CVE-2024-21410

CVE-2024-21410 is a critical improper authentication flaw (CWE-287, CVSS 9.8) in on-premises Microsoft Exchange Server that enables NTLM credential relay attacks against the server's authentication, for example over SMTP. An attacker who can induce an NTLM authentication exchange can relay captured credentials to Exchange and impersonate another user, achieving an elevation of privilege without holding valid credentials themselves. Organizations running vulnerable on-premises Exchange — particularly internet-facing servers where Extended Protection for Authentication is not enforced — are affected. Exploitation is confirmed in the wild: CISA added the flaw to its Known Exploited Vulnerabilities catalog on 2024-02-15, and the fix shipped in Microsoft's February 2024 Patch Tuesday release alongside roughly 90 other vulnerabilities; no public proof-of-concept code is known.

What to do: Apply Microsoft's February 2024 Exchange Server security update immediately, prioritizing internet-facing servers; note that the update enables Extended Protection for Authentication (EPA) by default, so verify EPA is active and that clients, appliances, and load balancers relying on NTLM/SMTP authentication still function. Because the flaw is on CISA's KEV catalog, federal and high-risk operators must apply the vendor mitigations promptly or discontinue use if patching is not possible.