Introduction
In April 2025, Microsoft released a significant security update addressing 134 vulnerabilities across its product range, marking the largest patch release of the year [2]. Among these [2] [5] [9], a critical vulnerability [2] [5] [9], CVE-2025-29824 [1] [2] [3] [4] [5] [6] [7] [8] [9], in the Windows Common Log File System (CLFS) driver [1] [2] [3] [4] [5] [6] [7] [8], has garnered particular attention due to its active exploitation by the RansomEXX ransomware group.
Description
Microsoft has released critical security updates addressing 134 vulnerabilities across various products [9], including Windows [2] [9], Office [1] [2] [9], Edge [9], and Azure [9], in its April 2025 Patch Tuesday rollout [1]. This marks the largest patch load of the year [2]. Among these [2] [5] [9], CVE-2025-29824 is a significant local Elevation of Privilege (EoP) vulnerability in the Windows Common Log File System (CLFS) driver [2] [4] [5] [6] [7], affecting all supported versions of Windows OS and Windows Server [2]. This critical flaw, classified as a “use after free” vulnerability [9], has a CVSSv3 score of 7.8 and allows authenticated local attackers to execute code with SYSTEM-level privileges by manipulating log file buffers, leading to memory corruption [1]. Currently, CVE-2025-29824 is under active exploitation by the RansomEXX ransomware gang, raising concerns for enterprise environments and critical infrastructure [2].
The exploitation process involves delivering malware through a malicious MSBuild file that leverages the CLFS flaw [1]. This vulnerability has been linked to targeted attacks against a limited number of organizations, including those in the IT and real estate sectors in the US [3], a financial institution in Venezuela [1], a Spanish software company [1] [3] [8], and retail organizations in Saudi Arabia [1]. Since 2022, Microsoft has patched an average of 10 vulnerabilities per year in the CLFS driver [7], with six of these being actively exploited. The last zero-day flaw in CLFS was addressed in December 2024 (CVE-2024-49138) [3]. Importantly [7], while patches for CVE-2025-29824 are available for most systems, updates for Windows 10 32-bit and x64 systems are not yet available and will be released as soon as possible [9]. Devices running Windows 11 [1], version 24H2 [1] [3], are not vulnerable due to enhanced security controls [1].
Security teams are advised to closely monitor the CLFS driver using EDR/XDR tools to mitigate risks until a patch is released for affected Windows 10 systems [4]. This includes observing processes interacting with clfs.sys [6], detecting anomalous behavior [6], and auditing systems for signs of abnormal dllhost.exe activity [1]. The US Cybersecurity and Infrastructure Security Agency (CISA) has added CVE-2025-29824 to its Known Exploited Vulnerabilities Catalog [9], urging users to apply patches before April 29, 2025 [9]. Microsoft recommends that organizations prioritize patching systems not on version 24H2 [1], enable cloud-delivered protection in Microsoft Defender Antivirus [1], run EDR in block mode [1], and utilize attack surface reduction rules in Microsoft Defender for Endpoint [1]. The latest security update can be applied through Windows Update [1], and a reboot is required for changes to take effect [1].
EoP vulnerabilities were the most common type addressed in this Patch Tuesday [4] [6], with Microsoft issuing updates for 49 such vulnerabilities [6], alongside 31 remote code execution and 17 information disclosure CVEs [6]. Notably, CVE-2025-26663 and CVE-2025-26670 are critical vulnerabilities that enable remote [5], unauthenticated attackers to execute code on affected systems via specially crafted LDAP messages [5], making them wormable due to the lack of required user interaction [5]. This marks the second EoP vulnerability in the Windows CLFS driver patched in 2025 [7], following the eight CLFS vulnerabilities patched in 2024. Experts anticipate that the discovery of CVE-2025-29824 may lead to the identification of additional vulnerabilities in CLFS in the near future [2].
Conclusion
The April 2025 Patch Tuesday release underscores the critical nature of timely security updates, particularly in light of active exploits like CVE-2025-29824. Organizations are urged to prioritize patching and employ robust monitoring strategies to mitigate risks. The ongoing discovery of vulnerabilities in the CLFS driver suggests a need for continued vigilance and proactive security measures to protect against future threats.
References
[1] https://cyberinsider.com/microsoft-fixes-actively-exploited-clfs-zero-day-used-in-ransomware-attacks/
[2] https://redmondmag.com/Articles/2025/04/08/April-Patch-Tuesday-25.aspx
[3] https://www.helpnetsecurity.com/2025/04/08/patch-tuesday-microsoft-zero-day-cve-2025-29824/
[4] https://ciso2ciso.com/microsoft-fixes-over-130-cves-in-april-patch-tuesday-source-www-infosecurity-magazine-com/
[5] https://www.computerweekly.com/news/366622332/Microsofts-April-2025-bumper-Patch-Tuesday-corrects-124-bugs
[6] https://www.infosecurity-magazine.com/news/microsoft-fixes-130-cves-april/
[7] https://www.tenable.com/blog/microsofts-april-2025-patch-tuesday-addresses-121-cves-cve-2025-29824
[8] https://www.csoonline.com/article/3957619/april-patch-tuesday-news-windows-zero-day-being-exploited-big-vulnerability-in-2-sap-apps.html
[9] https://thesecmaster.com/blog/breaking-down-the-latest-april-2025-patch-tuesday-report
