Microsoft Patch Tuesday for May 2023 — Fewest vulnerabilities disclosed in a month in three

Windows Network File System Remote Code Execution Vulnerability

Windows Pragmatic General Multicast (PGM) Remote Code Execution Vulnerability

Windows Kernel Elevation of Privilege Vulnerability

Microsoft SharePoint Server Spoofing Vulnerability

CVE-2023-24955 is a code injection vulnerability (CWE-94) in on-premises Microsoft SharePoint Server that enables remote code execution over the network (CVSS 3.1: 7.2, AV:N/AC:L/PR:H/UI:N). Exploitation requires authentication with high privileges — e.g., a SharePoint site administrator account — and no user interaction, so an attacker who has obtained elevated site credentials can send crafted requests that execute code on the SharePoint server. A successful attacker gains code execution in the context of the SharePoint service, with high impact on confidentiality, integrity, and availability, providing a foothold for lateral movement or ransomware deployment. Organizations running affected on-premises SharePoint Server releases are affected; the flaw was demonstrated at Pwn2Own and Microsoft patched it in the May 2023 Patch Tuesday updates. The bug is now exploited in the wild: CISA added it to the Known Exploited Vulnerabilities catalog on 2024-03-26 with known ransomware use, and EPSS places its 30-day exploitation probability at 85.4% (100th percentile).

Do: Apply Microsoft's May 2023 (or later) security updates for SharePoint Server immediately, prioritizing internet-facing servers; CISA's KEV entry requires federal agencies to apply vendor mitigations or discontinue use of the product. Audit and tighten accounts holding SharePoint site-administrator rights, and hunt for signs of exploitation such as unexpected site-admin activity or unusual process launches from SharePoint service accounts. Public reporting on the 2024 exploitation suggests it may be chained with SharePoint privilege-escalation flaw CVE-2023-29357 to achieve unauthenticated access, so ensure both flaws are patched.

CVE-2023-29336 is a use-after-free flaw (CWE-416) in Microsoft's Win32k kernel component that allows privilege escalation to SYSTEM. It is triggered by code running on a Windows host that causes the Win32k driver to reference freed kernel memory; the exact trigger path is not detailed in the available data, but as a kernel elevation-of-privilege issue it requires local code execution or an attacker already holding a foothold on the machine. A successful exploit grants SYSTEM privileges, giving the attacker full control of the compromised host. Because Win32k ships in every supported Windows client and server, effectively the entire Windows installed base is exposed to the flaw. The vulnerability is confirmed exploited in the wild — CISA added it to the KEV catalog on 2023-05-09 — and EPSS places its 30-day exploitation probability at 40.9% (99th percentile), though no public proof-of-concept is known and ransomware use is unconfirmed.

Do: Apply Microsoft's current cumulative Windows security updates (issued May 2023, per the CISA KEV required action) across all Windows clients and servers, prioritizing servers and systems exposed to untrusted users since the flaw is being actively exploited. Until patched, limit untrusted local code execution and restrict remote entry points such as RDP, because local privilege escalation flaws are commonly chained into full compromises. Verify update installation after deployment; specific affected build numbers and any ransomware involvement are not stated in the available data.