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high advisory

CVE-2026-40407 - Windows CLFS Driver Heap Overflow for Privilege Escalation

CVE-2026-40407 is a heap-based buffer overflow vulnerability in the Windows Common Log File System (CLFS) Driver, enabling a locally authenticated attacker to escalate privileges on the system.

CVE-2026-40407 is a heap-based buffer overflow vulnerability affecting the Windows Common Log File System (CLFS) Driver. This vulnerability allows an attacker with local access and valid credentials to escalate their privileges on a vulnerable system. The Common Log File System (CLFS) is a general-purpose logging service that can be used by software both in kernel-mode and user-mode to manage structured data records. A successful exploit of this vulnerability could allow an attacker to gain elevated system privileges, potentially leading to complete system compromise. Defenders should apply relevant patches as soon as possible.

Attack Chain

  1. The attacker gains initial access to the system with valid user credentials.
  2. The attacker crafts a malicious CLFS log file designed to trigger the heap overflow.
  3. The attacker interacts with the CLFS driver, causing it to parse the malicious log file.
  4. The CLFS driver allocates a heap buffer to store data from the log file.
  5. Due to insufficient bounds checking, the driver writes beyond the allocated buffer, causing a heap overflow.
  6. The heap overflow corrupts adjacent heap metadata or data structures.
  7. The attacker leverages the heap corruption to overwrite critical system data or function pointers.
  8. The attacker triggers the overwritten function pointer, leading to arbitrary code execution with elevated privileges.

Impact

Successful exploitation of CVE-2026-40407 allows a local attacker to elevate their privileges, potentially gaining SYSTEM level access. This could allow the attacker to perform a wide range of malicious activities, including installing programs, viewing, changing, or deleting data, or creating new accounts with full user rights. Given the widespread use of Windows, this vulnerability could potentially affect a large number of systems if exploited.

Recommendation

  • Apply the security update released by Microsoft to patch CVE-2026-40407 (reference: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40407).
  • Enable Sysmon process creation logging to monitor for unexpected processes spawned by CLFS-related binaries.
  • Deploy the Sigma rule provided below to detect potential exploitation attempts targeting CVE-2026-40407.

Detection coverage 2

Detect CVE-2026-40407 Exploitation - CLFS Driver Loading

medium

Detects CVE-2026-40407 exploitation by monitoring for abnormal CLFS driver loading, which could indicate malicious log file parsing.

sigma tactics: privilege_escalation techniques: T1068 sources: image_load, windows

Detect CVE-2026-40407 Potential Exploit - Suspicious CLFS File Creation

low

Detects CVE-2026-40407 potential exploit by monitoring for suspicious file creation involving CLFS log files.

sigma tactics: privilege_escalation techniques: T1068 sources: file_event, windows

Detection queries are available on the platform. Get full rules →