Microsoft Patch Tuesday for Oct. 2021 — Snort rules and prominent vulnerabilities

Microsoft Exchange Server Remote Code Execution Vulnerability

Windows Print Spooler Spoofing Vulnerability

Windows Hyper-V Remote Code Execution Vulnerability

Microsoft's Win32k kernel driver contains a use-after-free (CWE-416) local privilege escalation vulnerability (CVE-2021-40449) affecting Windows client versions from Windows 7 through Windows 11 21H2. A local attacker who can already execute code on a system can trigger the flaw — demonstrated in a public proof of concept via the NtGdiResetDC system call — to corrupt kernel memory and elevate to SYSTEM-level privileges with high impact on confidentiality, integrity, and availability (CVSS 7.8, local vector, no user interaction). Any Windows desktop or laptop running the affected versions is exposed to any unprivileged process or malware that gains a foothold on the device. The bug was exploited as a zero-day in the wild before being fixed; CISA added it to the KEV catalog on 2021-11-17 with known ransomware use, and public reporting ties exploitation to the MysterySnail RAT campaign (including the lightweight MysteryMonoSnail backdoor). EPSS places the flaw in the 99th percentile, with a 74.1% probability of exploitation activity within 30 days.

Do: Apply Microsoft's security updates addressing this Win32k flaw via Windows Update on all affected Windows 7, 8.1, 10, and 11 clients, per the CISA KEV required action, and verify inventories show no unpatched Windows 10 builds (1507–21H1) or Windows 11 21H2 endpoints. Because this was a zero-day exploited in the wild — including in campaigns involving MysterySnail and ransomware use noted by CISA — treat patching as urgent on all endpoints. Until patched, limit execution of untrusted or unprivileged local code on these systems.

CVE-2021-40450 is an elevation-of-privilege vulnerability in Win32k, the Windows kernel-mode graphics/window-manager subsystem, affecting Windows 10 (builds 1809 through 21H1), Windows 11 21H2, and Windows Server 2019, 2004, 20H2, and 2022. It is triggered locally: per the CVSS vector (AV:L/PR:L/UI:N), an attacker must already be able to execute code on the target as a low-privileged user, with no user interaction required, and then abuses the Win32k flaw to escalate. Successful exploitation yields SYSTEM-level privileges, giving full control of the host (reading or modifying any data, installing software, disabling security controls), and such flaws are commonly chained with an initial remote-code-execution or phishing foothold to fully compromise a machine. Any organization or individual running the affected Windows builds is exposed in principle, though practical risk concentrates on hosts where an attacker can first gain a local foothold. The flaw was addressed in Microsoft's October 2021 Patch Tuesday; it was added to CISA's Known Exploited Vulnerabilities catalog on 2022-04-25, confirming in-the-wild exploitation (no public PoC is known, EPSS estimates roughly 2.1% probability of exploitation in the next 30 days, and any ransomware association is unconfirmed).

Do: Apply Microsoft's October 2021 security updates (per vendor instructions) to all affected Windows 10, Windows 11, and Windows Server hosts, prioritizing endpoints exposed to untrusted users and servers where an attacker could chain this elevation with a remote-code-execution or phishing foothold. Because the flaw is KEV-listed with confirmed in-the-wild exploitation, verify installed patch/build levels across the estate and check hosts with any indicators of compromise for local privilege-escalation activity; if patching is delayed, limit local code execution by untrusted users and monitor for suspicious SYSTEM-level process activity.

Microsoft Excel Remote Code Execution Vulnerability