Code execution via incorrect JIT optimization
Published Aug 12, 2021 · Updated Aug 10, 2026
Memory corruption in Microsoft Windows JScript9 allows remote attackers to execute code through crafted JavaScript in affected builds. The JIT compiler performs an incorrect optimization on attacker-controlled script, producing an out-of-bounds write in jscript9.dll. Exploitation requires a user to open attacker-controlled web content in Internet Explorer, and resulting code runs with that user's rights.
Summary
What happened
Memory corruption in Microsoft Windows JScript9 allows remote attackers to execute code through crafted JavaScript in affected builds. The JIT compiler performs an incorrect optimization on attacker-controlled script, producing an out-of-bounds write in jscript9.dll. Exploitation requires a user to open attacker-controlled web content in Internet Explorer, and resulting code runs with that user's rights.
The record
- CVE
- CVE-2021-34480
- Published
- Aug 12, 2021
- Updated
- Aug 10, 2026
- Vendor
- Microsoft
- Product
- Windows 10 Version 1507
- Classifications
- CWE-787, T1203
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Aug 12, 2021CVE publishedPublication date reported by the CVE source.
- Aug 10, 2026Record 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=10.0.10240.0 <10.0.10240.19022
What conditions does exploitation require?
What is affected?
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.
Public exploit references
- Internet Explorer JIT Optimization Memory Corruption proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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