CERT Polska, the Polish Computer Emergency Response Team (CERT), warned that attackers have begun exploiting a critical vulnerability in Zimbra Collaboration Suite (ZCS).

ZCS is a popular email and collaboration software suite used by hundreds of millions of people and organizations worldwide, including thousands of businesses and hundreds of government agencies.

The Zimbra security team released version 10.1.20 on July 20 to patch the vulnerability (tracked as CVE-2026-73570), which allows unauthenticated attackers to gain remote code execution by exploiting a command injection weakness in the SNMP monitoring component when SNMP notifications are enabled.

"Due to improper sanitization of untrusted input during SNMP notification processing, an unauthenticated attacker can send specially crafted SMTP requests that may result in execution of arbitrary operating system commands as the Zimbra user," it explained.

Internet security watchdog Shadowserver now tracks over 12,100 Zimbra servers exposed online, most of them in Europe (4,382) and Asia (4,492).

However, there is no information on how many of them are honeypots or have already been patched against the CVE-2026-73570 security flaw.

Flagged as actively exploited

On Monday, the Polish CERT team reported that threat actors are now exploiting CVE-2026-73570 in attacks.

"The CERT Polska team reports on an actively used OS Command Injection vulnerability in the Zimbra Collaboration Suite," it warned.

CERT Polska also asked admins to check their logs for suspicious activity, such as the Zimbra service restarting on its own, and for files created in the /opt/zimbra/jetty/webapps/, /opt/zimbra/jetty_base/webapps/, and /tmp/ folders by user zimbra over the last 30 days.

Zimbra flaws are frequently targeted in the wild and have been used to breach many vulnerable email servers in recent years.

For instance, Russian Winter Vivern cyber spies used a reflected XSS exploit in February 2023 to steal emails belonging to NATO-aligned individuals and organizations from Zimbra webmail portals.

In October 2024, US and UK cyber agencies warned that APT29 hackers (tracked as Midnight Blizzard and Cozy Bear and linked to Russia's Foreign Intelligence Service) were targeting vulnerable Zimbra servers by exploiting a security issue previously abused to steal email account credentials.

More recently, in March, Seqrite Labs researchers also revealed that APT28 hackers (a state-backed threat group linked to Russia's military intelligence service) were exploiting a stored cross-site scripting (XSS) vulnerability in attacks targeting Ukrainian government ZCS servers.

Overall prevention scores can hide what happens after initial access. Once attackers are using valid credentials, prevention drops sharply.

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