CVE-2025-66376 is a stored Cross-site Scripting (XSS) vulnerability classified under CWE-79, affecting Zimbra Collaboration Suite (ZCS) versions 10.0 prior to 10.0.18 and 10.1 prior to 10.1.13. The vulnerability occurs in the Classic UI of ZCS when processing HTML email messages containing malicious Cascading Style Sheets (CSS) @import directives. These directives can be crafted to inject arbitrary scripts that execute in the context of the victim's browser when the email is viewed, bypassing input validation and sanitization mechanisms. The vulnerability does not require any authentication or user interaction, making it remotely exploitable by sending a specially crafted email to a target user. The CVSS v3.1 base score is 7.2, reflecting network attack vector, low attack complexity, no privileges required, no user interaction, and a scope change with partial impact on confidentiality and integrity but no impact on availability. Although no known exploits have been reported in the wild, the potential for attackers to steal session tokens, perform actions on behalf of users, or exfiltrate sensitive information is significant. The lack of available patches at the time of reporting necessitates immediate attention from administrators. This vulnerability highlights the risks of insufficient input neutralization in web-based collaboration platforms, especially those handling rich content such as HTML emails with embedded CSS. Organizations relying on Zimbra Collaboration for email and calendaring services must assess their exposure and apply mitigations promptly to prevent exploitation.

For European organizations, the impact of CVE-2025-66376 can be substantial due to the widespread use of Zimbra Collaboration Suite in enterprises, government agencies, and educational institutions. Exploitation could enable attackers to execute arbitrary scripts within users’ browsers, leading to theft of authentication cookies, session hijacking, unauthorized access to sensitive emails, and potential lateral movement within networks. The partial compromise of confidentiality and integrity could result in data breaches, intellectual property theft, and disruption of communication workflows. Given the vulnerability’s ability to be exploited without authentication or user interaction, attackers can target large numbers of users via phishing or spear-phishing campaigns. This risk is heightened in sectors with stringent data protection requirements under GDPR, where breaches could lead to regulatory penalties and reputational damage. Additionally, the scope change indicated in the CVSS vector suggests that the vulnerability affects components beyond the initial user context, potentially impacting other users or services. The absence of known exploits in the wild provides a window for proactive defense, but also means attackers may develop exploits rapidly once the vulnerability becomes widely known. Therefore, European organizations must prioritize remediation to maintain secure collaboration environments.

1. Upgrade Zimbra Collaboration Suite to versions 10.0.18 or later and 10.1.13 or later as soon as patches become available to eliminate the vulnerability. 2. Implement strict input validation and sanitization on all HTML email content, especially filtering or blocking CSS @import directives within emails. 3. Deploy Content Security Policy (CSP) headers that restrict the execution of inline scripts and loading of external stylesheets from untrusted sources to reduce the risk of script injection. 4. Monitor email gateways and security appliances for suspicious emails containing unusual CSS constructs or obfuscated payloads indicative of exploitation attempts. 5. Educate users about the risks of opening emails from unknown or untrusted senders and encourage reporting of suspicious messages. 6. Conduct regular security assessments and penetration testing focused on webmail interfaces and email clients to detect similar injection flaws. 7. Use multi-factor authentication (MFA) to limit the impact of session hijacking in case of successful exploitation. 8. Review and harden browser security settings and extensions to mitigate the execution of malicious scripts. 9. Maintain up-to-date backups and incident response plans to quickly recover from potential breaches. 10. Coordinate with Zimbra support and security advisories to stay informed about emerging threats and patches.