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Microsoft says Windows 11 version 24H2 replaces the older jscript9.dll with jscript9legacy.dll to address vulnerabilities and improve security. The replacement changes a compatibility detail: persistent JScript execution context is disabled by default. Applications that depend on definitions surviving from one script to the next may therefore fail unless an administrator opts in to persistence.
What Microsoft changed
Microsoft identifies jscript9legacy.dll as a newer JScript engine DLL. Its support article says the file replaces jscript9.dll starting with Windows 11 version 24H2. Microsoft gives addressing vulnerabilities and improving security as the reasons, but does not publish a quantified security improvement, vulnerability count or incident reduction for this change.
The documented scope is Windows 11 version 24H2 (all editions), Windows 11 version 25H2 (all editions) and Windows Server 2025. The support article, KB 5105752, was originally published June 18, 2026.
Why some JScript applications can break
Behavior of the older engine
Microsoft describes the older jscript9.dll as retaining global definitions and execution context between scripts. A function or other definition loaded by one script could remain available when another script ran.
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Default behavior in JScript9Legacy
In jscript9legacy.dll, that execution context does not persist by default. Definitions created by a script can be discarded when that script finishes. An application may consequently fail when it expects a later script to find functions, variables or polyfills loaded earlier.
This is a conditional compatibility issue, not a claim that every JScript program fails. Scripts that are self-contained or explicitly recreate their state may continue to work normally.
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What the February 2026 updates provide
Microsoft says updates released on and after February 24, 2026 address the persistence issue by providing an opt-in compatibility setting. Persistence remains disabled by default, so installing an applicable update does not automatically turn it on for every process.
Enable persistence for an affected application
Use this setting only when a specific application requires cross-script persistence. Microsoft advises backing up the registry before editing it.
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- Sign in with administrative rights and back up the registry.
- Open Registry Editor and go to
HKEY_LOCAL_MACHINESoftwarePoliciesMicrosoftInternet ExplorerMainFeatureControlFEATURE_ENABLE_PERSISTENCE. - Create a
DWORD (32-bit) Valuewhose name is the affected executable’s process name, such asexample.exe. - Set that DWORD’s value data to
1. - Close and restart the affected application so it starts with the new policy.
Microsoft’s documented configuration uses the process name, not a script filename. Confirm the executable name in Task Manager or the application’s documentation before creating the value.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Enable it for every process
Microsoft also documents an asterisk value for broad compatibility. Under the same FEATURE_ENABLE_PERSISTENCE key, create a DWORD (32-bit) Value named * and set it to 1. This enables persistence for all processes covered by the policy, so it should be used only when the wider scope is understood and acceptable. A per-process value is the narrower choice.
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How the two engine states compare
| State | Default context persistence | Cross-script compatibility | Configuration scope |
|---|---|---|---|
jscript9.dll |
Retained global definitions and execution context, according to Microsoft’s description | Scripts could use definitions loaded by earlier scripts | Built-in behavior of the older engine |
jscript9legacy.dll default |
Disabled | Applications relying on shared state may fail | Default for the documented Windows releases |
jscript9legacy.dll with policy |
Enabled | Restores the required persistence behavior | Named process, or all processes with * |
What this change is not
- It is not a hardware upgrade or a separate product to install.
- It does not mean Microsoft enabled persistent context globally.
- It does not mean all JScript applications are broken.
- It is different from Microsoft’s October 13, 2020 guidance about blocking the older
Jscript.dllin Internet Explorer 11. That earlier control concerned legacy Internet Explorer scripting, not this JScript9Legacy replacement.
Troubleshoot before changing the registry
- Verify that the computer is running one of the documented releases: Windows 11 24H2, Windows 11 25H2 or Windows Server 2025.
- Install updates released on or after February 24, 2026.
- Identify whether the failure occurs only when one script depends on definitions loaded by another.
- Apply a named-process policy first, restart the application, and test again.
- If the application still fails, remove or review the policy and consult the application vendor; the setting cannot repair unrelated JScript errors.
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