Microsoft Patches Two Zero Days This Month

Remote Procedure Call Runtime Remote Code Execution Vulnerability

CVE-2023-23397 is an elevation of privilege vulnerability in Microsoft Outlook caused by improper input validation (CWE-20) combined with authentication bypass via spoofed authentication data on the channel (CWE-294), allowing an attacker to force Outlook to authenticate to an attacker-controlled SMB/WebDAV server. It is triggered when Outlook processes a crafted email or calendar object — for example a meeting or task reminder whose sound property points to an attacker-supplied UNC path — and requires no user interaction. That authentication exchange leaks the victim's NTLM credential hash, which the attacker can crack offline or relay to authenticate as the victim and access resources such as Exchange mailboxes, effectively escalating privileges. Affected software spans Microsoft 365 Apps, Microsoft Office (including the Long Term Servicing Channel), and Microsoft Outlook, which are deployed across enterprises, governments, and militaries worldwide. It is actively exploited in the wild — added to CISA's Known Exploited Vulnerabilities catalog on 2023-03-14 with a 97.4% EPSS — and Microsoft has warned of exploitation by Russia-aligned threat actors in campaigns against government and military mail servers, with patches shipped in Microsoft's March 2023 security updates.

Do: Apply Microsoft's March 2023 security updates to Microsoft 365 Apps, Office/LTSC, and Outlook immediately, per CISA's required action. As interim mitigation, enable Extended Protection for Authentication or add accounts to the Protected Users group to block the NTLM credential leak, and audit calendar and task reminder sound properties for UNC paths (Microsoft published an audit/cleanup script for this) while watching for unexpected outbound SMB/WebDAV connections from hosts running Outlook.

CVE-2023-24880 is a security feature bypass (incorrect authorization, CWE-863) in Windows SmartScreen: when a user opens a file carrying Mark-of-the-Web — such as an email attachment or a downloaded file — Windows fails to display the SmartScreen 'Open File – Security Warning' prompt that normally precedes execution. An attacker who can get a user to open a crafted malicious file therefore gains the ability to run attacker-chosen code on that machine without the SmartScreen warning, defeating a core client-side defense; the bypass was typically chained with a lure or another flaw to deliver malware, including loaders feeding ransomware operations. Any user or organization running the affected Windows 10, Windows 11, or Windows Server builds is affected, though exploitation requires local access and user interaction (CVSS 4.4, local attack vector). The flaw was a zero-day exploited in the wild before Microsoft patched it in the March 2023 Patch Tuesday release — one of two actively exploited flaws fixed that month — and Google researchers reported a ransomware gang exploited it; CISA added it to the Known Exploited Vulnerabilities Catalog on 2023-03-14 with known ransomware use, and EPSS rates the probability of exploitation within 30 days at 78.2% (100th percentile). No public proof-of-concept is known, but confirmed in-the-wild exploitation means defenders should treat it as actively targeted.

Do: Apply the March 2023 Windows security updates to all affected Windows 10 (1607, 1809, 20H2, 21H2, 22H2), Windows 11 (21H2, 22H2), and Windows Server 2016/2019/2022 systems via Windows Update, WSUS, or your patch-management tooling, and verify coverage in your inventory — this is CISA's required action for KEV. Until patched, treat any host whose users open untrusted email attachments or downloads as exposed, since the bypass silently skips the SmartScreen warning; updating is the only complete fix, with no published configuration workaround.