
Zimbra Email Servers Under Attack: CVE-2026-73570 Enables Web Shell Deployment and Credential Theft
In July 2026, threat actors exploited CVE-2026-73570, an unauthenticated command injection vulnerability in Zimbra Collaboration Suite, to deploy web shells and harvest authentication secrets from internet-facing email servers. The critical flaw (CVSS 8.9) enabled attackers to execute remote code through specially crafted SMTP requests without authentication when SNMP notifications were enabled. Microsoft observed multi-stage attacks across multiple regions and industries between July 28 and August 7, 2026, where attackers deployed JSP web shells, established persistence mechanisms, escalated privileges, and exfiltrated mailbox data and authentication tokens including zimbraPreAuthKey and zimbraTwoFactorAuthSecret. This incident highlights the accelerating trend of targeting email infrastructure as attackers increasingly focus on business-critical communication systems and identity-related credentials that enable broader enterprise compromise and long-term persistence across hybrid cloud environments.
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