Patch today
Exploited in the wild
Confirmed attacks are already happening. Emergency change window.
No actively exploited vulnerabilities in this release.
Updated monthlyMicrosoft Patch Tuesday
Microsoft Patch Tuesday
May 2021
Microsoft's May 2021 Patch Tuesday fixes 55 vulnerabilities across Windows, Microsoft Office and Developer Tools - 4 rated Critical, 3 publicly disclosed before the patch shipped. No active exploitation this month, so lead with the 4 Critical remote code execution fixes.
Released May 11, 2021, 10:00 PT3 publicly disclosed35 KB articles to deploy
May 2021 Patch Tuesday at a glance
55
Microsoft CVEs
excl. Edge & Azure Linux
4
Critical
7% of the release
0
Actively exploited
none this month
3
Publicly disclosed
known before the patch
9.9
Highest CVSS
CVE-2021-28476
35
KB articles
what you actually deploy
Why this number differs from other roundups. Microsoft's May 2021 release document lists 158 entries. 66 are Azure Linux (Mariner) OS package updates and 37 are republished Chromium fixes for Edge - neither is a Microsoft product vulnerability. The 55 above is the real Patch Tuesday set. See the breakdown →
Priority order
Patch these first
Ranked by exploitation reality, not score alone. Nothing is under active attack this month, so the Critical remote-code-execution fixes lead.
Patch this week
Publicly disclosed
Details are already public. Working exploits typically follow within days.
CVE-2021-31204.NET and Visual Studio Elevation of Privilege VulnerabilityCVE-2021-31200Common Utilities Remote Code Execution VulnerabilityCVE-2021-31207Microsoft Exchange Server Security Feature Bypass VulnerabilityPatch this cycle
Critical remote code execution
No known exploitation yet, but network-reachable and high impact.
CVE-2021-31166HTTP Protocol Stack Remote Code Execution VulnerabilityCVE-2021-26419Scripting Engine Memory Corruption VulnerabilityCVE-2021-28476Windows Hyper-V Remote Code Execution VulnerabilitySee all 4 Critical →The numbers
What’s in this release
Where the 158 entries actually come from
Most roundups quote a single number. These are three unrelated release streams.
55
66
37
55 Microsoft products66 Azure Linux (Mariner)37 Edge (Chromium)
Severity
Microsoft channel only · 55 CVEs
| Critical | 4 |
|---|---|
| Important | 50 |
| Moderate | 1 |
| Low | 0 |
Impact type
What an attacker gains if it works
| Remote Code Execution | 22 |
|---|---|
| Elevation of Privilege | 11 |
| Information Disclosure | 10 |
| Spoofing | 9 |
| Security Feature Bypass | 2 |
| Denial of Service | 1 |
Most-affected components
Where this month’s fixes concentrate - filter by any of them in the table below
Microsoft Office SharePoint7Microsoft Office Excel5Windows Container Manager Service5Microsoft Exchange Server4Visual Studio Code3Windows Wireless Networking3Microsoft Graphics Component2Microsoft Windows Codecs Library2Skype for Business and Microsoft Lync2.NET Core & Visual Studio1Internet Explorer1Jet Red and Access Connectivity1
Every patch
Browse all 158 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-2021-31204 | .NET and Visual Studio Elevation of Privilege Vulnerability | EoP | 7.3 | Disclosed | .NET Core & Visual Studio | — |
| Important | CVE-2021-31200 | Common Utilities Remote Code Execution Vulnerability | RCE | 7.2 | Disclosed | Open Source Software | — |
| Moderate | CVE-2021-31207 | Microsoft Exchange Server Security Feature Bypass Vulnerability | Bypass | 6.6 | Disclosed | Microsoft Exchange Server | 5003435 |
| Critical | CVE-2021-31166 | HTTP Protocol Stack Remote Code Execution Vulnerability | RCE | 9.8 | More likely | Windows HTTP.sys | 5003173 |
| Critical | CVE-2021-26419 | Scripting Engine Memory Corruption Vulnerability | RCE | 7.5 | More likely | Internet Explorer | 5003165 +10 |
| Important | CVE-2021-28474 | Microsoft SharePoint Server Remote Code Execution Vulnerability | RCE | 8.8 | More likely | Microsoft Office SharePoint | 5001916 +2 |
| Important | CVE-2021-31181 | Microsoft SharePoint Remote Code Execution Vulnerability | RCE | 8.8 | More likely | Microsoft Office SharePoint | 5001916 +2 |
| Important | CVE-2021-31170 | Windows Graphics Component Elevation of Privilege Vulnerability | EoP | 7.8 | More likely | Microsoft Graphics Component | 5003169 +3 |
| Important | CVE-2021-31188 | Windows Graphics Component Elevation of Privilege Vulnerability | EoP | 7.8 | More likely | Microsoft Graphics Component | 5003169 +13 |
| Critical | CVE-2021-28476 | Windows Hyper-V Remote Code Execution Vulnerability | RCE | 9.9 | — | Role: Windows Hyper-V | 5003169 +13 |
| Critical | CVE-2021-31194 | OLE Automation Remote Code Execution Vulnerability | RCE | 8.8 | — | Windows OLE | 5003169 +13 |
| Important | CVE-2021-27068 | Visual Studio Remote Code Execution Vulnerability | RCE | 8.8 | — | Visual Studio | — |
| Important | CVE-2021-28455 | Microsoft Jet Red Database Engine and Access Connectivity Engine Remote Code Execution Vulnerability | RCE | 8.8 | — | Jet Red and Access Connectivity | 4484527 +17 |
| Important | CVE-2021-28465 | Web Media Extensions Remote Code Execution Vulnerability | RCE | 7.8 | — | Microsoft Windows Codecs Library | — |
| Important | CVE-2021-31165 | Windows Container Manager Service Elevation of Privilege Vulnerability | EoP | 7.8 | — | Windows Container Manager Service | 5003173 |
| Important | CVE-2021-31167 | Windows Container Manager Service Elevation of Privilege Vulnerability | EoP | 7.8 | — | Windows Container Manager Service | 5003169 +2 |
| Important | CVE-2021-31168 | Windows Container Manager Service Elevation of Privilege Vulnerability | EoP | 7.8 | — | Windows Container Manager Service | 5003173 |
| Important | CVE-2021-31169 | Windows Container Manager Service Elevation of Privilege Vulnerability | EoP | 7.8 | — | Windows Container Manager Service | 5003173 |
| Important | CVE-2021-31175 | Microsoft Office Remote Code Execution Vulnerability | RCE | 7.8 | — | Microsoft Office Excel | 5001914 +5 |
| Important | CVE-2021-31176 | Microsoft Office Remote Code Execution Vulnerability | RCE | 7.8 | — | Microsoft Office | 5001920 +1 |
| Important | CVE-2021-31177 | Microsoft Office Remote Code Execution Vulnerability | RCE | 7.8 | — | Microsoft Office Excel | 5001914 +3 |
| Important | CVE-2021-31179 | Microsoft Office Remote Code Execution Vulnerability | RCE | 7.8 | — | Microsoft Office Excel | 5001914 +5 |
| Important | CVE-2021-31180 | Microsoft Office Graphics Remote Code Execution Vulnerability | RCE | 7.8 | — | Microsoft Office Word | 4464542 +2 |
| Important | CVE-2021-31187 | Windows WalletService Elevation of Privilege Vulnerability | EoP | 7.8 | — | Windows WalletService | 5003169 +5 |
| Important | CVE-2021-31190 | Windows Container Isolation FS Filter Driver Elevation of Privilege Vulnerability | EoP | 7.8 | — | Windows Container Isolation FS Filter Driver | 5003171 |
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
Nothing is under active attack this month, so rank by exploitability forecast and reachability rather than raw score. A Critical flaw on an internet-facing service outranks a higher-scoring one behind authentication.
Patch Management vs Vulnerability Management: What's the Difference? →Stage the rollout in rings
This release means 35 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
37 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 35 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 55 vulnerabilities in May 2021, across Windows, Microsoft Office, Developer Tools and Azure. 4 are rated Critical, 3 were publicly disclosed before the patch shipped, and 0 were already being exploited. Note that Microsoft's release document lists 158 entries in total, but 66 of those are Azure Linux package updates and 37 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 37 Edge entries alongside Microsoft product fixes would inflate the headline number and imply work for your Windows Update rings that does not exist.