WARNING: New Unpatched Microsoft Exchange Zero-Day Under Active Exploitation

Security researchers are warning of previously undisclosed flaws in fully patched Microsoft Exchange servers being exploited by malicious actors in real-world attacks to achieve remote code execution on affected systems.

The two vulnerabilities, which are formally yet to be assigned CVE identifiers, are being tracked by the Zero Day Initiative as ZDI-CAN-18333 (CVSS score: 8.8) and ZDI-CAN-18802 (CVSS score: 6.3).

Successful exploitation of the flaws could be abused to gain a foothold in the victim's systems, enabling adversaries to drop web shells and carry out lateral movements across the compromised network.

There detected webshells, mostly obfuscated, being dropped to Exchange servers. Using the user-agent, we detected that the attacker uses Antsword, an active Chinese-based open source cross-platform website administration tool that supports web shell management.

The cybersecurity company theorized that the attacks are likely originating from a Chinese hacking group owing to the web shell's encoding in simplified Chinese (Windows Code page 936).

Also deployed in the attacks is the China Chopper web shell, a lightweight backdoor that can grant persistent remote access and allow attackers to reconnect at any time for further exploitation.

It's worth noting that the China Chopper web shell was also deployed by Hafnium, a suspected state-sponsored group operating out of China, when the ProxyShell vulnerabilities were subjected to widespread exploitation last year.

At least more than one organization has been the victim of an attack campaign leveraging the zero-day flaws. Additional details about the bugs have been withheld in light of active exploitation.

In the interim, as temporary workarounds, it's recommended to add a rule to block requests with indicators of compromise using the URL Rewrite Rule module for IIS servers.

In Autodiscover at FrontEnd, select tab URL Rewrite, and then select Request Blocking
Add string ".*autodiscover\.json.*\@.*Powershell.*" to the URL Path, and
Condition input: Choose {REQUEST_URI}
I can confirm significant numbers of Exchange servers have been backdoored - including a honeypot