CVE-2023-28252

What is the Windows Common Log File System Driver?

The Common Log File System (CLFS) is a Windows kernel-mode driver (CLFS.sys) that provides a high-performance logging infrastructure used internally by many Windows subsystems — including the Transaction Manager, Active Directory, and Exchange Server. CLFS manages binary log files with a complex, parser-heavy format. Because user-space applications can interact with CLFS directly through system calls, any memory corruption in the kernel-mode parser is reachable from an unprivileged process. CLFS has been exploited repeatedly in zero-day attacks since 2022 due to the complexity of its binary file format handling and the large attack surface exposed to user space.

Overview

CVE-2023-28252 is a heap-based buffer overflow (CWE-122) in the Windows CLFS kernel driver that allows a local attacker with standard user privileges to escalate to SYSTEM. It was patched in April 2023 Patch Tuesday as an actively exploited zero-day — simultaneously added to the CISA KEV catalog — and was the third CLFS-based zero-day exploited in ransomware attacks within six months. Kaspersky GReAT researchers Boris Larin discovered the in-the-wild exploitation and attributed it to the Nokoyawa ransomware group.

Affected Versions

Technical Details

A heap-based buffer overflow (CWE-122) occurs when a program writes beyond the bounds of a heap-allocated buffer, overwriting adjacent heap memory. In the CLFS driver, the vulnerability arises from insufficient bounds checking when parsing specific structured fields in CLFS binary log files (.blf files). A crafted log file triggers the overflow when CLFS processes the malformed structure in kernel mode.

By controlling what data is written beyond the buffer's bounds — targeting a subsequent kernel heap allocation — an attacker corrupts kernel data structures. Exploitation typically follows a heap grooming pattern:

1. Groom the kernel heap: allocate objects to position a known structure adjacent to the vulnerable allocation.
2. Trigger the overflow: craft a CLFS log file that causes the overflow to overwrite the adjacent structure.
3. Corrupt a security-relevant field: target a token pointer, function pointer, or privilege field in the overwritten structure.
4. Escalate to SYSTEM: execute the corrupted path, gaining kernel-level code execution or SYSTEM-level privileges.

The AC:L (low complexity) rating means that once a working exploit is built, it is reliably reproducible without special conditions or timing requirements.

Discovery

Boris Larin of Kaspersky's GReAT (Global Research & Analysis Team) discovered CVE-2023-28252 being actively exploited in the wild and reported it to Microsoft. Kaspersky's analysis documented the CLFS exploit as a component of Nokoyawa ransomware campaigns, where it provided the post-exploitation privilege escalation step.

Exploitation Context

Nokoyawa is a ransomware-as-a-service operation active in 2022–2023, targeting enterprises in retail, healthcare, and financial sectors primarily in Asia and South America. The group's use of a CLFS zero-day represents a significant investment — indicating they obtained the exploit from a broker or shared-infrastructure supplier that maintains Windows LPE capabilities.

CVE-2023-28252 is the third CLFS zero-day used in ransomware operations in rapid succession:

• CVE-2022-37969 (September 2022): CLFS zero-day patched after exploitation
• CVE-2023-23376 (February 2023): CLFS zero-day exploited in ransomware deployments
• CVE-2023-28252 (April 2023): CLFS zero-day, Nokoyawa ransomware

The pattern reflects both the attack surface's richness (complex kernel binary parser reachable from unprivileged user space) and the ransomware ecosystem's consistent demand for reliable Windows LPE primitives for post-exploitation use.

Remediation

1. Apply the April 2023 Windows cumulative update — patches CVE-2023-28252. Systems without this update are at high risk from ransomware deployments that chain initial access with CLFS-based privilege escalation.
2. Prioritize patching servers — CLFS-based LPE is particularly dangerous on file servers and backup hosts where ransomware operators seek to encrypt the widest possible data set.
3. Deploy behavioral endpoint detection — monitoring for low-privileged processes spawning SYSTEM-privileged children, or unexpected manipulation of Windows token privileges, detects CLFS LPE exploitation patterns.
4. Maintain current Windows cumulative updates — three CLFS zero-days in six months confirms this subsystem is actively researched by exploit developers; staying current eliminates known exploit paths.