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Defense Digests

Microsoft Exchange Vulnerability Actively Exploited in the Wild

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Dataprise Defense Digest 

ID: 2024-0002-1

Severity: Critical (9.8)

Published: February 15, 2023 


On February 13, 2024, Microsoft released a critical vulnerability (9.8 CVSS) being tracked as CVE-2024-21410 for a vulnerability found in Exchange Server. As of this time, the vulnerability has been, and is currently being exploited in the wild. The exploit is described as a privilege escalation technique that would allow the bad actor to successfully complete pass-the-hash attacks. Upon further investigation into the matter, Microsoft has uncovered that the initial vector of the attack was being allowed due to Extended Protection for Authentication (EPA) not being enabled by default in Exchange Server 2019. The company has since addressed this vulnerability with the release of Exchange Server 2019 Cumulative Update 14 (CU14). Russian state-affiliated hacking crews such as APT28 (aka Forest Blizzard) have a history of exploiting flaws in Microsoft Outlook to stage NTLM relay attacks.

Dataprise is aware of the critical nature of this vulnerability and is conducting a review of all available analyses of these vulnerabilities and the potential impact on our clients. This has been a major exercise as the investigation requires that a specific order of actions is taken to achieve the response objectives. Right now, our teams are working to confirm whether the recommended mitigation steps can be applied without causing any customer-facing service interruptions.


According to Microsoft, a bad actor could exploit this flaw and relay the legitimate user’s Net-NTLMv2 hash against the weakened server to then authenticate as the user. Microsoft notes that “An attacker could target an NTLM client such as Outlook with an NTLM credentials-leaking type vulnerability. The leaked credentials can then be relayed against the Exchange server to gain privileges as the victim client and to perform operations on the Exchange server on the victim’s behalf”. No further details have been released pertaining to the parties that are actively exploiting this vulnerability. Dataprise highly recommends reading the knowledge article provided by Microsoft and implementing the Cumulative Update to patch this critical vulnerability.


Microsoft has provided the following knowledge article explaining the fix for the existing vulnerability and includes the download location for the Exchange Server 2019 Cumulative Update.

Microsoft has provided the below knowledge article outlining the necessary steps and script to employ the fix for older Exchange Server versions.



  • Josh McNulty, vCISO 
  • Stephen Jones, VP, Cybersecurity

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