The Cybersecurity and Infrastructure Security Agency (CISA) has issued a critical alert concerning a zero-day cross-site scripting (XSS) vulnerability in Synacor’s Zimbra Collaboration Suite (ZCS), identified as CVE-2025-27915. This flaw is currently being actively exploited, posing significant risks to organizations utilizing this widely adopted email and collaboration platform.
Understanding the Vulnerability
The vulnerability resides in the Classic Web Client component of ZCS and is due to inadequate sanitization of HTML content within Internet Calendar System (ICS) files. Classified under CWE-79, which pertains to improper neutralization of input during web page generation, this flaw allows attackers to embed malicious JavaScript code within ICS files. When a user views an email containing such a crafted ICS entry, the embedded script executes automatically through an `ontoggle` event handler within a `
Mechanism of Exploitation
Exploitation of this vulnerability requires minimal user interaction. Simply viewing a specially crafted email triggers the malicious code execution. Attackers can leverage this to bypass standard security controls by using legitimate calendar file functionalities to deliver malicious payloads. Once executed, the malicious script can perform actions such as creating email filters that redirect incoming messages to attacker-controlled addresses, facilitating data exfiltration and continuous surveillance of victim communications.
Risk Assessment
The vulnerability affects the following ZCS versions:
– ZCS 10.1.9
– ZCS 10.0.15
– ZCS 9.0.0 Patch 46
The impact is classified as cross-site scripting, with a CVSS 3.1 score of 5.4, indicating a medium severity level. Exploitation prerequisites include the victim viewing a crafted email containing a malicious ICS calendar entry in the Classic Web Client, with user interaction required and the attacker needing a valid account or email delivery capability.
Mitigation Strategies
CISA has set October 28, 2025, as the mandatory remediation deadline for federal agencies under Binding Operational Directive (BOD) 22-01. Organizations are urged to apply vendor-provided mitigations, implement applicable cloud service guidance, or discontinue product usage if effective mitigations are unavailable.
Administrators should monitor the official Zimbra Security Center and the National Vulnerability Database for updated mitigation guidance and patches. Additional email security controls, including enhanced attachment scanning and user awareness training focused on suspicious calendar invitations and ICS file attachments, are recommended to bolster defenses against such exploits.
Conclusion
The active exploitation of CVE-2025-27915 underscores the critical need for organizations to promptly address vulnerabilities within their collaboration platforms. By implementing the recommended mitigations and maintaining vigilant security practices, organizations can protect their systems and sensitive information from potential breaches.
