New Proof of Concept for CVE-2021-42321 Released (Microsoft Exchange Remote Code Execution Vulnerability)


FortiGuard Labs is aware of a new proof of concept that is leveraging CVE-2021-42321, a Microsoft Exchange Server Remote Code Execution Vulnerability. The proof of concept, released by security researcher @jannggg on Twitter is a post authentication remote code execution vulnerability. Patches for CVE-2021-42321 were released by Microsoft on November 9th, and the vulnerability is rated as IMPORTANT.

What is the CVSS Score?

This vulnerability has a CVSS Base Score of 8.8.

Does the Attacker Need to be Authenticated?

Yes. The attacker needs to be authenticated to the Microsoft Exchange Server.

What Versions of Software are Affected?

Microsoft has released security updates for for the following versions of Microsoft Exchange:

Exchange Server 2013

Exchange Server 2016

Exchange Server 2019

Is this Being Exploited In the Wild?

Yes, Microsoft states that exploitation is limited to targeted attacks.

Has the Vendor Issued a Patch?

Yes, Microsoft issued a patch on November 9th. For further information on the vulnerability, including a link towards the available patches, please refer to the "Released: November 2021 Exchange Server Security Updates" link in the APPENDIX.

Any Suggested Mitigation?

As there have been reports of exploitation in the wild, including proof of concept code now available, it is imperative that patches are applied to affected systems as soon as possible. Also, to determine which machines may be behind on updates with respect to this latest patch, Microsoft has made available a PowerShell script that will help inventory potentially vulnerable machines on the network. Please refer to the "Exchange Server Health Checker" in the APPENDIX for this script.

What is the the Status of Coverage?

Coverage is being investigated at this time for feasibility. This threat signal will be updated once there is further information available.