CVE-2025-54391 is a critical vulnerability affecting the Zimbra Collaboration Suite (ZCS), specifically targeting the EnableTwoFactorAuthRequest SOAP endpoint. This vulnerability allows an attacker who already possesses valid user credentials to bypass the Two-Factor Authentication (2FA) mechanism that is intended to protect user accounts. The flaw lies in the endpoint's failure to require a valid authentication token or proof of access to an existing 2FA method before permitting the configuration of an additional 2FA method. Consequently, an attacker can add a new 2FA method—such as a third-party authenticator app or email-based 2FA—without proper authorization. This effectively circumvents the 2FA protection, granting unauthorized access to accounts that would otherwise be secured by multi-factor authentication. The vulnerability is classified under CWE-284 (Improper Access Control), indicating a failure to enforce correct access restrictions. The CVSS v3.1 base score is 9.1, reflecting a critical severity with network attack vector, low attack complexity, no privileges required, no user interaction, and high impact on confidentiality and integrity, but no impact on availability. No patch links are currently available, and no known exploits in the wild have been reported as of the publication date (September 16, 2025). The affected versions are not specified, which suggests that the vulnerability may impact multiple or all versions of Zimbra Collaboration Suite unless otherwise mitigated.

For European organizations, this vulnerability poses a significant risk, particularly for enterprises, government agencies, and service providers that rely on Zimbra Collaboration Suite for email and collaboration services. The ability to bypass 2FA undermines a critical security control designed to protect sensitive communications and data. Unauthorized access could lead to data breaches, exposure of confidential information, spear-phishing campaigns, and lateral movement within networks. Given the widespread adoption of Zimbra in various sectors across Europe, including public administration and education, the impact could be substantial. The compromise of user accounts protected by 2FA may also erode trust in organizational security postures and lead to regulatory compliance issues under GDPR, especially if personal data is exposed. The lack of a patch at the time of disclosure increases the urgency for organizations to implement compensating controls to mitigate potential exploitation.

European organizations should take immediate steps to mitigate this vulnerability beyond waiting for an official patch. First, restrict access to the Zimbra SOAP API endpoints by implementing network-level controls such as IP whitelisting and segmentation to limit exposure to trusted internal networks only. Second, enforce strict monitoring and logging of 2FA configuration changes and authentication events to detect suspicious activities promptly. Third, consider temporarily disabling the EnableTwoFactorAuthRequest SOAP endpoint if feasible, or applying custom access controls or web application firewall (WAF) rules to block unauthorized requests targeting this endpoint. Fourth, enhance user credential security by enforcing strong password policies and considering additional layers of authentication such as hardware tokens or biometric factors where possible. Finally, maintain close communication with Zimbra vendors and subscribe to security advisories to apply patches immediately upon release. Conduct regular security awareness training to alert users about potential phishing attempts that could leverage compromised credentials.