CVE-2025-49456

Executive Summary

CVE-2025-49456 is a race condition vulnerability in the installer of certain Zoom Clients for Windows. The flaw allows an unauthenticated attacker with local access to impact application integrity by exploiting a time-of-check/time-of-use (TOCTOU) race condition during the installation process. With a CVSS score of 6.2 (Medium), the vulnerability requires local access and high attack complexity, but no privileges or user interaction. The EPSS score is 0.0%, indicating very low exploitation likelihood in the wild. No public proof-of-concept, exploit code, or patch is currently available. The vendor (Zoom) has not released a fix or official advisory details.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N
• Base Score: 6.2 (Medium)
• Attack Vector: Local (AV:L) – attacker must have physical or logical local access to the system.
• Attack Complexity: High (AC:H) – successful exploitation requires precise timing and knowledge of the installation process.
• Privileges Required: None (PR:N) – no authentication needed.
• User Interaction: None (UI:N) – no victim action required.
• Scope: Unchanged (S:U) – vulnerability does not affect other components.
• Confidentiality Impact: None (C:N) – no data disclosure.
• Integrity Impact: High (I:H) – attacker can modify files or configuration during installation.
• Availability Impact: None (A:N) – no denial of service.

Threat Level: Medium. The high attack complexity and local access requirement reduce immediate risk, but integrity compromise could lead to persistent backdoors or privilege escalation.

Deep Dive & Mechanics

The vulnerability is a classic TOCTOU race condition in the Zoom installer for Windows. During the installation process, the installer performs a series of file operations (e.g., copying, moving, or setting permissions) based on the state of the filesystem at a given time. An attacker with local access can monitor the installation and, at the precise moment between a check (e.g., “does file X exist?”) and a use (e.g., “write to file X”), replace or modify a target file or directory. This can cause the installer to write malicious content to a privileged location, overwrite legitimate binaries, or alter configuration files.

Key factors:

• The race window is narrow, requiring high attack complexity.
• The attacker must have local access (e.g., via a compromised user account or physical access).
• The impact is limited to integrity; no confidentiality or availability loss.
• The vulnerability affects only the installer, not the running Zoom client.

Vulnerable vs. Patched Code

No specific code snippets are available because no patch or exploit has been published. However, the vulnerable pattern is a TOCTOU race condition in the installer’s file operations. A typical vulnerable code pattern (pseudocode) might look like:

// Vulnerable: TOCTOU race
if (file_exists(target_path)) {
    // Attacker can replace target_path between check and use
    delete_file(target_path);
}
copy_file(source, target_path);

A patched version would use atomic operations or file locking:

// Patched: Use atomic rename or lock
HANDLE hLock = CreateFile(target_path, GENERIC_WRITE, 0, NULL, OPEN_EXISTING, FILE_FLAG_DELETE_ON_CLOSE, NULL);
if (hLock != INVALID_HANDLE_VALUE) {
    // Lock acquired, safe to proceed
    MoveFileEx(source, target_path, MOVEFILE_REPLACE_EXISTING | MOVEFILE_WRITE_THROUGH);
    CloseHandle(hLock);
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a Windows system where Zoom installer is being run.
• Ability to run a monitoring script or tool (e.g., PowerShell, Python) with low privileges.
• Knowledge of the installer’s file operations (e.g., target directory C:\Program Files\Zoom\).

Exploitation Steps:

1. Identify the installation process and target files (e.g., Zoom.exe, ZoomLauncher.exe).
2. Use a file system watcher (e.g., ReadDirectoryChangesW API) to detect when the installer checks for a file.
3. At the moment between the check and the write, replace the target file with a malicious binary (e.g., a backdoor).
4. The installer writes the malicious file to the privileged location, granting persistence or privilege escalation.

Example PowerShell script (conceptual):

# Monitor for file check events
$watcher = New-Object System.IO.FileSystemWatcher
$watcher.Path = "C:\Program Files\Zoom"
$watcher.Filter = "*.*"
$watcher.EnableRaisingEvents = $true

Register-ObjectEvent $watcher "Changed" -Action {
    # Race condition: replace file immediately
    Copy-Item "C:\malicious\backdoor.exe" "C:\Program Files\Zoom\Zoom.exe" -Force
}

Blue Team

Detection:

• Monitor for unusual file system activity during Zoom installations (e.g., rapid file modifications in C:\Program Files\Zoom\).
• Use Sysmon Event ID 11 (FileCreate) and Event ID 2 (FileCreateStreamHash) to detect file replacements.
• Enable Windows Defender Attack Surface Reduction (ASR) rules to block untrusted processes from writing to protected folders.

Mitigation:

• Deploy Zoom installer via managed deployment tools (e.g., SCCM, GPO) to ensure integrity.
• Use application whitelisting (e.g., AppLocker) to prevent unauthorized binaries from executing.
• Apply the principle of least privilege; users should not have write access to installation directories.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): None available (no known malicious samples).
• File Paths: C:\Program Files\Zoom\Zoom.exe (potential replacement), C:\Program Files\Zoom\ZoomLauncher.exe
• Process Names: ZoomInstaller.exe, Zoom.exe (if replaced)
• Registry Keys: HKLM\SOFTWARE\Zoom (modified by installer)
• Network Indicators: None (local vulnerability only)

Detection Engineering

YARA Rule

rule CVE_2025_49456_Zoom_Installer_Race {
    meta:
        description = "Detects potential exploitation of CVE-2025-49456 via file replacement during Zoom installation"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-49456"
    strings:
        $s1 = "ZoomInstaller.exe" nocase
        $s2 = "C:\\Program Files\\Zoom\\" nocase
        $s3 = "MoveFileEx" nocase
        $s4 = "CreateFile" nocase
    condition:
        (uint16(0) == 0x5A4D) and  // PE file
        (1 of ($s1, $s2)) and
        (1 of ($s3, $s4))
}

Sigma Rule

title: Zoom Installer Race Condition Exploitation
id: 8f3a2b1c-4d5e-6f7a-8b9c-0d1e2f3a4b5c
status: experimental
description: Detects file replacement attempts during Zoom installation, indicative of CVE-2025-49456 exploitation
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-49456
logsource:
    product: windows
    service: sysmon
    category: file_event
detection:
    selection:
        EventID: 11
        TargetFilename|startswith: 'C:\Program Files\Zoom\'
        Image|endswith: '\ZoomInstaller.exe'
    condition: selection
falsepositives:
    - Legitimate Zoom updates
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-49456 Zoom Installer Race Condition - Potential Exploitation"; flow:to_server; content:"ZoomInstaller"; nocase; pcre:"/ZoomInstaller\.exe/i"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

• Restrict local access to systems where Zoom is installed (e.g., enforce least privilege, disable guest accounts).
• Monitor for unusual file system activity during Zoom installations using Sysmon or EDR.
• Use application whitelisting to prevent unauthorized binaries from executing in C:\Program Files\Zoom\.

Long-Term Fix:

• Await an official patch from Zoom. In the meantime, consider using the Zoom web client or a managed deployment tool that verifies file integrity.
• Apply the principle of atomic file operations in custom scripts or deployment tools.

Workaround:

• Install Zoom using a privileged account and ensure no other users are logged in during installation.
• Use Group Policy to restrict write access to the Zoom installation directory.

Valters IT