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CVE-2021-28310Win32k Elevation of Privilege VulnerabilityUpdated monthlyMicrosoft Patch Tuesday
Microsoft Patch Tuesday
April 2021
Microsoft's April 2021 Patch Tuesday fixes 108 vulnerabilities across Windows, Developer Tools and Microsoft Office - 19 rated Critical, 4 publicly disclosed before the patch shipped, one already exploited in the wild. Patch CVE-2021-28310 first.
Released April 13, 2021, 10:00 PT1 actively exploited4 publicly disclosed47 KB articles to deploy
April 2021 Patch Tuesday at a glance
108
Microsoft CVEs
excl. Edge & Azure Linux
19
Critical
18% of the release
1
Actively exploited
CVE-2021-28310
4
Publicly disclosed
known before the patch
9.8
Highest CVSS
CVE-2021-28480
47
KB articles
what you actually deploy
Why this number differs from other roundups. Microsoft's April 2021 release document lists 233 entries. 86 are Azure Linux (Mariner) OS package updates and 39 are republished Chromium fixes for Edge - neither is a Microsoft product vulnerability. The 108 above is the real Patch Tuesday set. See the breakdown →
Priority order
Patch these first
Ranked by exploitation reality, not score alone. This month's exploited flaw is rated Important - a severity-sorted list would bury it.
Patch this week
Publicly disclosed
Details are already public. Working exploits typically follow within days.
CVE-2021-27091RPC Endpoint Mapper Service Elevation of Privilege VulnerabilityCVE-2021-28458Azure ms-rest-nodeauth Library Elevation of Privilege VulnerabilityCVE-2021-28437Windows Installer Information Disclosure VulnerabilityCVE-2021-28312Windows NTFS Denial of Service VulnerabilityPatch this cycle
Critical remote code execution
No known exploitation yet, but network-reachable and high impact.
CVE-2021-28480Microsoft Exchange Server Remote Code Execution VulnerabilityCVE-2021-28481Microsoft Exchange Server Remote Code Execution VulnerabilityCVE-2021-28483Microsoft Exchange Server Remote Code Execution VulnerabilitySee all 19 Critical →The numbers
What’s in this release
Where the 233 entries actually come from
Most roundups quote a single number. These are three unrelated release streams.
108
86
39
108 Microsoft products86 Azure Linux (Mariner)39 Edge (Chromium)
Severity
Microsoft channel only · 108 CVEs
| Critical | 19 |
|---|---|
| Important | 88 |
| Moderate | 1 |
| Low | 0 |
Impact type
What an attacker gains if it works
| Remote Code Execution | 56 |
|---|---|
| Elevation of Privilege | 19 |
| Information Disclosure | 17 |
| Denial of Service | 9 |
| Security Feature Bypass | 5 |
| Spoofing | 2 |
Most-affected components
Where this month’s fixes concentrate - filter by any of them in the table below
Windows Remote Procedure Call Runtime27Visual Studio Code6Microsoft Windows Codecs Library5Microsoft Exchange Server4Microsoft Graphics Component4Microsoft Office Excel4Role: Windows Hyper-V4Windows Installer4Microsoft Windows Speech3Windows Diagnostic Hub3Windows TCP/IP3Azure DevOps2
Every patch
Browse all 233 entries
Sorted by risk by default: exploited first, then publicly disclosed, then severity, then CVSS. Search by CVE, title, component or KB number.
| Title | Status | KB | |||||
|---|---|---|---|---|---|---|---|
| Important | CVE-2021-28310 | Win32k Elevation of Privilege Vulnerability | EoP | 7.8 | Exploited | Windows Win32K | 5001330 +3 |
| Important | CVE-2021-27091 | RPC Endpoint Mapper Service Elevation of Privilege Vulnerability | EoP | 7.8 | Disclosed | Windows Registry | 5001335 +3 |
| Important | CVE-2021-28458 | Azure ms-rest-nodeauth Library Elevation of Privilege Vulnerability | EoP | 7.8 | Disclosed | Open Source Software | — |
| Important | CVE-2021-28437 | Windows Installer Information Disclosure Vulnerability | Info Disc. | 5.5 | Disclosed | Windows Installer | 5001330 +13 |
| Moderate | CVE-2021-28312 | Windows NTFS Denial of Service Vulnerability | DoS | 3.3 | Disclosed | Windows NTFS | 5001330 +2 |
| Critical | CVE-2021-28480 | Microsoft Exchange Server Remote Code Execution Vulnerability | RCE | 9.8 | More likely | Microsoft Exchange Server | 5001779 |
| Critical | CVE-2021-28481 | Microsoft Exchange Server Remote Code Execution Vulnerability | RCE | 9.8 | More likely | Microsoft Exchange Server | 5001779 |
| Critical | CVE-2021-28483 | Microsoft Exchange Server Remote Code Execution Vulnerability | RCE | 9.0 | More likely | Microsoft Exchange Server | 5001779 |
| Critical | CVE-2021-28482 | Microsoft Exchange Server Remote Code Execution Vulnerability | RCE | 8.8 | More likely | Microsoft Exchange Server | 5001779 |
| Important | CVE-2021-28319 | Windows TCP/IP Driver Denial of Service Vulnerability | DoS | 7.5 | More likely | Windows TCP/IP | 5001330 +3 |
| Important | CVE-2021-28324 | Windows SMB Information Disclosure Vulnerability | Info Disc. | 7.5 | More likely | Windows SMB Server | 5001330 |
| Important | CVE-2021-27072 | Win32k Elevation of Privilege Vulnerability | EoP | 7.0 | More likely | Windows Win32K | 5001330 +9 |
| Important | CVE-2021-28325 | Windows SMB Information Disclosure Vulnerability | Info Disc. | 6.5 | More likely | Windows SMB Server | 5001330 +9 |
| Important | CVE-2021-28442 | Windows TCP/IP Information Disclosure Vulnerability | Info Disc. | 6.5 | More likely | Windows TCP/IP | 5001330 +3 |
| Critical | CVE-2021-28329 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28330 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28331 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28332 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28333 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28334 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28335 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28336 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28337 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28338 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
| Critical | CVE-2021-28339 | Remote Procedure Call Runtime Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows Remote Procedure Call Runtime | 5001330 +13 |
Your move
What your IT team should do
The same five steps every month, in the order that reduces risk fastest.
Triage by exploitability, not CVSS
CVE-2021-28310 is rated Important and scores 7.8 - a CVSS-sorted list would bury the one flaw attackers are already using. Sort by exploited, then publicly disclosed, then severity.
Patch Management vs Vulnerability Management: What's the Difference? →Stage the rollout in rings
This release means 47 distinct update packages to deploy. Pilot on ring 0 within 24 hours and widen to production by day seven - never push the whole set everywhere at once.
What is Ring-Based Patch Deployment? A Guide to Progressive Rollouts →The Patch Management Lifecycle: 6 Stages from Discovery to Verification →Test the known-issue candidates
Check the Windows release health dashboard before promoting past ring 1, and have the rollback path written down before you need it. Kernel, networking and authentication fixes are the ones that historically break things.
12 Patch Management Best Practices for 2026 →Don't stop at the operating system
39 Chromium fixes for Edge shipped alongside this release on their own updater, and Adobe, SAP and Oracle release in the same week. OS-only patching leaves most of your real attack surface untouched.
What is Third-Party Patch Management and Why OS Patching Isn't Enough →Verify installed state, then evidence it
Confirm the patch is actually present on each endpoint rather than trusting that "deployment succeeded". Keep the resulting report - it is the artefact auditors ask for, and reconstructing it later is far more work than capturing it now.
Patch Management: The Complete Guide →Trinetri does steps two through five automatically - detecting the 47 KB articles across every endpoint, staging them in rings, verifying installed state and producing the compliance evidence. See patch management →
Archive
Previous Patch Tuesdays
Every month Microsoft has published back to January 2016, each with the same breakdown and the full CVE list.
October 2026October 13, 2026September 2026September 8, 2026August 2026August 11, 2026July 2026July 14, 2026June 2026June 9, 2026May 2026May 12, 2026April 2026April 14, 2026March 2026March 10, 2026February 2026February 10, 2026January 2026January 13, 2026December 2025December 9, 2025November 2025November 11, 2025October 2025October 14, 2025September 2025September 9, 2025August 2025August 12, 2025July 2025July 8, 2025June 2025June 10, 2025May 2025May 13, 2025April 2025April 8, 2025March 2025March 11, 2025February 2025February 11, 2025January 2025January 14, 2025December 2024December 10, 2024November 2024November 12, 2024
Older months104 releases, October 2024 – January 2016
2024
2023
2022
2021
2020
2019
2018
2017
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Patch Tuesday FAQ
Patch Tuesday falls on the second Tuesday of every month, with updates going live at 10:00 a.m. Pacific Time. Microsoft introduced the practice in October 2003, following the Blaster worm, to cut distribution costs and give administrators a predictable cadence instead of patches arriving at random. The 10:00 a.m. timing is deliberate: it leaves a full working week to remediate before the weekend. Internally Microsoft calls it the "B" release; optional "C" and "D" preview releases follow later in the month. The next Patch Tuesday is November 10, 2026.
Microsoft fixed 108 vulnerabilities in April 2021, across Windows, Developer Tools, Microsoft Office and Azure. 19 are rated Critical, 4 were publicly disclosed before the patch shipped, and 1 was already being exploited. Note that Microsoft's release document lists 233 entries in total, but 86 of those are Azure Linux package updates and 39 are republished Chromium fixes for Edge, which is why other sources sometimes quote a much larger number.
Exploit Wednesday is the day after Patch Tuesday. Attackers compare the newly released patches against the previous version to locate the flaw each one fixes, then build working exploits from that difference - often within 24 to 48 hours. It is the reason the deployment window matters as much as the patch itself: every hour a Critical remote code execution fix sits undeployed is an hour of measurable exposure.
A workable target for most organisations: actively exploited vulnerabilities within 24 hours, publicly disclosed ones within 72 hours, remaining Critical fixes within seven days, and everything else within the month. CISA's Known Exploited Vulnerabilities catalogue sets binding deadlines for US federal agencies and is a reasonable benchmark for everyone else. Stage the rollout in rings rather than deploying everywhere at once, so a regression is caught on a pilot group instead of in production.
An out-of-band update is a patch released outside the monthly cycle because the risk will not wait for the next Patch Tuesday. Microsoft issues them for actively exploited flaws with no mitigation - most famously during the WannaCry outbreak in 2017, when it took the unusual step of patching versions of Windows that were already out of support.
Yes. Adobe aligned its releases with Patch Tuesday in November 2012, and both SAP's Security Patch Day and Oracle's quarterly Critical Patch Updates are coordinated to the same week. Part of the reason is defensive: staggered releases let attackers reverse-engineer one vendor's patch to find the same underlying flaw in another's still-unpatched product. For IT teams it means the second Tuesday is rarely just a Microsoft problem.
Edge is built on Chromium, so most of its security fixes originate with the Chromium project and are republished by Microsoft under their own CVE identifiers. They ship on Chromium's cadence rather than the Patch Tuesday cycle, and they are delivered by Edge's own updater rather than Windows Update. Counting the 39 Edge entries alongside Microsoft product fixes would inflate the headline number and imply work for your Windows Update rings that does not exist.