Patch tracker / KB5055557
KB5055557: Windows Server 2012 R2 Security Update (April 2025)
KB5055557 is a High security update for Windows Server 2012 R2, released 2025-04-08. It closes 55 CVEs with a maximum CVSS of 8.8. At least one of them is actively exploited (CISA KEV) and linked to ransomware, so it is urgent, and Microsoft lists 1 known issue: test it before broad deployment. The most likely of them to be attacked is CVE-2025-26663, which FIRST.org rates at 14% probability of exploitation in the next 30 days.
Before you deploy: Microsoft lists 1 known issue for this update.
Our data comes from Microsoft, CISA and other public sources, and it can be wrong or out of date. If you are unsure, check the linked source before you act, and tell us when we are wrong so we can fix it.
Update summary
Fixes 55 CVEs. At least one is actively exploited (CISA KEV). Most severe CVSS 8.8 (High). EPSS exploit probability up to 14%. One is linked to ransomware. Among the top half of tracked Microsoft updates by EPSS exploit probability.
A newer security update for Windows Server 2012 R2 has shipped since this one: KB5126836 (2026-10-13). For cumulative update families the newer package includes these fixes, so installing that one is usually the shorter path. Verify against your own update rings before you skip this package.
Microsoft documents 1 known issue with this update.
- We are currently not aware of any issues with this update. For the most up-to-date information about known issues for Windows Server 2012 R2, please...
KB5055557 is the April 8, 2025 rollup for Windows 10 version 1809 and Windows Server 2019, closing 57 CVEs with a highest Severity of High (CVSS 8.8). One of these CVEs is on the CISA Known Exploited Vulnerabilities list and is linked to ransomware activity, so attackers are already using it against unpatched systems. Server 2019 domain controllers and 1809 endpoints that skipped this cycle remain directly exposed.
Do this: Deploy KB5055557 to all Windows 10 1809 and Server 2019 machines now, prioritizing internet-facing and domain controller systems given the ransomware-linked KEV entry.
AI-generated from public data, 2026-07-27. Verify against the vendor advisory before acting.
Senserva AI Opinion and rich prompt for KB5055557
Copy this prompt into Claude, ChatGPT, or Copilot. The facts are included, sourced from this page.
Full tracking and change history for KB5055557
- Released
- 2025-04-08
- Last changed
- 2026-09-30
- Changes tracked
- 7
Senserva began recording day-to-day change history on 2026-07-07, and refreshes it several times a day. Release dates are historical where Microsoft publishes them; revision history is tracked from that date forward, so it does not include changes made before then.
Microsoft revised KB5055557 on 2026-09-30: CVEs fixed 57 to 55.
- 2026-08-07 CVEs fixed 57 to 55
- 2026-09-29 CVEs fixed 55 to 57
- 2026-09-30 CVEs fixed 57 to 55
Recorded by Senserva on the dates shown, from the MSRC release feed. This is what moved and when, which neither the vendor advisory nor NVD publishes.
Check if KB5055557 is installed
Elevated PowerShell. No output means it is not installed.
Known issues, from Microsoft
Quoted from Microsoft's support article for KB5055557 (checked 2026-10-15). Microsoft lists 1 known issue. Verify against the article before acting.
We are currently not aware of any issues with this update. For the most up-to-date information about known issues for Windows Server 2012 R2, please go to the Windows release health dashboard .
CVEs fixed by this update
Sorted exploited-first, then by CVSS. Every CVE links to its Senserva page with the full risk facts, references, and a ready-to-paste AI prompt.
Scope and sources
- Affected products
- Also released in April 2025
KB5055518
59 CVEs · High · exploitedKB5055519
73 CVEs · High · exploitedKB5055521
60 CVEs · High · exploitedKB5055523
82 CVEs · High · exploitedKB5055526
77 CVEs · High · exploitedKB5055527
73 CVEs · High · exploitedKB5055528
65 CVEs · High · exploitedKB5055547
43 CVEs · High · exploitedThe full month, summarized: Microsoft Patch Tuesday.
- Authoritative references
See this and every Microsoft update ranked by real-world risk on the Microsoft Patch Tracker.