Introduction

Microsoft has officially replaced the legacy JScript scripting engine with the more modern JScript9Legacy engine in Windows 11 version 24H2 and later. This strategic move is aimed at bolstering the security of the operating system against increasingly sophisticated cyber threats while also improving performance and maintaining stability for legacy applications.

Why Did Microsoft Make This Change?

JScript (jscript.dll) was introduced in 1996 as Microsoft’s implementation of ECMAScript. It was widely used in Internet Explorer and for automation tasks on Windows.
Over the years, JScript became outdated, failing to meet modern security standards. It was frequently targeted by severe vulnerabilities such as arbitrary code execution, buffer overflows, and cross-site scripting (XSS) attacks.
Although Internet Explorer has been retired, JScript persisted to ensure backward compatibility with legacy workflows and enterprise applications.

What’s New with JScript9Legacy?

JScript9Legacy is a modernized version based on JScript9 (the Chakra engine), which was used in newer Microsoft browsers like Internet Explorer 9 and EdgeHTML.
Designed for use outside browsers, this engine supports legacy scripting needs while offering better security and compatibility.
Key Advantages:
- Enhanced Protection Against XSS and Web Vulnerabilities: JScript9Legacy adheres to modern JavaScript standards, reducing the risk of exploitation through malicious documents, emails, or websites.
- Improved Performance: The new architecture delivers faster and more stable script execution compared to the old JScript engine.
- Stricter Execution Policies: It introduces tighter controls over JavaScript objects and script execution, reducing the risk of malicious code execution or unauthorized memory access via outdated scripts.
- Better Compatibility: Improved support for newer web standards ensures that applications and enterprise workflows run more smoothly on the latest Windows platform.

Impact on Users and Enterprises

Automatic Transition: Windows 11 users on version 24H2 and above will automatically use JScript9Legacy for all scripting tasks that previously relied on JScript, with no manual intervention required.
Backward Compatibility: Legacy scripts will continue to function as usual. In case of any issues, enterprises can contact Microsoft for guidance on reverting to the old engine via the Services Hub.
Enhanced Security: Removing JScript significantly reduces the risk of attacks exploiting known vulnerabilities, especially critical ones like CVE-2024-38178 related to remote code execution.
No Effect on Older Windows Versions: This change only applies to Windows 11 24H2 and later; older versions will continue using the legacy JScript engine.

Technical Compatibility Notes

Some specific cases involving COM or workflows using 64-bit data types may encounter precision issues when passing data between objects. Microsoft recommends contacting technical support if such issues arise.

Conclusion

Microsoft’s adoption of JScript9Legacy in Windows 11 marks a significant advancement in operating system security while still ensuring stability for legacy applications and workflows. Users and enterprises benefit from a safer, more efficient environment without disruption to their daily operations.

Windows 11 Switches to JScript9Legacy: A Big Step In Security