CVE-2021-40450
CVE-2021-40450 is a security flaw in Microsoft Win32k, a part of Windows, Microsoft’s operating system. Someone who already has a foothold can use Microsoft Win32k to take full control of the machine. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 25 Apr 2022.[1]
Microsoft Win32k is in things households have, such as phones, computers, browsers and everyday apps, so the fix matters at home as much as at work. Fixed in version 10.0.17763.2237, 10.0.18363.1854, 10.0.19043.1288 and 10.0.20348.288 and others.
Description
CISA calls it a privilege escalation in Microsoft Win32k. In its words: “Microsoft Win32k contains an unspecified vulnerability that allows for privilege escalation.”[1]
It scores 7.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by the organisation that published the CVE using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| Windows 10 Version 1809 | 10.0.17763.2237 |
| Windows Server 2019 | 10.0.17763.2237 |
| Windows Server 2019 (Server Core installation) | 10.0.17763.2237 |
| Windows 10 Version 1909 | 10.0.18363.1854 |
| Windows 10 Version 21H1 | 10.0.19043.1288 |
| Windows Server 2022 | 10.0.20348.288 |
| Windows 10 Version 2004 | 10.0.19041.1288 |
| Windows Server version 2004 | 10.0.19041.1288 |
Every device with Microsoft Win32k needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 25 Apr 2022, 194 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 16 May 2022 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 2%. That’s in the top 25% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Microsoft has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Update Microsoft Win32k now.On Windows: Settings, Windows Update, Check for updates, then restart. Office updates come the same way, or from any Office app: File, Account, Update Options. Then turn on automatic updates, so the next fix arrives on its own.
- Check you’re on 10.0.17763.2237 or later.Fixed in version 10.0.17763.2237, 10.0.18363.1854, 10.0.19043.1288 and 10.0.20348.288 and others. Anything older is what attackers are going after.
- If there’s no update, stop using it.CISA’s advice when a flaw being used in attacks has no fix is to stop using the product until it does.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 7.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 2%.[4] |
See also
Other Microsoft Win32k flaws
- CVE-2016-0165: privilege escalation ·
- CVE-2023-29336: privilege escalation ·
- CVE-2015-2360: privilege escalation ·
- CVE-2018-8589: privilege escalation ·
- CVE-2014-4113: privilege escalation ·
- CVE-2021-41357: privilege escalation ·
Other Microsoft flaws
- CVE-2026-96951, Microsoft Windows: command injection ·
- CVE-2026-65660, Microsoft SharePoint: code injection ·
- CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
- CVE-2026-81963, Microsoft Windows: link following ·
- CVE-2019-1068, Microsoft SQL Server: remote code execution ·
Similar flaws
- CVE-2026-111357, Ivanti Connect Secure: missing authentication for critical function ·
- CVE-2026-97229, GitLab Community Edition and Enterprise Edition: integer overflow or wraparound ·
- CVE-2026-76418, Cisco Secure Firewall ASA Software and Cisco Secure FTD Software: unchecked return value to NULL pointer dereference ·
- CVE-2026-48356, Adobe Commerce and Magento: unrestricted upload of file with dangerous type ·
- CVE-2026-111497, Google Brotli: unchecked return value ·
Added the same day
- CVE-2022-29464, WSO2 products: unrestrictive upload of file ·
- CVE-2022-26904, Microsoft Windows: user profile service privilege escalation ·
- CVE-2022-21919, Microsoft Windows: user profile service privilege escalation ·
- CVE-2022-0847, Linux Kernel: privilege escalation ·
- CVE-2021-41357, Microsoft Win32k: privilege escalation ·
Every Microsoft flaw on CISA’s list · Everything added in April 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Win32k Privilege Escalation Vulnerability”. Known Exploited Vulnerabilities Catalog, added 25 Apr 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-40450 Detail”. National Vulnerability Database, published 13 Oct 2021.
- ^ Microsoft. Advisories and fixes: portal.msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.016, percentile 75.5, 31 Oct 2026.
- CVE Program. “CVE-2021-40450”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.