Microsoft April Patch Tuesday highlights zero-day risks

Microsoft's latest Patch Tuesday release has stirred interest among cybersecurity professionals due to an unusual delay and some noteworthy vulnerabilities. A 40-minute delay in releasing updates, particularly from a technology giant like Microsoft, has sparked curiosity about potential underlying issues. Tyler Reguly, Associate Director for Security R&D at Fortra, noted the peculiarities, pointing out the absence of Windows 10 security updates initially, which left many questioning what might have gone wrong.

This month's patch addressed 121 Common Vulnerabilities and Exposures (CVEs), with 11 rated critical and 110 considered important. However, it was the elevation of privilege vulnerabilities that dominated the updates, comprising 40.5% of the total patched, followed by remote code execution (RCE) vulnerabilities at 25.6%. Such security flaws, especially those related to the Windows Common Log File System (CLFS) Driver, are gaining traction among ransomware operators, according to Satnam Narang, Senior Staff Research Engineer at Tenable.

The current highlight of concern is CVE-2025-29824, an elevation of privilege bug in the CLFS Driver. This lone zero-day vulnerability, patched in April's release, has been notably active, marking a consistent focus in recent Patch Tuesday releases. Since 2022, 32 CLFS vulnerabilities have been addressed, with six being exploited in the wild, making them particularly attractive for attackers, especially ransomware operators.

Narang explained that post-compromise activities commonly require elevation of privilege vulnerabilities for actions like lateral movement within a system, which naturally attracts targeted attacks. Interestingly, while remote code execution flaws generally dominate Patch Tuesday releases, elevation of privilege vulnerabilities have led in the context of zero-day exploitation, accounting for over half of exploited zero-days so far in 2025.

Another focal point in April's release was vulnerabilities within Windows Remote Desktop Services (RDP), including CVE-2025-26671, CVE-2025-27480, and CVE-2025-27482. The critical nature of the latter two and their marking as "Exploitation More Likely" by Microsoft, despite requiring an attacker to win a race condition, poses a speculative risk for cybersecurity teams.

Tyler Reguly also highlighted the importance of prioritising vulnerabilities using Microsoft's Exploitability Index over the Common Vulnerability Scoring System (CVSS). For him, vulnerabilities like CVE-2025-27472, although lower on the CVSS scale, caught attention due to its potential to bypass Microsoft's SmartScreen, a feature common in threat attacks through Mark of the Web (MOTW). Reguly suggested that this metric might not always accurately reflect the criticality of threats, emphasizing the role of intuition in cybersecurity decision-making.

For cybersecurity professionals, particularly those at the Chief Information Security Officer (CISO) level, recurring vulnerabilities in frequently targeted technologies such as Office, Edge, CLFS, and MOTW should drive inquiries with vendors. As attackers evolve and vulnerabilities persist, there's an imperative for vendors to aid in proactive defence mechanisms.

This month's Patch Tuesday serves as a reminder of the dynamic nature of cybersecurity threats and the importance of timely and strategic patching, particularly as vulnerabilities are disclosed and scrutinised by potential malicious actors. Organisations are urged to remain vigilant, prioritise critical update implementations, and sustain dialogues with cybersecurity vendors to adaptively safeguard against emerging threats.