Hackers have been exploiting a critical vulnerability in the Zimbra Collaboration Suite in an attempt to obtain email backups and authentication credentials of vulnerable organzations, Microsoft has warned.

The vulnerability, tracked as CVE-2026-73570, lets attackers remotely issue operating system commands without authentication. Zimbra maintainer Synacor issued a patch on July 20, but didn’t disclose the vulnerability for more than three weeks after that. The security-focused Shadowserver Foundation said last week that its scans found that 274 separate instances of the Zimbra Collaboration Suite had been compromised. The number of servers running the software has fluctuated from 19,000 in the week following the patch to about 12,000 in the weeks following that. Currently, Shadowserver is tracking about 10,000 instances.

Look, ma, no authorization

From July 28 to August 7, Microsoft said Wednesday, the company detected two distinct scanning tools probing the Internet for vulnerable endpoints. The attackers first validated their exploit worked by sending HTTP, requests and DNS, ICMP, and out-of-band identity checks to domains hosted on public services. The probes allowed the attackers to confirm the exploit successfully executed commands on vulnerable servers without actually compromising them. Eventually, the attackers began using their command injection capability to install malicious payloads. Microsoft wrote:

Following successful exploitation, observed activity included deployment of JSP web shells and reverse shells, privilege escalation, persistent remote-access tooling, and memory-backed execution. Threat actors also accessed email and collected authentication and mailbox data, with archive creation and subsequent transfer activity observed. The activity included both automated payload delivery and hands-on-keyboard operations on compromised mail servers. Microsoft observed affected organizations in more than one region and industry. Based on the environments investigated, exploitation was not limited to a single sector or geographic area.

CVE-2026-73570 allows remote attackers with no credentials to run operating system commands through a crafted email that targets the ZCS SNMP notification path but only when an optional zimbra-snmp package is in place and SNMP notifications are enabled.