CISA Warns: Old Windows Flaw (CVE-2021-43226) Exploited in New Attacks
A privilege escalation vulnerability in Microsoft Windows systems is once again being actively exploited, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) has warned, adding the flaw to its official catalog of known exploited vulnerabilities. The issue — CVE-2021-43226 — was first identified in 2021, but in recent weeks has reemerged in active attacks, including campaigns distributing ransomware.
The vulnerability resides in the Common Log File System (CLFS) driver, responsible for handling event logs generated by both the operating system and applications. When exploited locally, it allows an attacker to bypass security restrictions and obtain elevated privileges, potentially leading to full system compromise. The flaw becomes particularly dangerous when paired with remote code execution, such as through a vulnerable network service or a phishing email — enabling attackers to advance deeper into the target network.
CLFS is a core Windows component found on nearly all workstations and servers, making it a valuable target for adversaries — especially when deployed on systems processing sensitive data, managing mission-critical applications, or administering cloud environments. Exploitation of CVE-2021-43226 requires no user interaction beyond the initial execution of malicious code with basic privileges.
Experts warn that once a system is breached, such vulnerabilities allow attackers to rapidly escalate privileges, potentially reaching domain administrator level, thereby gaining complete control over the corporate network. Organizations with limited resources and weak incident response capabilities are at particular risk, as they often lack centralized patch management and coordinated defense mechanisms.
As a mitigation measure, CISA urges administrators to apply Microsoft’s latest security updates and ensure that endpoint protection tools can detect and block exploitation attempts.
Where immediate patching is not feasible, it is advised to restrict access to the CLFS driver or isolate vulnerable systems from the network. It is equally critical to decommission unsupported Windows versions, which no longer receive security updates and therefore remain exposed.
Administrators should also analyze system logs for anomalies related to CLFS activity and configure alerts for indicators of compromise tied to this vulnerability. Federal agencies and contractors in the U.S. are required to comply with CISA’s Binding Operational Directive (BOD) 22-01, mandating systematic vulnerability management and coordinated remediation efforts.
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