Description

CISA has directed US federal agencies to urgently patch a high severity vulnerability in Zimbra Collaboration Suite identified as CVE 2025 66376. This flaw affects the classic web interface and is actively being exploited in attacks. The issue is a stored cross site scripting vulnerability that allows remote unauthenticated attackers to inject malicious code into email content. When users open such emails the malicious script can execute within their browser. This can lead to session hijacking data theft and unauthorized access to sensitive information within the email environment. The reason this vulnerability is critical is due to the widespread use of Zimbra across government and private organizations worldwide. The flaw can be exploited using crafted HTML emails that abuse CSS import functionality making it easy for attackers to deliver payloads without requiring authentication. Since email is a primary communication channel this increases the chances of successful exploitation. Past incidents show that Zimbra vulnerabilities have been repeatedly targeted by threat actors including state backed groups making this a high risk issue. CISA has added this vulnerability to its known exploited vulnerabilities catalog and has given agencies a strict deadline to apply patches or mitigations. Organizations are strongly advised to follow vendor guidance update systems immediately and discontinue use if fixes are not available. This vulnerability highlights the importance of timely patching monitoring email systems and strengthening security controls to prevent similar attacks