The Cybersecurity and Infrastructure Security Agency (CISA) has ordered U.S. government agencies to patch an actively exploited vulnerability in Zimbra Collaboration Suite (ZCS) within three days.
The Zimbra security team patched the security flaw (tracked as CVE-2026-73570) in version 10.1.20, released on July 20.
Successful exploitation allows unauthenticated attackers to gain remote code execution by exploiting a command injection weakness in the SNMP monitoring component when SNMP notifications are enabled on the targeted system.
"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.
CISA's warning comes after CERT Polska, the Polish Computer Emergency Response Team (CERT), first flagged the vulnerability as targeted in the wild last Monday.
While threat security watchdog Shadowserver tracks more than 12,000 Zimbra servers exposed on the Internet, there is no information on how many are honeypots or have already been secured against attacks exploiting the CVE-2026-73570 flaw.
On Friday, CISA confirmed CERT Polska's alert, added the flaw to its KEV catalog, and ordered U.S. Federal Civilian Executive Branch (FCEB) agencies to secure their systems within three days, by August 24.
Although CISA didn't share any information on these ongoing attacks, the Polish CERT team asked security teams to check logs for suspicious activity, such as the Zimbra service restarting unexpectedly, 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.
ZCS is a popular email and collaboration suite used by hundreds of millions of organizations and people worldwide, including hundreds of government agencies and thousands of businesses.
Zimbra security issues are commonly targeted in the wild and have been used to steal sensitive data from vulnerable email servers in recent years.
Most recently, Seqrite Labs researchers revealed in March that APT28 (a state-sponsored threat group linked to Russia's military intelligence service) was exploiting a stored cross-site scripting (XSS) vulnerability in attacks targeting Ukrainian government ZCS servers.
In October 2024, U.S. and UK cyber agencies warned that APT29 hackers (tracked as Midnight Blizzard and Cozy Bear) linked to Russia's Foreign Intelligence Service were targeting Zimbra servers using a flaw previously exploited to steal email account credentials.
Russian Winter Vivern cyber spies have also abused a reflected Cross-Site Scripting (XSS) vulnerability to steal emails belonging to NATO-aligned individuals and organizations via Zimbra webmail portals.
Once attackers have valid credentials, only 37% of their actions are blocked
Overall prevention scores can hide what happens after initial access. Once attackers are using valid credentials, prevention drops sharply.
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