TLP: CLEAR

Alert number: AL25-015

1. Audience
2. Purpose
3. Details
4. Suggested actions
5. References
6. About Cyber Centre

This Alert is intended for IT professionals and managers of notified organizations.

An Alert is used to raise awareness of a recently identified cyber threat that may impact cyber information assets, and to provide additional detection and mitigation advice to recipients. The Canadian Centre for Cyber Security (“Cyber Centre”) is also available to provide additional assistance regarding the content of this Alert to recipients as requested.

• On October 24, 2025, Microsoft published an out-of-band security update to a critical vulnerability in the Windows Server Update Service (WSUS) [1].
• CVE-2025-59287 involves the deserialization of untrusted data in WSUS, allowing an unauthorized attacker to execute code over a network.
• The WSUS Server Role is not enabled by default on Windows servers, and Windows servers that do not have this role enabled are not vulnerable. In response to Microsoft’s disclosure, the Cyber Centre released an update to AV25-666 on October 24, 2025 [2].
• The Cyber Centre is aware of active exploitation [3].

The Cyber Centre strongly recommends that organizations follow Microsoft customer guidance for mitigation advice:

• Apply the recommended update. If this is not possible, apply the following mitigations:
• If the WSUS Server Role is enabled on your server, disable it. Note that clients will no longer receive updates from the server if WSUS is disabled.
• Block inbound traffic to Ports 8530 and 8531 on the host firewall (as opposed to blocking only at the network/perimeter firewall) in order to render WSUS non-operational.

Microsoft adds that this update is cumulative, so organizations do not need to apply any previous updates before installing this one, as it supersedes all previous updates for affected versions. They suggest that if the October 2025 Windows security update has not been applied, that this out-of-band update should be applied. After installation, a reboot will be required.

In addition, the Cyber Centre also strongly recommends that organizations review and implement the Cyber Centre’s Top 10 IT Security Actions with an emphasis on the following topics [4].

• Patching operating systems and applications
• Isolating Web-Facing applications

Should activity matching the content of this Alert is discovered, recipients are encouraged to report via My Cyber Portal, or email contact@cyber.gc.ca.

1. Windows Server Update Service (WSUS) Remote Code Execution Vulnerability
2. Microsoft security advisory (AV25-666)
3. Exploitation of Windows Server Update Services Remote Code Execution Vulnerability (CVE-2025-59287)
4. Top 10 IT security actions to protect Internet connected networks and information (ITSM.10.089)

The Canadian Centre for Cyber Security (Cyber Centre) operates as part of the Communications Security Establishment. We are Canada’s national authority on cyber security, and we lead the government’s response to cyber security events. As Canada’s national computer security incident response team, the Cyber Centre works in close collaboration with government departments, critical infrastructure, Canadian businesses, and international partners to prepare for, respond to, mitigate, and recover from cyber events. We do this by providing authoritative advice and support and coordinating information sharing and incident response. The Cyber Centre is outward-facing, welcoming partnerships that help build a stronger, more resilient cyber space in Canada.

As always, please let us know if you have any questions or concerns or see unusual activity on your systems that you believe might be associated with this or any other vulnerability. Please send these communications to security.response@utoronto.ca.