Remote code execution via truncated HTML length
Published Jun 8, 2021 · Updated Oct 21, 2025
Out-of-bounds write in Microsoft Windows MSHTML allows remote attackers to execute code after a user opens malicious web content. MSHTML truncates an innerHTML string length in CTreePos but advances an index using the full TextData length, causing RemoveSplice to write beyond its allocation. Exploitation requires user interaction with attacker-controlled web content, and successful code execution remains constrained by the MSHTML process sandbox.
Summary
What happened
Out-of-bounds write in Microsoft Windows MSHTML allows remote attackers to execute code after a user opens malicious web content. MSHTML truncates an innerHTML string length in CTreePos but advances an index using the full TextData length, causing RemoveSplice to write beyond its allocation. Exploitation requires user interaction with attacker-controlled web content, and successful code execution remains constrained by the MSHTML process sandbox.
The record
- CVE
- CVE-2021-33742
- Published
- Jun 8, 2021
- Updated
- Oct 21, 2025
- Vendor
- Microsoft
- Product
- Windows 10 Version 1507
- Classifications
- CWE-787, CWE-823, T1203
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 1, 2021Exploitation reportedArmenian-user targeting with malicious Office documents
- Jun 8, 2021CVE publishedPublication date reported by the CVE source.
- Oct 21, 2025Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=10.0.0 <10.0.10240.18967
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Project Zero MSHTML innerHTML proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Reported exploitation
- CISA Known Exploited Vulnerabilities entryknown exploited catalog
- Armenian-user targeting with malicious Office documentsthreat actor
Technologies
Your stack
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Your stack
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