Microsoft Confesses Windows 11 25H2 Update Breaks Legacy Feature

Microsoft rolled out the latest Patch Tuesday updates for Windows 11, delivering KB5094126 and KB5093998, followed by dynamic patches KB5094149, KB5095971, and KB5094156. The company has already flagged two post‑release glitches: Office applications are behaving erratically and the Recycle Bin is misbehaving, with the possibility of additional issues emerging.

In a move that finally acknowledges a nearly year‑long problem, Microsoft revealed that a July 2025 change to the default scripting engine—switching to JScript9Legacy in Windows 11 24H2 and later—has persisted through the 25H2 release in October 2025, sparking widespread compatibility headaches.

The switch to JScript9Legacy was intended to tighten security by curbing legacy‑script vulnerabilities such as cross‑site scripting (XSS). By enforcing stricter execution policies and improving object handling, the new engine aims to thwart attackers who inject malicious code into legitimate sites.

Today Microsoft published a support article detailing the issue, first spotted by Neowin during routine monitoring. The company explains that JScript’s global definitions and execution context may fail to persist across scripts, potentially breaking older applications and web components that rely on legacy behavior.

Microsoft has traced the recent script execution failures to its decision to replace the legacy jscript9.dll engine with the newer jscript9legacy.dll. While the updated engine was introduced to patch security holes and bolster protection, it also altered how JScript manages its execution context, causing functions and definitions loaded by one script to become inaccessible to subsequent scripts once the first script finished running.

The firm explained that older Windows releases benefited from the previous JScript engine’s habit of preserving global definitions and execution state across scripts. In contrast, the new default behavior disables this retention, leading to compatibility issues for legacy workloads and scripts that rely on polyfills.

Microsoft has issued a fix in the KB5077241 update, but the patch was not automatically activated in later updates. Administrators must now manually enable persistent JScript execution context via a registry tweak. The company released the necessary setting today, which can be applied to individual processes or globally through the FEATURE_ENABLE_PERSISTENCE registry key.