This indicates an attack attempt to exploit an Unrestricted File Upload Vulnerability in Zimbra Collaboration.
This vulnerability is due to improper input validation. A remote, unauthenticated attacker can exploit this vulnerability by sending a crafted HTTP request to the target server. Successfully exploiting this vulnerability can result in uploading of web shells and remote code execution.
Zimbra Collaboration 8.8.15 before Patch 33
Zimbra Collaboration 9.0.0 before Patch 26
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.