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

Microsoft last week released Windows 11 KB5094126 and KB5093998 as the latest Patch Tuesday updates. Following that the company also published the accompanying dynamic updates under KB5094149, KB5095971, and KB5094156. So far the company has acknowledged two known issues that have popped up after the release which include bugged-out Office apps as well as the Recycle Bin; though there could be more at play too.

Speaking of bugs and issues, Microsoft seems to have finally acknowledged a problem that probably has been around for close to a year. That's because back in July of 2025 the company made a default change to the latest Windows 11 versions, wherein it switched to JScript9Legacy on Windows 11 24H2 and later releases. Hence following the release of version 25H2 in October 2025, JScript9Legacy also remained default-enabled. As a result there has been a compatibility issue ever since then.

For those wondering, by switching to JScript9Legacy Microsoft intended to improve the security of modern Windows PCs by reducing vulnerabilities tied to legacy scripting like cross-site scripting (XSS), among others. XSS exploits can allow cyber-attackers to attach malicious code onto legitimate websites and use them to execute the code when a potential victim loads such a website. Hence the new JScript9Legacy engine enforced stricter execution policies and improved object handling, which should help mitigate such attacks.

Microsoft today has published a new support article detailing the problem. Neowin spotted it while browsing. The company says that JScript global definitions and execution context may fail to persist across scripts, potentially breaking older dependent apps and web-based components that relied on this legacy behavior.

In the article Microsoft has confirmed that the issue stems from its move away from the older jscript9.dll engine in favor of jscript9legacy.dll. As mentioned above, while the newer engine was designed to address vulnerabilities and strengthen security it also changes how JScript handles execution context. As a result functions and definitions loaded by one script could no longer remain available to subsequent scripts once execution ended.

The company notes that some applications worked correctly on earlier Windows versions because the older JScript engine automatically retained global definitions and execution state between scripts. Under the newer model though that behavior is disabled by default causing certain legacy workloads and polyfill-dependent scripts to fail.

Microsoft says it addressed the problem via the KB5077241 update though the fix had not been enabled automatically in the following updates. As such admins must explicitly turn on persistent JScript execution context using a Registry setting that the tech giant shared today. The configuration can be applied to individual processes or system-wide through the FEATURE_ENABLE_PERSISTENCE registry key.

The steps have been outlined below:

1. Run the following command to create the feature control registry key:
   
   reg add "HKLM\Software\Policies\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_ENABLE_PERSISTENCE"
2. Under this key, create a new DWORD (32-bit) value.

   

3. Configure the value as follows:
   To enable persistence for specific processes only:
   Set the value to 1 for each target process name.
   To enable persistence for all processes:
   Add * as the key name and set its value to 1.

You can find the official support article here on Microsoft's website.