CVE-2021-33742
CVE-2021-33742 is a security flaw in Microsoft Windows, Microsoft’s operating system for PCs and servers. Attackers can run their own code on Microsoft Windows. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 3 Nov 2021.[1]
Microsoft Windows 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 10.0.17763.1999, 10.0.18363.1621, 10.0.19043.1052 and 10.0.19041.1052 and others.
Description
CISA calls it an MSHTML platform remote code execution in Microsoft Windows. In its words: “Microsoft Windows MSHTML Platform contains an unspecified vulnerability that allows for remote code execution.”[1]
The underlying weakness is out-of-bounds write (CWE-787), CWE-823, in MITRE’s list of common software weaknesses.[2]
It scores 7.5 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 | 10.0.17763.1999 |
| Windows Server 2019 | 10.0.17763.1999 |
| Windows 10 Version 1909 | 10.0.18363.1621 |
| Windows 10 Version 21H1 | 10.0.19043.1052 |
| Windows 10 Version 2004 | 10.0.19041.1052 |
| Windows 10 Version 20H2 | 10.0.19042.1052 |
| Windows 10 Version 1507 | 10.0.10240.18967 |
| Windows 10 Version 1607 | 10.0.14393.4467 |
Every device with Microsoft Windows needs the update.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 3 Nov 2021, 148 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 17 Nov 2021 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 59%. That’s in the top 1% 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 Windows 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 10.0.17763.1999 or later.Fixed in version 10.0.17763.1999, 10.0.18363.1621, 10.0.19043.1052 and 10.0.19041.1052 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 7.5.[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 59%.[4] |
See also
Other Microsoft Windows flaws
- CVE-2026-96951: command injection ·
- CVE-2026-85880: heap-based buffer overflow ·
- CVE-2026-81963: link following ·
- CVE-2008-4250: buffer overflow ·
- CVE-2026-32202: protection mechanism failure ·
- CVE-2025-60710: link following ·
Other out-of-bounds write flaws
- CVE-2026-86950, Apple devices: out-of-bounds write ·
- CVE-2026-53266, Linux Kernel: out-of-bounds write ·
- CVE-2026-87491, Chrome and other Chromium browsers: out of bounds write ·
- CVE-2026-53362, Linux Kernel: unspecified ·
- CVE-2022-0995, Linux Kernel: out-of-bounds write ·
Added the same day
- CVE-2021-42258, BQE BillQuick Web Suite: SQL injection ·
- CVE-2021-42013, Apache HTTP Server: path traversal ·
- CVE-2021-41773, Apache HTTP Server: path traversal ·
- CVE-2021-40539, Zoho ManageEngine: aDSelfService plus authentication bypass ·
- CVE-2021-40444, Microsoft MSHTML: remote code execution ·
Every Microsoft flaw on CISA’s list · Everything added in November 2021
References
- ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Windows MSHTML Platform Remote Code Execution Vulnerability”. Known Exploited Vulnerabilities Catalog, added 3 Nov 2021.
- ^ National Institute of Standards and Technology. “CVE-2021-33742 Detail”. National Vulnerability Database, published 8 Jun 2021.
- ^ Microsoft. Advisories and fixes: portal.msrc.microsoft.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.594, percentile 99.1, 31 Oct 2026.
- CVE Program. “CVE-2021-33742”. 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.