Microsoft has released a patch for a Windows zero day vulnerability that has been exploited by cybercriminals in ransomware attacks.\u00a0The vulnerability, identified as CVE-2023-28252, is a privilege escalation flaw affecting the Windows Common Log File System (CLFS) driver.CLFS is a general purpose logging service that can be used by dedicated client applications and that multiple clients can share to optimize log access. The vulnerability allows an attacker to elevate privileges to the system in low-complexity attacks without any user interaction.\u00a0Microsoft has credited Kaspersky Labs\u2019 Boris Larin, Mandiant\u2019s Genwei Jiang, and DBAPPSecurity WeBin Lab\u2019s Quan Jin for reporting the vulnerability.\u00a0Vulnerability used to deploy Nokoyawa ransomwareThe vulnerability has been used by a sophisticated cybercriminal group to deploy the Nokoyawa ransomware as a final payload, according to Kaspersky.\u00a0The group is known for its use of many similar but unique CLFS driver exploits that were likely developed by the same exploit author. \u201cSince at least June 2022, we\u2019ve identified five different exploits used in attacks on retail & wholesale, energy, manufacturing, healthcare, software development, and other industries,\u201d Kaspersky said in a blog post.\u00a0Nokoyawa ransomware was discovered in February 2022, sharing code with another ransomware family known as Karma. The initial version of the ransomware was written in the C programming language, and a second version of the ransomware was discovered in September 2022, written in the Rust programming language.The threat group behind the ransomware performs double extortion ransomware attacks: exfiltrating sensitive information from organizations, followed by file encryption and a ransom payment demand, security firm Zscaler said in a blog post.\u00a0The CLFS system vulnerabilityCVE-2023-28252 is a CLFS vulnerability that can be exploited when the system attempts to extend the metadata block. The vulnerability gets triggered by the manipulation of the base log file, according to Kaspersky.\u00a0The exploit uses the vulnerability to corrupt another specially crafted base log file object in a way that a fake element of the base log file gets treated as a real one.Attackers use Cobalt Strike Beacon as their main tool. It\u2019s launched with a variety of custom loaders aimed at preventing antivirus detection.\u201cIn this attack, cybercriminals used a newer version of Nokoyawa that is quite distinct from the JSWorm codebase. It\u2019s written in C and has encrypted strings. It was launched with an encrypted json config provided with a \u201c\u2013config\u201d command line argument,\u201d Kaspersky said.\u00a0Kaspersky said it will add more details about the vulnerability and triggers after 9 days of the release of the patch. This is to ensure that everyone has enough time to patch their systems before other actors develop exploits for CVE-2023-28252.There have been several vulnerabilities reported previously as well in the CLFS system.\u00a0Searching for \u201cWindows Common Log File System Driver Elevation Of Privilege Vulnerability\u201d shows that there have been at least 32 such vulnerabilities (not counting CVE-2023-28252) discovered since 2018, where three of them were detected in the wild as zero days (CVE-2022-24521, CVE-2022-37969, CVE-2023-23376), according to Kaspersky.\u00a0Microsoft released patches for 96 other security bugs as part of April\u2019s Patch Tuesday, including 45 remote code execution vulnerabilities.