CVE-2026-27860 is a security vulnerability classified as LDAP injection found in Open-Xchange GmbH's OX Dovecot Pro mail server software. The issue arises specifically when the configuration parameter auth_username_chars is set to an empty value. Under this condition, the software fails to properly neutralize special characters in LDAP queries used for authentication, allowing an attacker to inject arbitrary LDAP filter expressions. This injection can bypass intended restrictions on LDAP queries and enables an attacker to probe the LDAP directory structure, potentially gathering sensitive information about user accounts and organizational structure. The vulnerability does not directly lead to unauthorized access or modification of data but facilitates reconnaissance that could be leveraged in further attacks. The CVSS v3.1 base score is 3.7, reflecting low severity due to network attack vector, high attack complexity, no privileges required, no user interaction, and limited impact confined to confidentiality. No public exploits have been reported, and the vendor recommends not clearing out auth_username_chars or installing a fixed version once available. This vulnerability highlights the importance of input validation and proper sanitization of LDAP queries in authentication mechanisms.

The primary impact of this vulnerability is the potential for attackers to perform LDAP injection attacks that bypass certain authentication restrictions and probe the LDAP directory structure. This can lead to unauthorized information disclosure about user accounts and organizational hierarchy, which may aid attackers in planning more targeted attacks such as phishing, credential stuffing, or privilege escalation. Although the vulnerability does not directly compromise system integrity or availability, the reconnaissance advantage it provides can increase the risk of subsequent, more damaging attacks. Organizations relying on OX Dovecot Pro with LDAP authentication, especially those with sensitive or large-scale LDAP directories, may face increased exposure to information leakage. The low CVSS score reflects the limited direct impact, but the vulnerability should not be ignored as it weakens the security posture of affected systems.

To mitigate CVE-2026-27860, organizations should ensure that the auth_username_chars configuration parameter in OX Dovecot Pro is never set to an empty value, thereby preventing LDAP injection opportunities. Administrators should audit their current configurations to verify this setting is properly defined with allowed username characters. Additionally, organizations should monitor for vendor patches or updates addressing this vulnerability and apply them promptly once released. Employing network-level controls such as restricting access to LDAP servers and OX Dovecot Pro instances to trusted networks can reduce exposure. Implementing LDAP query logging and anomaly detection may help identify suspicious injection attempts. Finally, educating administrators about secure LDAP query construction and input validation best practices can prevent similar issues in future deployments.