CVE-2022-24521

What is the Windows Common Log File System (CLFS) Driver?

The Common Log File System (CLFS, clfs.sys) is a Windows kernel driver that provides a general-purpose, high-performance transaction logging infrastructure. It is used internally by Windows for application crash logging, transaction management, and other system services. Because CLFS runs in the kernel and parses structured file-based data, memory corruption bugs in CLFS can yield kernel privilege escalation. CLFS has been one of the most frequently exploited Windows kernel components in recent years, with attackers — particularly ransomware groups — returning repeatedly to CLFS vulnerabilities as their escalation primitive of choice.

Overview

CVE-2022-24521 is a privilege escalation vulnerability in the Windows CLFS kernel driver. A low-privileged local attacker can exploit the flaw to escalate to SYSTEM-level privileges. Microsoft confirmed active exploitation at time of disclosure; CISA added it to KEV the day after the April 2022 Patch Tuesday — a two-day turnaround indicating urgent, pre-patch exploitation. The ransomwareUse: true flag reflects confirmed use by ransomware operators who exploit CLFS vulnerabilities as post-intrusion privilege escalation tools.

Affected Versions

Technical Details

The specific vulnerability class in CVE-2022-24521 was not publicly detailed by Microsoft. The CLFS driver is known to suffer from out-of-bounds writes and integer overflows in its parsing of .blf (base log file) and container log file formats. The attacker interacts with CLFS via the CreateLogFile / AddLogContainerSet / ReadLogRecord API family to trigger memory corruption in kernel space.

• Attack vector: Local — requires an existing foothold on the target system
• Privileges required: Low — a standard unprivileged user account is sufficient
• User interaction: None — fully automated once local execution is available
• Impact: Full SYSTEM privilege escalation; unrestricted access to all OS resources
• Ransomware pattern: CLFS EoP vulnerabilities are routinely incorporated into ransomware affiliate toolkits as their standard Windows privilege escalation module, enabling escalation from a low-privilege initial access (phishing, VPN compromise) to SYSTEM before deploying encryption payloads

Discovery

Credited to CrowdStrike and Adam Podlosky/Amir Bazine. The pre-patch exploitation and immediate KEV addition confirm it was found in-the-wild before Microsoft was notified or could patch.

Exploitation Context

Confirmed active exploitation before the April 2022 patch. CLFS vulnerabilities are a high-rotation target for ransomware operators — Kaspersky documented a threat actor exploiting a series of CLFS bugs (including CVE-2022-37969 and related CVEs) across 2022–2023 in ransomware deployment chains. This pattern reflects organized, sustained research into CLFS by financially motivated threat actors.

Remediation

1. Apply the April 2022 Patch Tuesday cumulative update for your Windows version immediately
2. Treat CLFS patches as high-priority in patching workflows — this driver has been a recurring ransomware EoP vector
3. Enable Windows Defender Attack Surface Reduction rules to slow post-exploitation activity
4. Monitor for unexpected SYSTEM-privilege processes spawned from low-privilege user contexts
5. Implement network segmentation to limit the blast radius of any initial access that would precede this escalation