Threat Signal Report

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

description-logo Description

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.


Traffic Light Protocol

Color When Should it Be used? How may it be shared?


Not for disclosure, restricted to participants only.
Sources may use TLP:RED when information cannot be effectively acted upon by additional parties, and could lead to impacts on a party's privacy, reputation, or operations if misused. Recipients may not share TLP:RED information with any parties outside of the specific exchange, meeting, or conversation in which it was originally disclosed. In the context of a meeting, for example, TLP:RED information is limited to those present at the meeting. In most circumstances, TLP:RED should be exchanged verbally or in person.


Limited disclosure, restricted to participants’ organizations.
Sources may use TLP:AMBER when information requires support to be effectively acted upon, yet carries risks to privacy, reputation, or operations if shared outside of the organizations involved. Recipients may only share TLP:AMBER information with members of their own organization, and with clients or customers who need to know the information to protect themselves or prevent further harm. Sources are at liberty to specify additional intended limits of the sharing: these must be adhered to.


Limited disclosure, restricted to the community.
Sources may use TLP:GREEN when information is useful for the awareness of all participating organizations as well as with peers within the broader community or sector. Recipients may share TLP:GREEN information with peers and partner organizations within their sector or community, but not via publicly accessible channels. Information in this category can be circulated widely within a particular community. TLP:GREEN information may not be released outside of the community.


Disclosure is not limited.
Sources may use TLP:WHITE when information carries minimal or no foreseeable risk of misuse, in accordance with applicable rules and procedures for public release. Subject to standard copyright rules, TLP:WHITE information may be distributed without restriction.