CVE-2026-23618 is a stored cross-site scripting (XSS) vulnerability classified under CWE-79, affecting GFI Software's MailEssentials AI product versions prior to 22.4. The vulnerability arises from improper neutralization of user-supplied input during web page generation in the Spam Keyword Checking (Subject) conditions interface. Specifically, an authenticated user can inject malicious HTML or JavaScript code via the ctl00$ContentPlaceHolder1$pvSubject$TXB_SubjectCondition parameter on the /MailEssentials/pages/MailSecurity/ASKeywordChecking.aspx page. This input is stored persistently and later rendered without adequate sanitization in the management interface, allowing the injected script to execute in the context of any logged-in user who views the affected page. The attack vector requires the attacker to have authenticated access to the MailEssentials AI management interface, but does not require elevated privileges beyond that. The vulnerability can lead to session hijacking, unauthorized actions, or credential theft by executing arbitrary scripts within the victim's browser session. The CVSS 4.0 vector indicates network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:L - meaning low privileges, i.e., authenticated user), user interaction required (UI:P), low impact on confidentiality and integrity, and no impact on availability. No public exploits are currently known, and no patches are linked yet, but the issue is publicly disclosed and should be addressed promptly.

This vulnerability poses a moderate risk to organizations using vulnerable versions of GFI MailEssentials AI, particularly those that allow multiple administrators or users with authenticated access to the management interface. Successful exploitation could enable attackers to execute arbitrary scripts in the context of other logged-in users, potentially leading to session hijacking, theft of credentials or sensitive data, and unauthorized administrative actions. This could compromise the integrity and confidentiality of the email security management environment, potentially allowing attackers to manipulate spam filtering rules or disable protections. While availability impact is minimal, the breach of administrative controls could have cascading effects on email security posture. Organizations with multiple administrators or shared access environments are at higher risk. The requirement for authentication and user interaction limits the scope somewhat, but insider threats or compromised low-privilege accounts could be leveraged to exploit this vulnerability.

To mitigate CVE-2026-23618, organizations should first apply any available patches or updates from GFI Software that address this XSS vulnerability as soon as they are released. In the absence of a patch, administrators should restrict access to the MailEssentials AI management interface to trusted users only and enforce strong authentication mechanisms, including multi-factor authentication, to reduce the risk of compromised credentials. Additionally, implement strict input validation and output encoding on the affected parameter to neutralize HTML and JavaScript content before storage and rendering. Employ web application firewalls (WAFs) with custom rules to detect and block malicious payloads targeting the Spam Keyword Checking interface. Regularly audit and monitor administrative activities and logs for suspicious behavior indicative of exploitation attempts. Educate administrators about the risks of injecting untrusted content and the importance of cautious input handling. Finally, consider isolating the management interface within a secure network segment to limit exposure.