CVE-2025-41239

Executive Summary

CVE-2025-41239 is an information disclosure vulnerability affecting VMware ESXi, Workstation, Fusion, and VMware Tools. The flaw resides in the vSockets component, where uninitialized memory can be leveraged by a local attacker with administrative privileges on a virtual machine to leak sensitive data from processes communicating over vSockets. With a CVSS score of 7.1, this vulnerability poses a significant confidentiality risk, particularly in multi-tenant or shared virtualized environments. No public proof-of-concept or exploit code has been released, and the vendor has not yet issued a patch.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Local (L) – The attacker must have local administrative access to a VM.
• Attack Complexity (AC): Low (L) – No special conditions beyond local admin access.
• Privileges Required (PR): High (H) – Requires administrative privileges on the VM.
• User Interaction (UI): None (N) – No user action needed beyond initial access.
• Scope (S): Changed (C) – The impact may extend to other VMs or the host via shared vSockets.
• Confidentiality (C): High (H) – Memory leakage can expose sensitive process data.
• Integrity (I): None (N) – No data modification possible.
• Availability (A): None (N) – No denial of service.

Threat Level: High. While the attack requires local admin privileges, the potential for cross-VM or host memory leakage in virtualized environments elevates the risk.

Deep Dive & Mechanics

The vulnerability is rooted in the vSockets implementation, a mechanism for high-performance communication between virtual machines and the host. When vSockets allocates memory buffers for inter-process communication, it fails to initialize the memory before use. This uninitialized memory may contain residual data from previous operations, including credentials, encryption keys, or other sensitive information from other processes.

An attacker with local administrative access on a VM can craft a malicious vSockets client that reads from these uninitialized buffers. By repeatedly connecting and reading, they can harvest leaked data. The attack does not require any special network access or user interaction beyond the initial compromise.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, the vulnerable code pattern likely resembles:

// Vulnerable: buffer allocated but not zeroed
char *buffer = malloc(size);
// Data is read into buffer without initialization
recv(socket, buffer, size, 0);
// Attacker can read uninitialized memory

A patched version would initialize the buffer:

// Patched: buffer zeroed before use
char *buffer = calloc(1, size);
// or
char *buffer = malloc(size);
memset(buffer, 0, size);
recv(socket, buffer, size, 0);

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local administrative access to a VMware VM (ESXi, Workstation, or Fusion).
• VMware Tools installed on the VM.
• Knowledge of vSockets communication endpoints.

Exploitation Steps:

1. Identify vSockets endpoints used by target processes (e.g., via lsof or vmware-vmctrl).
2. Write a custom vSockets client that connects to the target endpoint.
3. Send a minimal request to trigger a response that uses uninitialized memory.
4. Read the response and extract leaked data (e.g., memory dumps, keys).
5. Repeat to harvest additional data.

Example Python script (conceptual):

import socket
import struct

# vSockets address family
AF_VSOCK = 40
# Example port
port = 1234

sock = socket.socket(AF_VSOCK, socket.SOCK_STREAM)
sock.connect((socket.VMADDR_CID_ANY, port))
# Send a small request
sock.send(b'\x00')
# Read response containing leaked memory
data = sock.recv(4096)
print(data.hex())
sock.close()

Blue Team

Detection:

• Monitor for unusual vSockets connections from VMs to the host or other VMs.
• Look for processes repeatedly connecting to vSockets endpoints without legitimate reason.
• Audit memory usage patterns in VMware Tools processes.

Hunting:

• Search for AF_VSOCK socket creation in system logs.
• Use process monitoring to detect unauthorized vSockets clients.
• Correlate with administrative privilege escalation events.

Indicators of Compromise (IOCs)

• Network: Unusual vSockets traffic between VMs or VM-to-host on non-standard ports.
• Process: Unknown or suspicious processes using AF_VSOCK sockets (e.g., custom Python scripts).
• File: Presence of custom vSockets client binaries or scripts in /tmp or user directories.
• Memory: VMware Tools processes showing unexpected memory allocation patterns.
• Log: Event ID 5156 (Windows) or audit logs showing AF_VSOCK connections.

Detection Engineering

YARA Rule

rule CVE_2025_41239_vSockets_Leak {
    meta:
        description = "Detects potential exploitation of CVE-2025-41239 via uninitialized vSockets memory"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-41239"
    strings:
        $s1 = "AF_VSOCK" ascii wide nocase
        $s2 = "VMADDR_CID_ANY" ascii wide nocase
        $s3 = "vSockets" ascii wide nocase
        $s4 = "malloc" ascii wide
        $s5 = "recv" ascii wide
    condition:
        (uint16(0) == 0x5A4D or uint32(0) == 0x464c457f) and
        (all of ($s1, $s2, $s3) or ($s4 and $s5))
}

Sigma Rule

title: CVE-2025-41239 vSockets Uninitialized Memory Exploitation
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation of CVE-2025-41239 via vSockets connections
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-41239
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\python.exe'
        CommandLine|contains: 'AF_VSOCK'
    condition: selection
falsepositives:
    - Legitimate VMware administration tools
level: high

Suricata Rule

alert ip any any -> any any (msg:"CVE-2025-41239 vSockets Uninitialized Memory Exploitation"; flow:to_server; content:"|00 00 00 00|"; depth:4; content:"|AF_VSOCK|"; distance:0; within:100; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Restrict local administrative access to VMs to only trusted personnel.
2. Monitor vSockets activity using the provided detection rules.
3. Isolate sensitive VMs from shared vSockets networks.

Long-term Fix:

• Apply vendor patches once released.
• Update VMware Tools to the latest version.
• Consider disabling vSockets if not required for operations.

Workarounds:

• Use host-based firewalls to block vSockets traffic between VMs.
• Implement memory integrity checks on VMware Tools processes.

Valters IT
DevSecOps & Cyber Threat Intelligence