Microsoft Confirms Long-Running Windows 11 JScript Compatibility Issue

Microsoft has recently acknowledged several issues affecting Windows users, including a Recycle Bin bug and Office app launch problems introduced by June Patch Tuesday updates. The company has now also documented a long-running compatibility issue related to JScript9Legacy that affects some legacy applications on newer Windows 11 releases.

According to Microsoft, the issue dates back to July 2025, when the company changed newer versions of Windows 11 to use the JScript9Legacy engine by default. The change affects Windows 11 version 24H2 and later releases, including Windows 11 25H2.

Microsoft introduced the newer engine as part of its ongoing effort to improve security and reduce risks associated with legacy scripting technologies. The updated jscript9legacy.dll component enforces stricter execution policies and improves object handling behavior, helping reduce exposure to vulnerabilities such as cross-site scripting (XSS) attacks.

Legacy Apps Can Break After JScript Engine Changes

While the security improvements provide better protection, they also create compatibility problems for some older applications and web-based components.

Microsoft says that JScript global definitions and execution context may fail to persist across scripts under the newer engine. Older applications often relied on behavior found in the previous jscript9.dll implementation, which retained global definitions and execution state between script executions.

Because JScript9Legacy disables that persistence by default, applications that depend on shared execution context, polyfills, or legacy scripting behaviors may stop working correctly. This can lead to failures in older enterprise software, internal web applications, and other legacy workloads.

Microsoft Provides Registry-Based Workaround

Microsoft says the compatibility issue was addressed through update KB5077241. However, the fix is not enabled automatically in subsequent Windows updates.

Administrators who need to restore the previous behavior must manually enable persistent JScript execution context through the Windows Registry using the FEATURE_ENABLE_PERSISTENCE key.

The workaround supports both application-specific and system-wide configurations:

• Enable persistence for individual applications by creating a DWORD value for each target process and setting it to 1.
• Enable persistence for all processes by creating a value named * and setting it to 1.
• Restart affected applications after applying the registry changes.

Microsoft recommends that administrators carefully test the workaround before deploying it broadly across production environments.

For detailed instructions on configuring the workaround, administrators can consult Microsoft’s official support documentation.

In other news, Microsoft recently fixed Windows Server 2016 update installation failures that prevented some systems from deploying the June 2026 security update successfully.

Via Neowin