Feature or Flaw? How Hackers Turned Edge's IE Mode into a Backdoor

The pursuit of backward compatibility is a double-edged sword in the world of technology. While essential for supporting legacy systems, it often creates unintended security vulnerabilities. Microsoft recently confirmed this reality by revamping the Internet Explorer (IE) mode within its modern Edge browser after threat actors were found to be actively exploiting the feature as a security bypass.

In August 2025, Microsoft received credible reports detailing how unknown attackers were leveraging IE mode—a feature designed to allow Edge users to render older, IE-dependent websites—to gain unauthorised access to user devices. This sophisticated attack was particularly alarming because it subverted the robust, modern security protections built into the Chromium-based Edge browser.

How Hackers Turned Edge's IE Mode into a Backdoor

The attack chain was ingenious in its simplicity and effectiveness. It began with social engineering: hackers would trick unsuspecting users into visiting a seemingly legitimate website. Once there, a deceptive pop-up or "flyout" would instruct the user to reload the page in IE mode for full functionality.

The moment the page was reloaded, the user's browser essentially dropped its modern defenses and embraced the outdated security context of Internet Explorer. The attackers then weaponised an unspecified zero-day exploit in IE's legacy JavaScript engine, Chakra, to achieve remote code execution. This first breach was quickly followed by a second exploit, which allowed the threat actors to elevate their privileges out of the browser's sandbox and seize complete control of the victim's device.

This break-out capability is what made the attacks so dangerous. By escaping the browser's confines, the adversaries could execute a full range of post-exploitation activities, including deploying malware, moving laterally across networks, and exfiltrating sensitive data. The IE mode, intended as a bridge to the past, had become a highly effective backdoor in the present.

In response to this active exploitation and the clear risk posed by the feature's ease of access, Microsoft has significantly tightened the reins on IE mode. The company has removed dedicated entry points like the toolbar button and context menu option that previously made the feature a simple click away.

Users now must take a deliberate, multi-step process to enable and use IE mode: navigating deep into settings, explicitly setting the "Allow sites to be reloaded in Internet Explorer mode" option, and manually adding specific sites to a compatibility list. Microsoft stated that these restrictions are necessary to balance legacy support with security, ensuring that the decision to use the older, less-secure technology is "significantly more intentional." The added friction serves as a major barrier, designed to frustrate even the most determined attackers, turning a one-click exploit vector into a complex, multi-step user-initiated process. This move underscores the principle that security often requires sacrificing convenience.