The Cybersecurity and Infrastructure Security Agency (CISA) has added a critical Microsoft zero-day vulnerability to the Known Exploited Vulnerabilities (KEV) Catalog, mandating that all Federal Civilian Executive Branch (FCEB) agencies patch the flaw no later than January 31, 2023. The vulnerability, tracked as CVE-2023-21674, is a Windows Advanced Local Procedure Call (ALPC) elevation of privilege (EOP) vulnerability. ALPC is an inter-process message-passing protocol allowing applications to access APIs and services, as well as make Remote Procedure Calls (RPC), requesting services from programs located in another system on a network. If successfully exploited, an attacker could perform a sandbox escape, escalating their local privileges to SYSTEM, giving them the permissions they need to carry out follow-on attacks. Once an actor has escalated their privileges, they could make configuration changes, view sensitive data, and create more privileged user accounts, as well as download malicious programs. EOP vulnerabilities are usually exploited in tandem with malware, as well as other vulnerabilities like remote code execution (RCE). This flaw affects millions of organizations across the world, and due to its low complexity, it can be exploited without any victim user interaction. CTIX analysts urge all Windows users to update to the most recent secure patch immediately to prevent exploitation.