Hackers can execute commands on a remote computer by sending malformed emails to a Zimbra mail server. It’s critical, but difficult to exploit.

In an email sent Wednesday afternoon, Proofpoint researcher Greg Lesnewich seemed to largely concur that the attacks weren’t likely to lead to mass infections that could install ransomware or espionage malware. The researcher provided the following details:

While the exploitation attempts we have observed were indiscriminate in targeting, we haven’t seen a large volume of exploitation attempts
Based on what we have researched and observed, exploitation of this vulnerability is very easy, but we do not have any information about how reliable the exploitation is
Exploitation has remained about the same since we first spotted it on Sept. 28th
There is a PoC available, and the exploit attempts appear opportunistic
Exploitation is geographically diverse and appears indiscriminate
The fact that the attacker is using the same server to send the exploit emails and host second-stage payloads indicates the actor does not have a distributed set of infrastructure to send exploit emails and handle infections after successful exploitation. We would expect the email server and payload servers to be different entities in a more mature operation.
Defenders protecting Zimbra appliances should look out for odd CC or To addresses that look malformed or contain suspicious strings, as well as logs from the Zimbra server indicating outbound connections to remote IP addresses.