JScript9Legacy Archives

Winsage

June 19, 2026

Microsoft finally admits its default Windows 11 25H2, 24H2 action broke key legacy component

Microsoft released Patch Tuesday updates for Windows 11, specifically KB5094126 and KB5093998, along with dynamic updates KB5094149, KB5095971, and KB5094156. Two issues have been acknowledged: malfunctioning Office applications and complications with the Recycle Bin. In July 2025, Microsoft changed the default settings of Windows 11 to JScript9Legacy in versions 24H2 and later, continuing with version 25H2 in October 2025. This change aimed to enhance security by addressing vulnerabilities related to legacy scripting, particularly cross-site scripting (XSS). A support article details a compatibility issue arising from the transition from jscript9.dll to jscript9legacy.dll, which affects how JScript manages execution context. Functions and definitions established by one script are no longer accessible to subsequent scripts, leading to failures in legacy applications. To address this, Microsoft released the KB5077241 update, which requires manual activation of persistent JScript execution context through a Registry setting. The steps to implement this solution involve creating a feature control registry key and configuring a DWORD value for specific processes or all processes.

Winsage

July 11, 2025

Microsoft enables JScript9Legacy scripting engine to improve Windows 11 security

Microsoft is phasing out JScript in Windows 11 24H2 in favor of JScript9Legacy to enhance security. JScript9Legacy is enabled by default and will manage all scripting processes previously reliant on JScript without requiring user action. This new scripting engine offers improved performance, compatibility with modern web standards, and advanced security features to mitigate risks such as cross-site scripting (XSS). The transition is designed to be seamless, although Microsoft has not provided procedures for reverting to JScript if compatibility issues arise. The change is limited to Windows 11 24H2, with no updates planned for earlier Windows versions.

Winsage

July 11, 2025

Windows 11 24H2 standardizes its scripting engine

Microsoft has announced that starting with Windows 11 version 24H2, the JScript9Legacy engine will be enabled by default for all scripting processes that previously relied on the classic JScript engine. This new engine offers improved protection against threats like cross-site scripting (XSS) and enhances performance. Users will not need to take any action, as existing scripts will continue to function normally. In case of compatibility issues, organizations can revert to the previous engine temporarily. The transition marks the retirement of JScript, which has been part of Windows since 1996, as it is now considered outdated and vulnerable. Microsoft has decided to discontinue support for JScript due to the retirement of Internet Explorer and the adoption of the Edge browser. This update applies only to Windows 11 version 24H2 and later, while older versions will still use the original JScript engine.