Urgent: Microsoft Issues Patches for 97 Flaws, Including Active Ransomware Exploit

Microsoft's WinVerifyTrust function improperly validates Windows Authenticode signatures on portable executable (PE) files, allowing crafted modifications to a signed binary — such as altered sections or checksum-table manipulation — to still pass signature verification. An attacker who tampers with a legitimately signed executable can distribute modified files that Windows and dependent security controls treat as authentic, gaining the trust of a valid digital signature and, ultimately, remote code execution in contexts that rely on signature checks. Any Microsoft Windows platform performing Authenticode verification through WinVerifyTrust is affected; the source data does not list specific Windows version ranges, but because WinVerifyTrust is core Windows functionality the exposure is very broad. The flaw was added to CISA's Known Exploited Vulnerabilities catalog on 2022-01-10, indicating confirmed in-the-wild exploitation, and EPSS puts its 30-day exploitation probability at about 44.6% (99th percentile), though no public PoC is known.

Do: Apply Microsoft's updates for CVE-2013-3900 across all Windows systems per vendor instructions, as required for CISA KEV entries. Where supported, enable Microsoft's stricter certificate padding check (EnableCertPaddingCheck) per Microsoft guidance to harden signature verification against this bypass. Prioritize patching endpoints and servers that execute or validate downloaded, emailed, or otherwise externally sourced signed binaries, given confirmed exploitation and high EPSS.

Secure Boot Security Feature Bypass Vulnerability

CVE-2022-24521 is an elevation-of-privilege flaw in the Windows Common Log File System (CLFS) driver, a kernel component, caused by an out-of-bounds write (CWE-787). A local attacker who already has limited privileges on an affected Windows machine can trigger the bug and gain elevated (SYSTEM/administrator) rights without any user interaction. Because the CLFS driver is part of the operating system, every user and service on an unpatched host is exposed to post-compromise escalation, which ransomware operators use to move from an initial foothold to full control. Affected products per the data include Windows 10 releases 1507, 1607, 1809, 1909, 20H2, 21H1, and 21H2, Windows 11 21H2, Windows 7, Windows 8.1, Windows RT 8.1, and Windows Server 2008. The flaw was patched in Microsoft's April 2022 Patch Tuesday, added to CISA's Known Exploited Vulnerabilities catalog on 2022-04-13, and is known to be used in ransomware campaigns, with public reporting tying exploitation to Cuba ransomware activity.

Do: Apply the April 2022 Microsoft security updates (Patch Tuesday, released April 12, 2022) to all affected Windows 10/11, Windows 7/8.1/RT 8.1, and Windows Server 2008 systems immediately, per the CISA KEV required action; note that Windows 7/8.1/RT 8.1 and Server 2008 may require Extended Security Updates to receive the fix. Because this is a local privilege escalation, prioritize hosts reachable for initial access, review endpoint telemetry for suspicious limited-user-to-SYSTEM activity, and watch for indicators associated with Cuba ransomware campaigns exploiting this flaw.

The Windows Common Log File System (CLFS) driver contains an input-validation flaw (CWE-20) that leads to an out-of-bounds write (CWE-787), allowing an attacker who already has a foothold or local access on a system to escalate privileges. It is triggered when the driver processes malformed or specially crafted common log file data, rather than via a remote network request on its own. Successful exploitation yields elevated (typically SYSTEM-level) privileges, giving the attacker full control of the host and making the bug a common post-exploitation link in ransomware chains. CISA lists the affected product broadly as 'Microsoft Windows', so essentially all Windows installations current at the time of disclosure were in scope. The flaw is confirmed exploited in the wild: CISA added it to the KEV catalog on 2022-09-14 with known ransomware use, EPSS puts 30-day exploitation probability at 28.3% (98th percentile), and no public PoC is known.

Do: Apply Microsoft's September 2022 (or later) cumulative updates to every Windows host per vendor instructions, prioritizing endpoints and servers exposed for user or remote access. Because this is a local privilege escalation, treat it as a post-exploitation risk: also harden and patch initial-access surfaces (RDP, VPN, other exposed services) and hunt for signs of compromise such as unexpected SYSTEM-spawned processes or anomalous activity involving clfs.sys. Ransomware operators are known to use this bug, so remediation should be treated as urgent.

Microsoft Message Queuing (MSMQ) Remote Code Execution Vulnerability

CVE-2023-23376 is a heap-based buffer overflow (out-of-bounds write, CWE-122/CWE-787) in the Windows Common Log File System (CLFS) kernel driver. A local attacker with low privileges can trigger the flaw when the driver mishandles CLFS log-file data, with no user interaction required. Successful exploitation elevates the attacker from a low-privileged user to SYSTEM/kernel-level privileges, which is why ransomware operators chain it with other bugs after gaining an initial foothold. All supported Windows 10 (1507, 1607, 1809, 20H2, 21H2, 22H2), Windows 11 (21H2, 22H2), and Windows Server 2008, 2012, 2016, and 2019 releases as of February 2023 are affected. The flaw is actively exploited in the wild: it was added to CISA's Known Exploited Vulnerabilities catalog on February 14, 2023 with known ransomware use, Microsoft shipped the fix in the February 2023 Patch Tuesday release, and EPSS estimates roughly an 11% probability of exploitation within the next 30 days (96th percentile).

Do: Apply the Microsoft security updates released on February 14, 2023 (February Patch Tuesday) to every Windows 10, Windows 11, and Windows Server system in the affected version list, prioritizing servers and endpoints exposed to ransomware-prone environments. Verify patch status via update history or vulnerability scanning, since exploitation requires only local low-privileged access and there is no substitution for patching. Given confirmed ransomware chaining, also hunt for signs of post-compromise privilege escalation on hosts that were unpatched before mid-February 2023.

CVE-2023-28252 is a heap-based buffer overflow (CWE-122) in the Microsoft Windows Common Log File System (CLFS) driver that allows privilege escalation. The flaw is triggered when the kernel's CLFS driver processes malformed or maliciously crafted log file data, corrupting heap memory; an attacker who can already run code on a target system (e.g., a low-privileged user or an attacker chained with another flaw such as a remote code execution bug) can leverage it to gain SYSTEM-level privileges. Because the CLFS driver ships with supported Windows client and server releases, essentially the entire Windows installed base is potentially exposed. The vulnerability is being actively exploited: CISA added it to the KEV catalog on 2023-04-11 with known ransomware use, and EPSS estimates a 49.0% probability of exploitation within 30 days (99th percentile). It was addressed in Microsoft's April 2023 security updates, and the required remediation action is to apply the vendor updates.

Do: Apply Microsoft's April 2023 (or later) cumulative security updates to all Windows clients and servers per vendor instructions, prioritizing high-value and domain infrastructure systems given the known ransomware use; there is no public PoC or known workaround, so patching is the primary mitigation. Note that this is a local elevation-of-privilege flaw, so also hunt for prior low-privilege access or exploitation chains on hosts, and confirm remediation by checking installed update levels across the estate.