Microsoft Exchange Vulnerability - CVE-2021-42321

THE THREAT

eSentire has identified publicly available Proof-of-Concept (PoC) exploit code, for the critical Microsoft Exchange vulnerability CVE-2021-42321. CVE-2021-42321 was announced as part of Microsoft’s November Patch Tuesday release. Exploitation would allow a remote threat actor, with previous authentication, to execute code on vulnerable servers.

Prior to the patch release, Microsoft confirmed that limited exploitation of CVE-2021-42321 had occurred. The release of PoC exploit code significantly increases the likelihood of widespread exploitation in the near future. Organizations that have not already applied the relevant security patches are recommended to do so immediately.

What we’re doing about it

• eSentire MDR for Log has rules in place to identify exploitation on CVE-2021-42321
• eSentire’s Managed Vulnerability Service (MVS) has a local plugin in place to identify this vulnerability
• eSentire security teams are tracking this threat for additional detection and prevention opportunities

What you should do about it

• After performing a business impact review, apply the relevant Exchange security patches
  • Alternative mitigations are not available

Additional information

At the time of this reporting, Microsoft has not shared details relating to the real-world exploitation of CVE-2021-42321. Based on limited public details, it appears that only a small number of attacks have occurred. All attacks to date are believed to have been performed by the same threat actor.

The release of PoC exploit code drastically increases the likelihood that multiple threat actor groups will adopt the exploit and use it in attacks in the near future. Additionally, technical details for CVE-2021-42321 have been released. eSentire has not observed exploitation at this time. It should be noted that in order for an attacker to successfully exploit this vulnerability, they would need to have gained prior authentication via a separate attack/vulnerability.

Impacted Products:

• Microsoft Exchange Server 2019 versions prior to: 15.2.986.14 & 15.2.922.19
• Microsoft Exchange Server 2016 versions prior to: 15.1.2375.17 & 15.1.2308.20

References:

1. https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-42321
2. https://techcommunity.microsoft.com/t5/exchange-team-blog/released-november-2021-exchange-server-security-updates/ba-p/2933169
3. https://peterjson.medium.com/some-notes-about-microsoft-exchange-deserialization-rce-cve-2021-42321-110d04e8852