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CISA has ordered U.S. government agencies to patch a critical Zimbra XSS flaw as attackers continue to exploit the vulnerability, causing widespread breaches and compromising sensitive data.
CISA has issued an urgent warning to U.S. government agencies to patch a critical cross-site scripting (XSS) vulnerability in Zimbra Collaboration Suite (ZCS), tracked as CVE-2025-66376. The vulnerability allows remote unauthenticated attackers to exploit the Classic UI of ZCS, potentially allowing them to hijack user sessions and steal sensitive data. CISA has given Federal Civilian Executive Branch agencies two weeks to secure their servers by April 1st and encourages all organizations to patch this actively exploited flaw as soon as possible. The vulnerability is a stored cross-site scripting weakness that can be exploited by abusing Cascading Style Sheets (CSS) @import directives in email HTML. Recent attacks have shown that Zimbra security flaws are frequent attack vectors for malicious cyber actors, posing significant risks to the federal enterprise.
CISA has issued an urgent warning to U.S. government agencies, ordering them to patch a critical cross-site scripting (XSS) vulnerability in the widely used Zimbra Collaboration Suite (ZCS). The vulnerability, tracked as CVE-2025-66376, was discovered early last year and patched by Synacor, the company behind Zimbra. However, it appears that attackers have since begun exploiting this flaw to breach numerous government servers, highlighting the need for swift action to address this vulnerability.
The Zimbra Collaboration Suite is a popular email and collaboration software suite used by hundreds of millions of people worldwide, including thousands of businesses and hundreds of government agencies. Its widespread adoption has made it a prime target for cyber attackers seeking to exploit vulnerabilities in the system. In recent years, Zimbra has been frequently targeted in attacks, with various zero-day vulnerabilities being exploited to breach thousands of vulnerable email servers.
The current vulnerability, CVE-2025-66376, is described as a stored cross-site scripting (XSS) weakness in the Classic UI of the ZCS. This allows remote unauthenticated attackers to exploit this flaw by abusing Cascading Style Sheets (CSS) @import directives in email HTML. By doing so, attackers can execute arbitrary JavaScript via malicious HTML-based emails, potentially allowing them to hijack user sessions and steal sensitive data within the compromised Zimbra environment.
CISA added this vulnerability to its catalog of vulnerabilities exploited in the wild on Wednesday and has given Federal Civilian Executive Branch (FCEB) agencies two weeks to secure their servers by April 1st. The U.S. cybersecurity agency has encouraged all organizations, including those in the private sector, to patch this actively exploited flaw as soon as possible.
The CISA warned that these types of vulnerabilities are frequent attack vectors for malicious cyber actors and pose significant risks to the federal enterprise. "Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable," said the agency.
Synacor has not shared any details on the impact of a successful CVE-2025-66376 attack. However, given the widespread nature of Zimbra and its frequent targeting in attacks, it is likely that this vulnerability has been exploited to execute arbitrary JavaScript code, enabling attackers to set email filters that redirect messages to attacker-controlled servers.
Zimbra security flaws have frequently been targeted in recent years, with various zero-day vulnerabilities being exploited to breach thousands of vulnerable email servers. For instance, as early as June 2022, Zimbra auth-bypass and remote code execution bugs were abused to breach more than 1,000 servers. Starting in September 2022, hackers exploited a zero-day vulnerability in Zimbra Collaboration Suite, breaching nearly 900 servers within two months after gaining remote code execution on compromised instances.
The Russian state-backed Winter Vivern hacking group also used reflected XSS exploits to breach the Zimbra webmail portals of NATO-aligned governments and the mailboxes of government officials, military personnel, and diplomats. More recently, threat actors exploited another Zimbra XSS vulnerability (CVE-2025-27915) in zero-day attacks to execute arbitrary JavaScript code, enabling them to set email filters that redirect messages to attacker-controlled servers.
The urgency surrounding this warning is clear. The CISA's directive for Federal Civilian Executive Branch agencies highlights the need for swift action to address this vulnerability. Given the widespread nature of Zimbra and its frequent targeting in attacks, it is essential for all organizations to patch this actively exploited flaw as soon as possible.
To protect themselves against such vulnerabilities, users must prioritize timely software updates and adhere to vendor instructions for mitigating these flaws. Furthermore, it is crucial for organizations to implement robust cybersecurity measures, including regular vulnerability assessments and penetration testing, to identify and address potential security weaknesses before they can be exploited by attackers.
In conclusion, the discovery of CVE-2025-66376 highlights the need for organizations to prioritize timely software updates and adhere to vendor instructions for mitigating vulnerabilities. By taking swift action to address this flaw, users can minimize their exposure to attacks that exploit Zimbra's security weaknesses.
CISA has ordered U.S. government agencies to patch a critical Zimbra XSS flaw as attackers continue to exploit the vulnerability, causing widespread breaches and compromising sensitive data.
Related Information:
https://www.ethicalhackingnews.com/articles/CISA-Urges-Feds-to-Patch-Zimbra-XSS-Flaw-as-Attackers-Exploit-Vulnerability-ehn.shtml
https://www.bleepingcomputer.com/news/security/cisa-orders-feds-to-patch-zimbra-xss-flaw-exploited-in-attacks/
https://cybersecuritynews.com/cisa-zimbra-collaboration-suite-xss-zero-day/
Published: Wed Mar 18 16:14:41 2026 by llama3.2 3B Q4_K_M
