Exploitation of CLFS Zero-Day Leads to Ransomware Attacks: A Detailed Analysis

Microsoft Threat Intelligence Center (MSTIC) and the Microsoft Security Response Center (MSRC) recently discovered a severe zero-day vulnerability in Windows Common Log File System (CLFS). This vulnerability, tracked as CVE-2025-29824, has been exploited by cybercriminals to escalate privileges and deploy ransomware. The targeted organizations span a range of industries including IT, real estate, finance, and retail, with specific incidents occurring in the United States, Venezuela, Saudi Arabia, and Spain. The exploit was primarily delivered through PipeMagic malware, which allowed attackers to gain privileged access and deploy ransomware in compromised environments. Microsoft released a patch for this vulnerability on April 8, 2025, but experts urge swift application of the update to mitigate risks.

Overview of the CLFS Zero-Day Vulnerability

The Common Log File System (CLFS) kernel driver in Windows contains a zero-day vulnerability that allows attackers to escalate privileges on a compromised machine. This exploit enables attackers with standard user privileges to gain elevated access and further exploit systems for malicious purposes. Microsoft identified the flaw and tracked it as CVE-2025-29824, fixing it with a security update released on April 8, 2025. Although the exact initial entry point remains unclear, subsequent analysis reveals the use of PipeMagic malware in exploiting this vulnerability.

Pre-Exploitation and Malware Deployment

The attack process begins with the deployment of PipeMagic malware. Attackers use legitimate system utilities such as the certutil tool to download malicious files from compromised third-party websites. The malware, often carried within an MSBuild file, is then executed and decrypted using various system functions. Upon execution, the malware activates and prepares the system for exploitation by escalating privileges through the CLFS vulnerability. The first critical step of the attack involves using dllhost.exe to load and deploy the exploit.

The CLFS Exploit Process

The exploitation targets the CLFS kernel driver by leveraging the NtQuerySystemInformation API to leak sensitive kernel data to user mode. However, from Windows 11, version 24H2, access to these system classes is restricted to users with SeDebugPrivilege, making it impossible for the exploit to function on that version. After successfully bypassing this restriction, the exploit uses memory corruption techniques and the RtlSetAllBits API to grant the process elevated privileges, enabling the attacker to inject malicious code into higher-level system processes.

Post-Exploitation and Ransomware Deployment

After gaining elevated privileges, the attackers proceed with a typical ransomware attack. They inject additional tools like procdump.exe into critical processes and dump sensitive memory (e.g., LSASS.exe) to extract user credentials. These credentials are then used for further system compromise. Ransomware payloads are deployed, with files being encrypted and renamed with a random extension. A ransom note with the name !_READ_ME_REXX2_!.txt is dropped on the infected system. Additionally, the ransomware actors utilize specific .onion domains, associated with the RansomEXX family, to communicate with their command-and-control infrastructure.

Mitigation and Protection Recommendations

Microsoft recommends immediate installation of security updates to address CVE-2025-29824, particularly for customers not running Windows 11, version 24H2, as they remain vulnerable. In addition to patching, organizations are advised to follow best practices for ransomware defense, including enabling cloud-delivered protection, employing endpoint detection and response (EDR) tools in block mode, and using advanced protection rules in Microsoft Defender.

What Undercode Say:

The exploitation of the CLFS zero-day vulnerability in Windows presents a serious threat to organizations worldwide. Notably, the discovery of PipeMagic malware being used as a vehicle for privilege escalation is alarming, as it demonstrates how attackers can bypass traditional security measures and escalate their privileges once inside the network. The post-exploitation phase, where attackers deploy ransomware, highlights the growing sophistication of cybercriminals who target vulnerable systems to launch widespread attacks.

Ransomware actors value privilege escalation vulnerabilities like CVE-2025-29824 because they allow for the rapid deployment of malicious software across large environments. This specific attack not only leverages a zero-day vulnerability but also shows the danger of relying solely on traditional perimeter defenses. Once a system is compromised and privileges are escalated, attackers can roam freely within a network, deploy ransomware, and exfiltrate valuable data before encrypting the entire environment.

The fact that the exploit bypasses some of the protections in newer versions of Windows (like Windows 11, version 24H2) underscores the need for continuous vigilance and patching. Systems running older versions or those with unpatched vulnerabilities are at greater risk. This suggests that threat actors are constantly adapting their tactics to bypass the latest security patches, making it crucial for organizations to implement layered defenses that extend beyond simple patch management.

Moreover, Microsoft’s advisory, which encourages organizations to apply security updates immediately, serves as a reminder of the critical importance of proactive cybersecurity practices. In particular, enabling automated defenses, such as Microsoft Defender’s cloud-delivered protection and Endpoint Detection and Response (EDR), can help organizations mitigate the impact of attacks before they escalate to full-blown ransomware incidents.

The rise of ransomware-as-a-service models, as seen in this attack, signals a broader shift in the cybercrime ecosystem. Ransomware is no longer limited to sophisticated groups with significant resources; it is increasingly being packaged and sold as a service, lowering the barrier to entry for less experienced criminals. This trend will likely result in an increase in such attacks, making robust defenses, such as those offered by Microsoft Defender, even more critical.

Organizations in sectors like finance, real estate, and IT must be especially cautious, as these industries are frequent targets for cybercriminals due to their wealth of sensitive information. In this case, the fact that such a large swath of industries was impacted by a single vulnerability demonstrates the widespread danger posed by advanced persistent threats (APTs) and the increasing sophistication of ransomware campaigns.

Lastly, the inclusion of specific indicators of compromise (IOCs), such as malicious command lines, domains, and file paths, offers vital clues for defenders in identifying and mitigating attacks early. However, as noted by Microsoft, these indicators can also trigger false positives in some cases. Therefore, threat intelligence platforms like Microsoft Sentinel can provide enhanced visibility into such activities, allowing defenders to respond swiftly and reduce the attack surface.

Fact Checker Results:

Vulnerability Validity: CVE-2025-29824 is indeed a real vulnerability that was discovered and patched by Microsoft on April 8, 2025.
Attack Confirmation: The use of PipeMagic malware in the exploitation of this vulnerability has been corroborated by multiple sources, including Kaspersky and ESET.
Ransomware Connection: The attack, including the deployment of ransomware and the use of .onion domains tied to RansomEXX, has been independently verified by Microsoft and other cybersecurity firms.