KB5066840: Cumulative security update for Internet Explorer: October 14, 2025

Note

• Important Certain versions of Microsoft Internet Explorer have reached the end of support. Note that some versions of Internet Explorer may be supported past the latest OS end date when Extended Security Updates (ESUs) are available. See Lifecycle FAQ - Extended Security Updates for a list of products offering ESUs.
• To continue receiving security and quality updates, we recommend that you upgrade to Microsoft Edge.

Note

• Windows Secure Boot certificate expiration
• Important: Secure Boot certificates used by most Windows devices are set to expire starting in June 2026. Microsoft has been updating these certificates on consumer and non-managed business devices for the past months. Devices that haven’t received the newer certificates will continue to start and operate normally, and standard Windows updates will continue to install. We will continue to install the newer certificates via Windows updates in the coming months.
• You can check your PC status on the Windows Security app. If you are an IT administrator, follow the guidance on the Secure Boot Playbook for Windows clients and Windows Server.

Note

Reminders

• The improvements that are included in this Internet Explorer update are also included in the October 2025 Monthly Rollup. Installing either this Internet Explorer update or the Monthly Rollup installs the same improvements.
• This update is not applicable for installation on a device on which the Monthly Rollup from October 2025 (or a later month) is already installed. This is because that update contains all the same improvements that are included in this Internet Explorer update.
• If you use update management processes other than Windows Update and you automatically approve all security update classifications for deployment, this update and the October 2025 Monthly Rollup are deployed. We recommend that you review your update deployment rules to make sure that the desired updates are deployed.
• If you install a language pack after you install this update, you must reinstall this update. Therefore, we recommend that you install any language packs that you need before you install this update. For more information, see Add language packs to Windows.