtual desktop exclusively for screen sharing content
• Consider using browser-based Zoom when possible for more isolated screen sharing
• Implement a "clean desktop" policy where users prepare a dedicated workspace before sharing

# Configuration example - Create a clean desktop script for pre-meeting preparation
#!/bin/bash
# Close sensitive applications before Zoom screen sharing
pkill -f "password-manager"
pkill -f "email-client"
pkill -f "chat-application"
# Switch to dedicated screen sharing virtual desktop
wmctrl -s 2
echo "Desktop prepared for secure screen sharing"