Microsoft released crucial out-of-band security updates on October 24, 2025, to address a critical vulnerability in the Windows Server Update Service (WSUS). This vulnerability, identified as CVE-2025-59287, has a CVSS score of 9.8, indicating its severity, and had already been actively exploited in the wild prior to the patch.
Three researchers, MEOW, f7d8c52bec79e42795cf15888b85cbad, and Markus Wulftange from CODE WHITE GmbH, were credited with discovering and reporting this serious flaw. The vulnerability arises from an unsafe deserialization of untrusted data in WSUS, permitting unauthorized attackers to execute code remotely on affected systems.
Given the nature of the flaw, if exploited, attackers could send a specially crafted event that triggers this unsafe deserialization. This method enables remote code execution, potentially granting an attacker SYSTEM privileges. Microsoft has previously cautioned developers against using the BinaryFormatter for deserialization because it poses a security risk with untrusted inputs; indeed, it was removed from .NET 9 in August 2024.
To mitigate this vulnerability, Microsoft released security updates for several supported versions of Windows Server, including Windows Server 2012, 2016, 2019, and others. After installing the patch, a system reboot is recommended for it to take effect.
In light of the urgency, users unable to immediately apply the update can protect themselves by disabling the WSUS Server Role if it is enabled and blocking inbound traffic to the necessary ports (8530 and 8531) on their firewalls. Microsoft specifically advised not to revert these actions until the update is successfully installed.
Further, the Dutch National Cyber Security Centre (NCSC) confirmed they received reports about exploitation of this vulnerability, which began on the same day Microsoft released the patch. A malicious actor reportedly transmitted a payload targeting a specific customer, executing arbitrary commands on the server while avoiding detection.
Microsoft emphasized that this vulnerability does not affect servers lacking the WSUS Server Role. Users are urged to check Microsoft’s CVE page for detailed remediation guidance.
Given the existence of a proof-of-concept exploit and reported attack activity, it is critical for affected users to apply the security patch promptly to safeguard their systems. The U.S. Cybersecurity and Infrastructure Security Agency (CISA) has also added this vulnerability to its Known Exploited Vulnerabilities catalog, mandating federal agencies to address it by November 14, 2025.
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