Why Microsoft Is Finally Dropping Internet Explorer: Timeline, Impact, and Security Risks

Planned End of Support for Internet Explorer 11 in Microsoft 365

On 17 August 2022 Microsoft announced that one year later (17 August 2023) all Microsoft 365 (formerly Office 365) applications will cease to support Internet Explorer 11.

Phased Withdrawal Timeline

30 Nov 2020 – Teams web client removed IE 11 support.

9 Mar 2021 – Legacy Microsoft Edge (Edge Legacy) support ended.

17 Aug 2021 – Core Microsoft 365 services (Office 365, OneDrive, Outlook, SharePoint, Teams desktop) stopped working with IE 11.

17 Aug 2023 – Final cutoff: IE 11 will be unable to authenticate or render any Microsoft 365 web pages.

Technical Rationale

IE 11 is a component of Windows 10/11 and is not updated independently of the OS. Its rendering engine (Trident) does not implement modern web standards (HTML5, CSS3, ES6) and lacks security mitigations such as SameSite‑strict cookies, CSP, and modern TLS defaults. Maintaining compatibility requires Microsoft to keep a legacy code path in its cloud services, increasing testing overhead and security surface.

Impact on Users and Administrators

Authentication flows that rely on legacy Active Directory Federation Services (ADFS) may fail when accessed via IE 11.

Custom line‑of‑business web apps that depend on IE‑only features (e.g., document.all, ActiveX, VBScript) will stop loading.

Enterprise policies that enforce IE 11 as the default browser must be updated to Edge, Chrome, or Firefox.

Migration Path

Microsoft Edge provides an “IE mode” that emulates Trident for intranet sites. Administrators can enable it via Group Policy:

# Example registry key to enable IE mode
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge]
"InternetExplorerIntegrationLevel"=dword:00000001

Edge IE mode is intended as a temporary bridge; Microsoft has indicated it may be removed in a future release, so legacy applications should be modernized (e.g., replace ActiveX with standard JavaScript APIs).

Security Considerations

Security research highlights that IE 11’s architecture makes it vulnerable to drive‑by attacks, memory‑corruption exploits, and lack of sandboxing. Continuing to use IE 11 as a default browser exposes users to higher risk compared with browsers that implement process isolation and frequent security patches.

Conclusion

IE 11 will remain part of the Windows OS but will no longer be supported by Microsoft 365 after August 2023. Organizations should audit internal web applications for IE‑specific dependencies, transition users to a modern browser, and, where necessary, use Edge’s IE mode only as a short‑term compatibility solution.