Attacker exploits undisclosed Microsoft Exchange zero-day vulnerability

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Researchers from Vietnamese cybersecurity outfit GTS have uncovered a malicious campaign that exploits a yet-to-be-revealed zero-day vulnerability in Microsoft Exchange.

The researchers assume that a Chinese threat group is responsible for the attacks, which are based on the code page of the web shells. Investigations show that the user agent with which the web shells were installed also belongs to Antsword, a China-based open-source website admin tool for web shells with support for web shell management.

To carry out the attacks, the attackers formed two zero-day chains to use Chinese Chopper web shells on compromised servers for persistence and data theft while moving laterally to other systems on the victim’s networks.

The bugs tracked as ZDI-CAN-18333 and ZDI-CAN-18802 have been reported to Microsoft. However, Microsoft has not yet disclosed information about the two vulnerabilities and has yet to assign them a CVE ID to track them.

Until Microsoft releases security updates to fix the two zero-days, GTSC has provided a temporary mitigation that can help block attack attempts. This temporary mitigation includes adding a new IIS server rule using the URL Rewrite Rule module in Autodiscover at FrontEnd while selecting tab URL Rewrite and then Request Blocking; adding the string “.*Autodiscover\.json.*\@,*PowerShell.*” to the URL path; condition input: Choose {REQUEST _URL}.

Admins interested in checking if their Exchange servers have already been compromised by this exploit can run the following PowerShell command to search IIS log files for compromise indicators.

The sources for this piece include an article in BleepingComputer.

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