CVE-2024-27443 is a Cross-Site Scripting (XSS) vulnerability identified in the CalendarInvite feature of the Zimbra Collaboration Suite (ZCS) versions 9.0 and 10.0, specifically within the classic webmail user interface. The root cause is improper input validation of the calendar header fields in email messages. An attacker can exploit this by sending a crafted email containing a malicious calendar header with embedded JavaScript code. When a user opens or previews this email in the vulnerable Zimbra interface, the malicious script executes within the context of the user's session. This execution can lead to unauthorized actions such as session hijacking, theft of sensitive information, or manipulation of the webmail interface. The vulnerability requires no prior authentication but does require user interaction to trigger the payload. The CVSS 3.1 score of 6.1 reflects a medium severity, with the attack vector being network-based and low attack complexity. The scope is changed (S:C) because the vulnerability affects resources beyond the initially vulnerable component, potentially impacting user data confidentiality and integrity. No patches or official fixes are currently linked, and no active exploitation has been reported. The vulnerability is classified under CWE-79, which covers improper neutralization of input leading to XSS. Given Zimbra's deployment in many enterprise and government environments, this vulnerability poses a significant risk if left unmitigated.

For European organizations, the impact of CVE-2024-27443 can be significant, especially for those relying on Zimbra Collaboration Suite for email and calendar services. Successful exploitation could allow attackers to execute arbitrary JavaScript in the context of a victim's session, potentially leading to session hijacking, unauthorized access to sensitive emails and calendar data, and lateral movement within the organization's network. This could result in data breaches, exposure of confidential communications, and disruption of business operations. The vulnerability's requirement for user interaction means phishing or social engineering campaigns could be used to deliver the malicious payload, increasing the risk to end users. Organizations in sectors such as government, finance, healthcare, and critical infrastructure—where Zimbra is commonly deployed—may face heightened risks due to the sensitivity of their data and regulatory compliance obligations under GDPR. Additionally, the cross-site scripting flaw could be leveraged as a foothold for further attacks, including malware delivery or credential theft, amplifying the potential damage.

European organizations should implement the following specific mitigations: 1) Immediately review and apply any official patches or updates from Zimbra addressing this vulnerability once available. 2) If patches are not yet available, configure email filtering solutions to detect and quarantine emails containing suspicious or malformed calendar headers to reduce exposure. 3) Educate users about the risks of interacting with unexpected or suspicious calendar invites and emails, emphasizing caution before opening or previewing such messages. 4) Consider disabling or restricting the use of the classic webmail interface in favor of more secure or updated clients if feasible. 5) Implement Content Security Policy (CSP) headers on the webmail interface to limit the execution of unauthorized scripts. 6) Monitor logs for unusual activity related to calendar invites or webmail sessions that could indicate exploitation attempts. 7) Employ multi-factor authentication (MFA) to reduce the impact of session hijacking. 8) Conduct regular security assessments and penetration tests focusing on webmail and calendar functionalities to identify residual risks.