CVE-2025-46736 is a medium severity vulnerability affecting Umbraco CMS, a widely used open-source .NET content management system. The vulnerability is classified under CWE-204: Observable Response Discrepancy. It arises from differences in the timing of post-login API responses, which can be analyzed by an attacker to determine whether a specific user account exists in the system. This is a form of user enumeration attack that does not require authentication or user interaction and can be exploited remotely over the network. The affected versions include all Umbraco CMS versions prior to 10.8.10 and versions from 11.0.0-rc1 up to but not including 13.8.1. The issue was patched in versions 10.8.10 and 13.8.1. No known workarounds exist, so updating to a patched version is the primary remediation. The CVSS v3.1 base score is 5.3, reflecting a medium severity level, with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and limited impact on confidentiality (C:L), with no impact on integrity or availability. Although this vulnerability does not directly allow account takeover or system compromise, it facilitates reconnaissance by confirming valid usernames, which can be leveraged in subsequent targeted attacks such as password guessing, phishing, or social engineering.

For European organizations using Umbraco CMS, this vulnerability poses a risk primarily in the form of information disclosure. Attackers can enumerate valid user accounts, which undermines user privacy and can aid in further attacks like credential stuffing or brute force attempts. This is particularly concerning for organizations handling sensitive or regulated data, such as those in finance, healthcare, or government sectors, where user account confidentiality is critical. While the vulnerability itself does not allow direct system compromise, the information gained can be a stepping stone for more severe attacks. Additionally, organizations subject to GDPR must consider the implications of user data exposure, as even indirect disclosure of user existence could be viewed as a data protection concern. The absence of known exploits in the wild reduces immediate risk, but the ease of exploitation and network accessibility mean that attackers could develop exploits rapidly if the vulnerability becomes widely known.

The primary and most effective mitigation is to upgrade Umbraco CMS to version 10.8.10 or later, or 13.8.1 or later, where the vulnerability is patched. Organizations should prioritize patch management processes to ensure timely updates. In environments where immediate patching is not feasible, implementing rate limiting and anomaly detection on login endpoints can help reduce the risk of automated enumeration attacks. Additionally, deploying Web Application Firewalls (WAFs) with custom rules to detect and block suspicious timing-based probes may provide temporary protection. Organizations should also review and harden authentication mechanisms, including enforcing strong password policies and multi-factor authentication, to mitigate risks from subsequent attacks leveraging enumerated usernames. Monitoring logs for unusual login attempts or enumeration patterns is recommended to detect potential exploitation attempts early.