Microsoft Windows Server Update Service (WSUS) Deserialization of Untrusted Data Vulnerability
CVE-2025-59287 is a security vulnerability in Microsoft Windows, added to CISA KEV on October 24, 2025. Patch priority: Patch this week.
What it is
Microsoft Windows Server Update Service (WSUS) contains a deserialization of untrusted data vulnerability that allows for remote code execution.
Who's affected
Affects anyone running Windows workstations or servers. In a small CPA, legal, or dental practice, Windows is typically the platform your accounting, document management, or practice management software runs on — exploitation gives an attacker access to whatever client files and credentials live on those machines.
What to do
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Book a free 20-minute triage call.
Insurance readiness
Your insurer will ask if this is patched.
Unpatched entries on CISA's Known Exploited Vulnerabilities list are exactly what cyber-insurance carrier questionnaires probe for, and knowing which controls they check is how you keep a renewal from stalling.
Get the free carrier questionnaire →Severity
CVSS base score: 9.8 — CRITICAL
Weakness classification: CWE-502
Microsoft fix
- KB5070883 (Security Update)
- KB5070884 (Security Update)
- KB5070892 (Standalone Security Update)
- KB5070881 (Security Update)
- KB5070893 (Standalone Security Update)
- KB5070879 (Security Update)
- KB5070882 (Security Update)
- KB5070887 (Security Update)
- KB5070886 (Security Update)
Microsoft Exploitability Index (assessed at update release): Exploitation More Likely
This CVE is on CISA's Known Exploited Vulnerabilities catalog. Microsoft's index reflects their release-time forecast, not current observed activity, and is not automatically revised upward when exploitation appears later.
Vendor workaround
Vendor-published workaround
The following workarounds might be helpful in your situation. In all cases, Microsoft strongly recommends that you install the updates for this vulnerability as soon as possible even if you plan to leave either of these workarounds in place:
If you are unable to install the October 23, 2025 out-of-band update, you can take any of the following actions to be protected against this vulnerability:
- 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) to render WSUS non-operational.
Important: Do NOT undo either of these workarounds until after you have installed the update.
Vendor-published mitigation
Mitigation refers to a setting, common configuration, or general best-practice, existing in a default state, that could reduce the severity of exploitation of a vulnerability.
The following mitigating factors might be helpful in your situation:
The WSUS Server Role is not enabled by default on Windows servers. Windows servers that do not have the WSUS server role enabled are not vulnerable to this vulnerability. If the WSUS server role is enabled, the server will become vulnerable if the fix is not installed before the WSUS server role is enabled.
A workaround reduces exposure while you patch — it is not a substitute for the update. Install the fix above as soon as you can.
CISA due date
Federal deadline: November 14, 2025. Federal agencies must complete the required action by this date. For private SMBs the deadline is advisory — but treat it as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).
EPSS
EPSS score: 99.9% — higher than 100% of all scored CVEs.
EPSS estimates the probability of exploitation activity in the next 30 days. Every entry on this site is already confirmed exploited, so read it as relative urgency among the things you still have open, not as permission to wait.
Other exploited Microsoft Windows entries
More CISA KEV entries for Microsoft Windows. Same product line, same actively-exploited status.
Source
This entry comes from CISA's Known Exploited Vulnerabilities catalog, public US-Government data we pull daily. The plain-language framing and the what-to-do guidance above are ours. View the original entry on cisa.gov.