CISA Warns of Zimbra Collaboration Suite (ZCS) XSS Zero-Day Vulnerability Actively Exploited in Attacks
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CISA has issued a critical warning regarding a zero-day cross-site scripting (XSS) vulnerability in Synacor\u2019s Zimbra Collaboration Suite (ZCS), designated as CVE-2025-27915<\/a>.\u00a0<\/p>\n This vulnerability has been actively exploited in attacks and poses significant risks to organizations using the popular email and collaboration platform.<\/p>\n The vulnerability exists within the Classic Web Client component of Zimbra Collaboration Suite<\/a> and stems from insufficient sanitization of HTML content in ICS (Internet Calendar System) files.\u00a0<\/p>\n The security flaw is classified under CWE-79, which specifically addresses improper neutralization of input during web page generation.<\/p>\n When users view email <\/a>messages <\/a>containing malicious ICS entries, embedded JavaScript code executes automatically through an ontoggle event handler within a <details> tag.\u00a0<\/p>\n This exploitation vector allows attackers to run arbitrary JavaScript code within the victim\u2019s authenticated session context.\u00a0<\/p>\n The attack mechanism bypasses standard security controls by leveraging legitimate calendar file functionality to deliver malicious payloads.<\/p>\n The vulnerability\u2019s exploitation requires minimal user interaction \u2013 simply viewing a specially crafted email message triggers the malicious code execution.\u00a0<\/p>\n This low barrier to exploitation makes it particularly dangerous for widespread attacks targeting multiple organizations simultaneously.<\/p>\nZimbra Collaboration Suite (ZCS) XSS Flaw<\/strong><\/h2>\n