CVE-2021-36948

CVE-2021-36948 is an elevation-of-privilege flaw in the Microsoft Windows Update Medic Service (WaaSMedicSvc), the built-in service that keeps Windows Update functioning; Microsoft has not publicly detailed the underlying bug. A local attacker who can execute code on a target machine can abuse the service to elevate their privileges to higher integrity levels, typically SYSTEM, enabling full control of the host such as installing software, modifying accounts, and disabling defenses. Any Windows system running the Windows Update Medic Service is affected; CISA lists the impacted product simply as 'Microsoft Windows' without version detail, and the service ships with Windows 10 and later. The flaw was added to CISA's Known Exploited Vulnerabilities catalog on 2021-11-03, indicating confirmed exploitation in the wild, with no public proof-of-concept code known and ransomware use undetermined. Microsoft's EPSS model assigns a 26.7% probability of exploitation within 30 days (98th percentile), so patching urgency is high.

What to do: Apply Microsoft's November 2021 Patch Tuesday cumulative updates (per the vendor's advisory) via Windows Update or your patch management platform, prioritizing servers, workstations, and multi-user hosts where local privilege escalation is most damaging. Since CISA lists this in KEV, federal and regulated environments must patch by the catalog deadline; as an interim mitigation, restrict untrusted local code execution and review whether any local accounts show unexpected SYSTEM-level activity.