CVE-2026-53410

Executive Summary

CVE-2026-53410 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 7.0 (High), indicating significant potential for impact, though exploitation requires local access and a race condition to be won. No public proof-of-concept (PoC) or exploit code is currently available, and the patch status remains unknown. The EPSS score is 0.0%, suggesting low immediate exploitation activity in the wild.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
• Base Score: 7.0 (High)
• Attack Vector (AV): Local (L) – The attacker must have local access to the system.
• Attack Complexity (AC): High (H) – Exploitation requires winning a race condition, which is non-trivial.
• Privileges Required (PR): Low (L) – The attacker needs only low-level privileges (e.g., a standard user account).
• User Interaction (UI): None (N) – No user interaction is required beyond the attacker's own actions.
• Scope (S): Unchanged (U) – The vulnerability does not cross a trust boundary.
• Confidentiality (C): High (H) – Successful exploitation could lead to full disclosure of sensitive data.
• Integrity (I): High (H) – The attacker could modify system files or configurations.
• Availability (A): High (H) – The attacker could cause a denial of service or system instability.

Threat Level: High. While the attack complexity is high, the potential for full system compromise makes this a critical concern for organizations using Zoom Clients on Windows.

Deep Dive & Mechanics

The vulnerability is a classic TOCTOU race condition. During the installation or uninstallation of Zoom Clients for Windows, the software performs a check on a file or registry key (e.g., checking permissions or existence) and then later uses that resource. An attacker with local access can exploit the window between the check and the use to replace the legitimate resource with a malicious one (e.g., a symbolic link or a malicious DLL). This can lead to privilege escalation, as the Zoom installer/uninstaller runs with elevated privileges (e.g., SYSTEM or Administrator).

Attack Scenario:

1. An authenticated local user (low privileges) initiates the Zoom uninstallation process.
2. The Zoom uninstaller checks a file path (e.g., C:\ProgramData\Zoom\uninstall.exe) to verify it is safe.
3. Before the uninstaller uses that file, the attacker quickly replaces it with a symbolic link pointing to a system-critical file (e.g., C:\Windows\System32\config\SAM).
4. The uninstaller, running with high privileges, follows the symbolic link and performs an operation (e.g., delete or modify) on the target file, leading to privilege escalation or system compromise.

Vulnerable vs. Patched Code

Since no patch details are available, we can only hypothesize the vulnerable code pattern. A typical TOCTOU vulnerability in Windows installers might look like this:

Vulnerable Code (C++ pseudo-code):

// Check if the file is safe
if (CheckFilePermissions(L"C:\\ProgramData\\Zoom\\temp.dll")) {
    // Use the file - race window here
    LoadLibrary(L"C:\\ProgramData\\Zoom\\temp.dll");
}

Patched Code (Hypothetical):

// Use a secure API that atomically checks and uses the file
HANDLE hFile = CreateFile(L"C:\\ProgramData\\Zoom\\temp.dll", 
                          GENERIC_READ, FILE_SHARE_READ, NULL, 
                          OPEN_EXISTING, FILE_FLAG_OPEN_REPARSE_POINT, NULL);
if (hFile != INVALID_HANDLE_VALUE) {
    // Use the file handle directly, avoiding TOCTOU
    LoadLibraryFromHandle(hFile);
    CloseHandle(hFile);
}

The patched version uses CreateFile with FILE_FLAG_OPEN_REPARSE_POINT to prevent following symbolic links, and then uses the handle directly, eliminating the race window.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Windows 10/11 with Zoom Client installed (vulnerable version).
• Local user account with low privileges.
• Ability to run scripts or executables.

Exploitation Steps:

1. Identify the vulnerable process: Monitor the Zoom installer/uninstaller process (e.g., ZoomInstaller.exe or ZoomUninstaller.exe) to find the race window.
2. Create a malicious symbolic link: Use a tool like mklink or a PowerShell script to create a symbolic link that points to a high-value target (e.g., C:\Windows\System32\config\SAM).
3. Race the condition: Use a tight loop to repeatedly replace the legitimate file with the symbolic link during the installation/uninstallation process.

Example PowerShell Script (Conceptual):

# Monitor for Zoom uninstaller process
while ($true) {
    if (Get-Process -Name "ZoomUninstaller" -ErrorAction SilentlyContinue) {
        # Create symbolic link
        cmd /c "mklink C:\ProgramData\Zoom\temp.dll C:\Windows\System32\config\SAM"
        Start-Sleep -Milliseconds 10
        Remove-Item "C:\ProgramData\Zoom\temp.dll" -Force
    }
}

Note: This is a conceptual example. Actual exploitation would require precise timing and may need to be run multiple times.

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual symbolic link creation events (Event ID 4663 for file system, or Sysmon Event ID 1 for process creation with mklink).
• Look for processes running with high integrity (e.g., SYSTEM) accessing user-writable directories like C:\ProgramData\Zoom\.
• Use Sysmon to log file creation and deletion events in the Zoom installation directory.

Defense:

• Apply the principle of least privilege to the Zoom installer/uninstaller.
• Use Windows Defender Application Control (WDAC) or AppLocker to restrict execution of untrusted binaries.
• Enable Windows Defender Credential Guard to protect high-value targets like the SAM file.
• Monitor for TOCTOU patterns using ETW (Event Tracing for Windows) for file operations.

Indicators of Compromise (IOCs)

• File System:
  • Unexpected symbolic links in C:\ProgramData\Zoom\ or C:\Users\<username>\AppData\Local\Zoom\.
  • Creation of files named temp.dll or similar in Zoom directories.
• Process:
  • ZoomInstaller.exe or ZoomUninstaller.exe spawning child processes like cmd.exe or powershell.exe.
  • High number of file operations (create/delete) in a short time from a low-privilege process.
• Registry:
  • Unusual modifications to HKLM\SOFTWARE\Zoom or HKCU\Software\Zoom during installation/uninstallation.
• Network:
  • No specific network IOCs, as this is a local privilege escalation.

Detection Engineering

YARA Rule

rule CVE_2026_53410_TOCTOU_Zoom {
    meta:
        description = "Detects potential TOCTOU exploitation attempts against Zoom Clients for Windows"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-53410"
    strings:
        $s1 = "mklink" ascii wide nocase
        $s2 = "ZoomInstaller" ascii wide nocase
        $s3 = "ZoomUninstaller" ascii wide nocase
        $s4 = "temp.dll" ascii wide nocase
        $s5 = "CreateSymbolicLink" ascii wide nocase
    condition:
        (any of ($s1, $s5) and any of ($s2, $s3)) or
        (any of ($s2, $s3) and $s4)
}

Sigma Rule

title: CVE-2026-53410 - TOCTOU Race Condition in Zoom Clients
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of TOCTOU vulnerability in Zoom Clients for Windows
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-53410
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\ZoomInstaller.exe'
            - '\ZoomUninstaller.exe'
        ParentImage|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
    condition: selection
falsepositives:
    - Legitimate administrative scripts
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-53410 - Potential TOCTOU Exploitation via Zoom"; flow:to_server; content:"ZoomInstaller"; nocase; content:"mklink"; nocase; distance:0; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected systems: If possible, disconnect vulnerable Zoom Clients from the network until a patch is applied.
2. Restrict local access: Limit the number of users with local access to systems running Zoom Clients.
3. Monitor for exploitation: Deploy the provided YARA, Sigma, and Suricata rules to detect potential exploitation attempts.

Long-term Fixes:

1. Apply vendor patch: Monitor Zoom's security advisories for an official patch. Once available, apply it immediately.
2. Upgrade Zoom Client: If a patched version is released, upgrade all affected clients.
3. Implement application whitelisting: Use AppLocker or WDAC to prevent unauthorized executables from running.
4. Enable advanced auditing: Configure Windows audit policies to log file system and process creation events for Zoom directories.

Workarounds (if no patch available):

• Disable automatic updates and uninstall Zoom Clients until a patch is released.
• Use a third-party security tool to monitor for TOCTOU attacks.
• Implement file system permissions to prevent low-privilege users from writing to Zoom installation directories.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert