Introduction

In April 2025, Microsoft released a cumulative update that introduced a new, empty folder named INLINECODE0 at the root of the C: drive on Windows systems. This unexpected addition has raised questions among users and IT administrators regarding its purpose and implications for system security. This article delves into the reasons behind the creation of the INLINECODE1 folder, its role in mitigating specific vulnerabilities, and the broader impact on Windows security practices.

Background: The inetpub Folder and IIS

Traditionally, the INLINECODE2 folder is associated with Microsoft's Internet Information Services (IIS), a web server platform used to host websites and web applications. When IIS is installed, the INLINECODE3 directory is created to store web content, logs, and related files. However, with the April 2025 update, this folder began appearing on systems regardless of whether IIS was installed or enabled.

The April 2025 Update and CVE-2025-21204

The creation of the INLINECODE4 folder is directly linked to Microsoft's efforts to address a security vulnerability identified as CVE-2025-21204. This vulnerability involves improper link resolution before file access ('link following') in the Windows Update Stack, which could allow local attackers with low privileges to escalate their permissions and manipulate file operations on the system. To mitigate this risk, Microsoft implemented a security measure that involves the creation of the INLINECODE5 folder with specific system-level permissions. This folder acts as a safeguard against potential exploits that could leverage symbolic links to gain unauthorized access or escalate privileges. Microsoft has explicitly advised users not to delete this folder, emphasizing its role in enhancing system protection. (bleepingcomputer.com)

Technical Details and Implications

The INLINECODE6 folder is created with SYSTEM ownership and specific permissions to prevent unauthorized access or modification. While the folder itself is empty and does not consume significant disk space, its presence is crucial for the integrity of the security update. Deleting the folder may not cause immediate operational issues; however, it could expose the system to the vulnerabilities that the update aims to mitigate. Additionally, some users have reported that if the INLINECODE7 directory is created before the update is deployed, the cumulative updates may fail to install. (bleepingcomputer.com)

User Guidance and Best Practices

Given the security implications, users and IT administrators are advised to:

  • Do Not Delete the inetpub Folder: Regardless of whether IIS is active on the device, the folder should remain intact to ensure the effectiveness of the security update.
  • Recreate the Folder if Deleted: If the folder has been inadvertently removed, it can be restored by enabling IIS through the Windows Features control panel. Once IIS is installed, the INLINECODE8 folder will be recreated with the appropriate permissions. If IIS is not needed, it can be uninstalled afterward, and the folder will remain. (bleepingcomputer.com)
  • Monitor for Updates: Stay informed about any further advisories from Microsoft regarding this issue, as additional patches or guidance may be provided to address related concerns.

Conclusion

The introduction of the INLINECODE9 folder through the April 2025 Windows update underscores Microsoft's proactive approach to addressing security vulnerabilities. While its sudden appearance may have caused confusion, understanding its purpose in mitigating CVE-2025-21204 highlights the importance of adhering to recommended security practices. By maintaining the integrity of system components like the INLINECODE10 folder, users can contribute to a more secure computing environment.