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CISA Warns of Zimbra Collaboration Suite Vulnerability Exploited in Attacks

Cybersecuritynews •Abinaya • March 19, 2026

CISA has added a high-severity vulnerability affecting the Zimbra Collaboration Suite (ZCS) to its Known Exploited Vulnerabilities (KEV) catalog. Tracked as CVE-2025-66376, this security flaw is currently facing active exploitation in the wild. Organizations utilizing Zimbra must urgently prioritize remediation to prevent unauthorized access and potential data compromise. The vulnerability is a stored cross-site scripting […]

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This article is one source in a clustered incident — the cluster page carries the summary, timeline and every other outlet covering it.

APT28 Exploits Zimbra Vulnerability in Ongoing Attacks Against Ukraine Russian state-backed hackers from APT28 are actively exploiting a high-severity stored cross-site scripting vulnerability (CVE-2025-66376) in the Zimbra Collaboration Suite (ZCS) to target Ukrainian government entities. The flaw allows unauthenticated attackers to execute remote code and harvest sensitive information from compromised email accounts. The Cybersecurity and Infrastructure Security Agency (CISA) has added this vulnerability to its Known Exploited Vulnerabilities catalog and mandated that U.S. federal agencies secure their servers by April 1, 2026. The attacks involve phishing emails containing obfuscated JavaScript payloads that exploit the vulnerability when opened in a vulnerable Zimbra session. This campaign, named Operation GhostMail, has already targeted critical infrastructure, including the Ukrainian State Hydrology Agency. Security researchers have noted that Zimbra vulnerabilities have been frequently exploited in recent years, with multiple incidents reported involving state-sponsored actors. Organizations using Zimbra are urged to apply the available patches immediately to mitigate risks. 8 sources 81

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CISA Warns of Zimbra Collaboration Suite Vulnerability Exploited in Attacks

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