Microsoft Patches Six Zero-Day Security Holes

CVE-2021-28550 is a use-after-free memory corruption flaw in Adobe Acrobat DC and Acrobat Reader DC that an unauthenticated attacker can trigger by getting a victim to open a malicious PDF file. Successful exploitation allows arbitrary code execution in the context of the current user, giving the attacker the privileges of that user on the affected machine. Anyone running Acrobat or Acrobat Reader DC at or below versions 2021.001.20150, 2020.001.30020, or 2017.011.30194 is affected. The flaw was exploited as a zero-day in targeted attacks — Microsoft reported it being chained with Windows zero-days by an Austrian company's operators — and it is listed in CISA's Known Exploited Vulnerabilities catalog as of November 3, 2021. CISA's required action is to apply vendor updates, and defenders should treat exploited, user-targeted PDF attacks as the primary risk.

Do: Upgrade Acrobat and Acrobat Reader DC to versions later than 2021.001.20150, 2020.001.30020, and 2017.011.30194 on the respective tracks, per Adobe's May 2021 update and the CISA KEV required action. Until patched, avoid opening PDFs from untrusted sources and consider email-gateway filtering or sandboxing of PDF attachments. Check endpoint inventory for the affected version ranges and prioritize systems of users who handle unsolicited documents.

CVE-2021-31199 is an elevation-of-privilege vulnerability in the Microsoft Enhanced Cryptographic Provider, a core Windows component that provides cryptographic services to applications. It carries a CVSS 3.1 score of 5.2 (AV:L/AC:L/PR:L/UI:N/S:C), meaning it is triggered by an attacker who already runs low-privileged code locally on a vulnerable Windows system, with no user interaction required, and lets the attacker break out of the intended security scope to gain elevated privileges. The gain is higher privileges on the compromised host, typically used as a stepping stone in a broader intrusion or malware delivery chain. Exposure is broad: any unpatched Windows 7, 8.1, RT 8.1, Windows 10 (versions 1507 through 21H1), Windows Server 2008, or Windows Server 2004 system is affected, since the provider ships with Windows itself. Exploitation is confirmed in the wild: it was one of the actively exploited Windows zero-days fixed in Microsoft's mid-2021 Patch Tuesday release (part of the six/seven-zero-day coverage), Microsoft attributed targeted attacks to the Austrian firm DSIRF using its Subzero surveillance malware, and the flaw was added to CISA's KEV on 2021-11-03; EPSS estimates a 3.0% chance of exploitation in the next 30 days (86th percentile) and no public PoC is known.

Do: Apply Microsoft's security updates (the monthly Patch Tuesday cumulative updates covering this CVE) per vendor instructions for every in-scope Windows version — the flaw is in CISA KEV, so patching is mandatory for federal agencies and there is no documented workaround. Verify hosts have received the updated cumulative update, prioritizing multi-user endpoints, RDP/terminal servers, and workstations where untrusted code runs; if patching is delayed, hunt for signs of targeted intrusion consistent with DSIRF/Subzero activity.

CVE-2021-33739 is an elevation-of-privilege vulnerability in the Microsoft Desktop Window Manager (DWM) Core Library, the component that composites the Windows graphical desktop. A local attacker who can already execute code on a vulnerable machine — for example via a malicious application, an Office payload, or a chained remote code execution bug — can exploit the flaw in DWM to gain elevated privileges on the system. Successful exploitation yields high-impact results (CVSS 8.4, high for confidentiality, integrity and availability), and the bug is typically used to break out of a low-privileged context or complete a full compromise chain. Affected systems are Windows 10 versions 1909, 2004, 20H2 and 21H1, and Windows Server versions 2004 and 20H2, prior to the June 2021 security updates. The flaw was one of six zero-days Microsoft confirmed as actively exploited in the June 2021 Patch Tuesday release; it was added to CISA's Known Exploited Vulnerabilities catalog on 2021-11-03 and carries a 6.6% EPSS (93rd percentile), although no public PoC is known and ransomware use is undetermined.

Do: Apply the June 2021 (or later) Windows cumulative updates on Windows 10 1909/2004/20H2/21H1 and Windows Server 2004/20H2, consistent with CISA's required action to apply vendor updates. Because this is a local privilege escalation, prioritize hosts where untrusted or multi-user code runs (VDI, RDS, kiosks, jump servers) and environments with KEV-mandated remediation deadlines. Verify patch status against the June 2021 release, since the bug was confirmed exploited in the wild.

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