CISA has issued a vital warning concerning a zero-day cross-site scripting (XSS) vulnerability in Synacor’s Zimbra Collaboration Suite (ZCS), designated as CVE-2025-27915.
This vulnerability has been actively exploited in assaults and poses important dangers to organizations utilizing the favored electronic mail and collaboration platform.
Zimbra Collaboration Suite (ZCS) XSS Flaw
The vulnerability exists throughout the Traditional Net Shopper part of Zimbra Collaboration Suite and stems from inadequate sanitization of HTML content material in ICS (Web Calendar System) recordsdata.
The safety flaw is classed below CWE-79, which particularly addresses improper neutralization of enter throughout internet web page era.
When customers view electronic mail messages containing malicious ICS entries, embedded JavaScript code executes mechanically by means of an ontoggle occasion handler inside a tag.
This exploitation vector permits attackers to run arbitrary JavaScript code throughout the sufferer’s authenticated session context.
The assault mechanism bypasses normal safety controls by leveraging reliable calendar file performance to ship malicious payloads.
The vulnerability’s exploitation requires minimal consumer interplay – merely viewing a specifically crafted electronic mail message triggers the malicious code execution.
This low barrier to exploitation makes it notably harmful for widespread assaults focusing on a number of organizations concurrently.
Threat FactorsDetailsAffected ProductsZimbra Collaboration Suite (ZCS) 10.1.9ZCS 10.0.15ZCS 9.0.0 Patch 46ImpactCross-site scriptingExploit PrerequisitesVictim should view a crafted electronic mail containing a malicious ICS calendar entry within the Traditional Net Shopper; consumer interplay required; attacker wants a legitimate account or electronic mail supply capabilityCVSS 3.1 Score5.4 (Medium)
Mitigations
The profitable exploitation of CVE-2025-27915 permits attackers to carry out unauthorized actions inside compromised consumer accounts, together with the creation of malicious electronic mail filters that redirect incoming messages to attacker-controlled addresses.
This functionality facilitates complete knowledge exfiltration and ongoing surveillance of sufferer communications.
CISA has designated October 28, 2025, because the necessary remediation deadline for federal companies below Binding Operational Directive (BOD) 22-01.
Organizations should apply vendor-provided mitigations, implement relevant cloud service steerage, or discontinue product utilization if efficient mitigations stay unavailable.
The company emphasizes that this vulnerability’s lively exploitation standing requires speedy consideration from all Zimbra Collaboration Suite directors.
Safety groups ought to monitor the official Zimbra Safety Heart and Nationwide Vulnerability Database for up to date mitigation steerage and patches.
Organizations must also implement further electronic mail safety controls, together with enhanced attachment scanning and consumer consciousness coaching targeted on suspicious calendar invites and ICS file attachments.
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