CVE-2026-53411

Executive Summary

CVE-2026-53411 is a Time-of-Check to Time-of-Use (TOCTOU) race condition vulnerability in the installation and uninstallation process of certain Zoom Clients for Windows. This flaw allows an authenticated local user to escalate privileges on the affected system. The vulnerability has a CVSS score of 7.8 (High), indicating significant impact on confidentiality, integrity, and availability. As of this report, no patch is available, and no public proof-of-concept (PoC) or exploit code has been released. The EPSS score is 0.0%, suggesting low exploitation likelihood in the wild, but the risk remains for targeted attacks.

CVSS Breakdown & Threat Level

• CVSS Score: 7.8 (High)
• Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
• Attack Vector (AV): Local – The attacker must have local access to the system.
• Attack Complexity (AC): High – Exploitation requires precise timing and race condition manipulation.
• Privileges Required (PR): Low – The attacker needs initial low-privileged access.
• User Interaction (UI): None – No user action is required beyond the attacker's own actions.
• Scope (S): Unchanged – The vulnerability does not affect other components.
• Confidentiality (C): High – Full data access possible.
• Integrity (I): High – Full system modification possible.
• Availability (A): High – Potential for denial of service.

Threat Level: High. While exploitation is complex, successful privilege escalation can lead to full system compromise.

Deep Dive & Mechanics

The vulnerability resides in the installation and uninstallation process of Zoom Clients for Windows. A TOCTOU race condition occurs when the installer/uninstaller checks a file or registry key's state (e.g., permissions, existence) and then performs an operation based on that check. An attacker can manipulate the resource between the check and the use, causing the installer to perform privileged operations on attacker-controlled files.

Attack Scenario:

1. An authenticated low-privileged user triggers the Zoom installer or uninstaller.
2. The installer checks a temporary file or directory for a condition (e.g., "does this file exist?").
3. The attacker, via a separate thread or process, replaces or modifies that file/directory with a symbolic link to a privileged location (e.g., C:\Windows\System32).
4. The installer then performs an operation (e.g., write, delete, or move) on the attacker-controlled path, leading to privilege escalation.

Root Cause: Lack of proper locking or atomic operations during file system interactions in the installer/uninstaller.

Vulnerable vs. Patched Code

Since no patch is available, a hypothetical fix would involve:

• Vulnerable Code (Conceptual):

  if (File.Exists(tempPath))
  {
      File.Delete(tempPath); // TOCTOU: tempPath could be replaced with a symlink
  }
  

• Patched Code (Conceptual):

  using (var fs = new FileStream(tempPath, FileMode.Open, FileAccess.Read, FileShare.None))
  {
      // Perform operation with exclusive lock
      File.Delete(tempPath);
  }
  

  The patched version uses a file lock to prevent race conditions.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Local access to a Windows system with Zoom Client installed.
• Low-privileged user account.

Exploitation Steps:

1. Identify the vulnerable installer/uninstaller process (e.g., ZoomInstaller.exe).
2. Monitor file system operations using tools like Process Monitor.
3. Create a race condition script that continuously replaces a target file/directory with a symbolic link to a privileged location (e.g., C:\Windows\System32\evil.dll).
4. Trigger the installer/uninstaller while the script is running.
5. If successful, the installer will perform an operation on the privileged location, allowing privilege escalation.

Example Race Condition Script (PowerShell):

while ($true) {
    Remove-Item -Path "C:\Temp\zoom_temp" -Force -ErrorAction SilentlyContinue
    New-Item -ItemType SymbolicLink -Path "C:\Temp\zoom_temp" -Target "C:\Windows\System32"
    Start-Sleep -Milliseconds 10
}

Blue Team (Detection & Prevention)

Detection:

• Monitor for abnormal file system operations by Zoom installer/uninstaller processes.
• Detect creation of symbolic links by low-privileged users.
• Alert on high-frequency file operations targeting temporary directories.

Prevention:

• Apply the principle of least privilege to user accounts.
• Use Windows Defender Exploit Guard to block symbolic link creation.
• Implement application whitelisting for installer binaries.

Indicators of Compromise (IOCs)

• File System:
  • Creation of symbolic links in C:\Temp or %TEMP% directories.
  • Unusual file operations by ZoomInstaller.exe or ZoomUninstaller.exe.
• Process:
  • Multiple instances of ZoomInstaller.exe running concurrently.
  • High CPU usage by race condition scripts.
• Registry:
  • Unauthorized modifications to HKEY_LOCAL_MACHINE\SOFTWARE\Zoom or similar keys.

Detection Engineering

YARA Rule

rule CVE_2026_53411_TOCTOU_Zoom {
    meta:
        description = "Detects potential TOCTOU exploitation attempts against Zoom installer"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-53411"
    strings:
        $s1 = "ZoomInstaller.exe" nocase
        $s2 = "ZoomUninstaller.exe" nocase
        $s3 = "SymbolicLink" nocase
        $s4 = "\\Temp\\" nocase
    condition:
        (any of ($s1, $s2)) and ($s3 or $s4)
}

Sigma Rule

title: CVE-2026-53411 TOCTOU Race Condition in Zoom Installer
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of TOCTOU vulnerability in Zoom Client for Windows
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-53411
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\\ZoomInstaller.exe'
            - '\\ZoomUninstaller.exe'
        CommandLine|contains:
            - '\\Temp\\'
            - 'SymbolicLink'
    condition: selection
falsepositives:
    - Legitimate Zoom updates
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"CVE-2026-53411 Potential Zoom TOCTOU Exploit"; flow:to_server,established; content:"ZoomInstaller.exe"; http_uri; nocase; content:"SymbolicLink"; distance:0; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Restrict Local Access: Limit user accounts to least privilege.
2. Monitor Logs: Enable auditing for file system and process creation events.
3. Block Symbolic Links: Use Group Policy to restrict symbolic link creation to administrators.

Long-Term Fix:

• Await an official patch from Zoom. Apply it as soon as it becomes available.
• Consider using application control solutions (e.g., AppLocker) to whitelist trusted installer binaries.

Workarounds:

• Uninstall Zoom Client if not essential.
• Use the web-based version of Zoom instead of the desktop client.

Valters IT