Windows CLFS and five exploits used by ransomware operators (Exploit #1 – CVE-2022

CVE-2018-8611 is an elevation-of-privilege vulnerability in the Microsoft Windows kernel caused by improper handling of objects in memory (recorded as CWE-404, improper resource shutdown/release). It is a local flaw: an attacker who can already run code with limited privileges on a target machine can trigger it with a specially crafted application, with no user interaction required. Successful exploitation executes code in kernel context, elevating the attacker to SYSTEM/administrator and giving full control of the host, a typical post-initial-access step in ransomware chains. Nearly every Windows client and server version in support at disclosure is affected: Windows 7, 8.1, RT 8.1 and Windows 10 (1607 through 1809), plus Windows Server 2008 through 2019. The flaw is confirmed exploited in the wild, with CISA adding it to the KEV catalog on 2022-05-24 and an EPSS of 4.2% (90th percentile); the KEV record lists ransomware use as unknown, though public reporting ties a family of Windows kernel EoP exploits including this one to ransomware operators.

Do: Apply Microsoft's December 2018 security update for CVE-2018-8611 on all affected Windows 7, 8.1, RT 8.1, 10 and Server 2008-2019 hosts; there is no known workaround and patching is the required CISA KEV action, so verify the December 2018 (or later) monthly rollup is installed. Prioritize servers and workstations where limited-privilege users or untrusted workloads run (RDS, application servers, shared endpoints), and treat any remaining unpatched Windows 7/8.1/legacy-Server machine as high risk because ransomware operators have been observed chaining Windows kernel EoP exploits like this one after initial access.

CVE-2021-31955 is an information disclosure flaw in the Windows kernel (CWE-497) that leaks sensitive kernel information to a locally running attacker. It is triggered by executing a malicious, specially crafted application on an affected Windows system; the attack vector is local, requires only low privileges, and needs no user interaction. Successful exploitation gives the attacker access to sensitive system information (high confidentiality impact), which is typically valuable for reconnaissance or as part of a chained attack with other kernel bugs. Organizations running Windows 10 versions 1809 through 21H1 and Windows Server 2019/2004/20H2 prior to the June 2021 security updates are affected. The flaw was exploited in the wild — Microsoft fixed it among six actively exploited zero-days in June 2021 Patch Tuesday, CISA added it to the KEV catalog on 2021-11-03, and EPSS assigns it an 81.1% probability of exploitation within 30 days.

Do: Apply the June 2021 Windows cumulative security updates per vendor instructions to Windows 10 (1809, 1909, 2004, 20H2, 21H1) and Windows Server 2019/2004/20H2, then verify affected devices are running updated builds. Because this flaw was exploited in the wild and is on CISA's KEV list, patch it on a priority timeline and check for signs of local malicious code execution. Where patching is delayed, limit local code execution on affected hosts and prioritize internet-exposed servers.

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.

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.