CVE-2023-21674

CVE-2023-21674 is a use-after-free flaw (CWE-416) in the Windows Advanced Local Procedure Call (ALPC) facility, the kernel-level mechanism Windows uses for fast communication between processes and system services. An attacker who can already run code on a Windows machine can trigger the bug by sending crafted ALPC requests, corrupting memory in a privileged process. Successful exploitation allows a local, low-privileged attacker to elevate to SYSTEM/administrator privileges, typically to gain full control of the host or to complete an exploit chain after an initial compromise. Essentially all supported Windows client and server installations from Microsoft are affected; the source data does not list specific versions, and the fix shipped with Microsoft's January 2023 Patch Tuesday, which addressed a record 974 vulnerabilities including this and one other actively exploited Windows zero-day. CISA added the CVE to the Known Exploited Vulnerabilities catalog on 2023-01-10, confirming exploitation in the wild; no public proof-of-concept is known, ransomware use is undetermined, and EPSS estimates a 41.8% probability of exploitation in the next 30 days (99th percentile).

What to do: Apply Microsoft's January 2023 (released 2023-01-10) or later Windows cumulative security updates to all Windows clients and servers, per CISA's required action to 'apply updates per vendor instructions.' Because this is a local privilege escalation, prioritize hosts where low-privileged users or untrusted code execute, such as endpoints, RDS/VDI servers, and multi-user systems, and verify installation via your update-management inventory. Treat this as actively exploited; with no public PoC or documented workaround, timely patching is the primary mitigation.