CVE-2025-46736: Umbraco CMS Information Disclosure Flaw

CVE-2025-46736 Overview

Umbraco CMS, a popular free and open source .NET content management system, contains a user enumeration vulnerability that allows attackers to determine whether an account exists by analyzing the timing of post-login API responses. This timing-based attack vector exposes sensitive information about valid user accounts within the system, which can be leveraged for further attacks such as credential stuffing or targeted brute force attempts.

Critical Impact

Attackers can enumerate valid user accounts through timing analysis of authentication API responses, enabling targeted attacks against confirmed user accounts.

Affected Products

• Umbraco CMS versions prior to 10.8.10
• Umbraco CMS versions prior to 13.8.1

Discovery Timeline

• 2025-05-06 - CVE-2025-46736 published to NVD
• 2025-09-03 - Last updated in NVD database

Technical Details for CVE-2025-46736

Vulnerability Analysis

This vulnerability falls under CWE-204 (Observable Response Discrepancy), a category of information disclosure where an application provides different responses based on internal state that should remain hidden from users. In the case of Umbraco CMS, the authentication mechanism exhibits measurable timing differences between login attempts for existing versus non-existing accounts.

When a user attempts to authenticate, the system performs different internal operations depending on whether the username exists in the database. For valid usernames, the system may perform additional operations such as password hash retrieval and comparison, while for non-existent usernames, the process terminates earlier. This timing discrepancy, though potentially subtle, can be measured and analyzed by attackers to build a list of valid accounts.

The vulnerability affects the network-accessible authentication endpoints and requires no authentication or special privileges to exploit, making it accessible to any remote attacker with network access to the Umbraco CMS installation.

Root Cause

The root cause lies in the authentication flow's lack of constant-time response handling. The login API does not normalize response times between successful account lookups (even with wrong passwords) and failed account lookups, creating an observable timing oracle. The security patch introduces modifications to the SecuritySettings.cs configuration to address this behavior and ensure consistent response timing regardless of account existence.

Attack Vector

An attacker can exploit this vulnerability by sending multiple authentication requests to the Umbraco CMS login endpoint while precisely measuring response times. By comparing the response latencies for different usernames, the attacker can distinguish between valid and invalid accounts. This technique is typically automated using scripts that:

1. Send login requests with candidate usernames
2. Measure response times with high precision
3. Perform statistical analysis to identify timing patterns
4. Classify accounts as existing or non-existing based on response time distributions

The patch commits show modifications to the security configuration settings, adding System.ComponentModel.DataAnnotations to implement proper validation and timing normalization:

// See LICENSE for more details.

using System.ComponentModel;
+using System.ComponentModel.DataAnnotations;

namespace Umbraco.Cms.Core.Configuration.Models;

Source: GitHub Commit 14fbd206

Detection Methods for CVE-2025-46736

Indicators of Compromise

• Unusually high volume of failed login attempts from single IP addresses or ranges
• Sequential or automated patterns in username submissions during authentication attempts
• Rapid succession of login requests testing different usernames with identical or dummy passwords
• Abnormal traffic patterns targeting the Umbraco backoffice login endpoints

Detection Strategies

• Implement rate limiting detection on authentication endpoints to identify enumeration attempts
• Monitor for patterns of failed authentications where usernames vary but passwords remain constant
• Deploy web application firewall (WAF) rules to detect and block timing attack patterns
• Analyze authentication logs for sequential username testing patterns indicative of enumeration tools

Monitoring Recommendations

• Enable detailed logging for all authentication attempts including source IP, timestamp, and username
• Set up alerting thresholds for failed login attempts exceeding normal baseline activity
• Monitor response times on authentication endpoints for anomalous measurement activity
• Implement SentinelOne Singularity™ platform for real-time detection of reconnaissance activities against web applications

How to Mitigate CVE-2025-46736

Immediate Actions Required

• Upgrade Umbraco CMS to version 10.8.10 or later for the 10.x branch
• Upgrade Umbraco CMS to version 13.8.1 or later for the 13.x branch
• Review authentication logs for signs of previous enumeration attempts
• Consider implementing additional rate limiting on login endpoints as defense in depth

Patch Information

Umbraco has released security patches that address this timing attack vulnerability. The fixes are available in the following versions:

• Version 10.8.10 - Patch for the 10.x release branch
• Version 13.8.1 - Patch for the 13.x release branch

The patches can be obtained through the official GitHub security advisory. Additional commit details are available at commit 14fbd206 and commit 34709be6.

Workarounds

• No known workarounds are available for this vulnerability according to the vendor advisory
• Implement aggressive rate limiting on authentication endpoints as a partial mitigation
• Deploy a reverse proxy or WAF with timing attack detection capabilities
• Monitor and block IP addresses exhibiting enumeration behavior patterns

# Update Umbraco CMS using .NET CLI
dotnet add package Umbraco.Cms --version 13.8.1

# Verify installed version
dotnet list package | grep Umbraco