Critical Exchange Server Flaw (CVE-2024

CVE-2024-21412 is a security feature bypass (CWE-693) in how Microsoft Windows handles Internet Shortcut files: a crafted shortcut can make Windows skip the security warning prompt that normally appears before untrusted internet content is opened or downloaded. Triggering it requires user interaction — an attacker must deliver a malicious shortcut file, typically via email or a malicious website, and convince the user to open it, which is reflected in the CVSS vector's UI:R component. An attacker who succeeds gains a bypass of those prompts, making it easier to retrieve and execute malicious remote content with fewer warnings; the DarkGate malware operators used exactly this technique in zero-day campaigns to distribute their loader. Anyone running the affected Windows 10 (1809, 21H2, 22H2), Windows 11 (21H2, 22H2, 23H2), Windows Server 2019, or Windows Server 2022 (including 23H2) builds was exposed. The flaw was patched in Microsoft's February 2024 Patch Tuesday release (2024-02-13), the same day CISA added it to the KEV catalog, and it is under active exploitation with known ransomware association and a 95.4% EPSS score.

Do: Apply the February 2024 Windows cumulative security update (released 2024-02-13) or any later monthly cumulative update to every affected Windows 10, Windows 11, Windows Server 2019, and Windows Server 2022 build, and verify patch levels through your endpoint inventory. Because exploitation requires user interaction, as an interim control flag or block .url/Internet Shortcut attachments at email gateways and remind users not to open shortcuts from untrusted sources. Prioritize internet-facing and shared endpoints given the KEV listing and known ransomware use.

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.

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.

CVE-2024-21413 is an improper input validation flaw (CWE-20) in Microsoft Outlook, publicly dubbed "MonikerLink", in which Outlook mishandles a specially crafted hyperlink (a file:// moniker link) and bypasses the security prompt normally applied before opening such links. The flaw is triggered when a user opens or clicks a maliciously crafted link in an email, causing Outlook to invoke the target outside its protected handling. A successful attack can leak the user's NTLM credentials and can achieve remote code execution in the context of the current user; the flaw carries a critical CVSS 3.1 score of 9.8. Anyone running affected Outlook clients — Microsoft 365 Apps, Office 2016, Office 2019, and Office LTSC — is exposed, and the issue was fixed in Microsoft's February 2024 Patch Tuesday release. Exploitation is confirmed in the wild: CISA added the flaw to its Known Exploited Vulnerabilities catalog on 2025-02-06, EPSS assigns a ~95% exploitation probability (100th percentile), and a public PoC is available.

Do: Apply Microsoft's February 2024 (or later) security updates for Microsoft 365 Apps, Office 2016, Office 2019, and Office LTSC, and verify Outlook builds are current, per the KEV required action to apply vendor mitigations or discontinue use. As interim mitigation, restrict outbound SMB/NTLM from endpoints (e.g., block outbound port 445 or disable NTLM where feasible) to blunt credential leakage from crafted file:// links. Hunt for signs of exploitation, such as unexpected outbound SMB connections or NTLM authentication events following users clicking links in email.