Legacy IE Mode in Edge Opens Door to Hackers

In August 2025, cybersecurity researchers uncovered a sophisticated hacking campaign exploiting Microsoft Edge’s Internet Explorer (IE) mode to compromise users’ devices. 

By leveraging social engineering and zero-day vulnerabilities within IE’s outdated Chakra JavaScript engine, threat actors successfully bypassed modern browser protections. 

This discovery highlights the persistent risks of maintaining legacy compatibility features in today’s rapidly evolving digital landscape.

When compatibility becomes a vulnerability

Microsoft Edge’s IE mode was originally developed to provide compatibility for older web applications and technologies that relied on outdated frameworks such as ActiveX, Silverlight, or Flash. 

Many enterprises, government portals, and industrial systems still depend on these legacy components, making full deprecation impractical. 

However, attackers have now weaponized this compatibility feature to bypass modern browser security protections.

In this campaign, adversaries combined social engineering and zero-day exploits to manipulate unsuspecting users into reloading web pages in IE mode. 

Victims were first directed to spoofed websites that mimicked legitimate business or government sites. Once on these pages, a convincing flyout notification prompted users to “reload in Internet Explorer mode,” ostensibly to ensure proper site functionality.

This simple interaction transferred the browsing session from Edge’s secure Chromium-based environment to the outdated IE framework—an environment inherently more vulnerable to exploitation.

The anatomy of the exploit

At the core of the attack was the exploitation of the Chakra JavaScript engine used by Internet Explorer. 

Despite Microsoft’s previous hardening efforts, Chakra remains susceptible to memory corruption vulnerabilities. Upon activation of IE mode, attackers deployed a zero-day exploit targeting the engine to achieve remote code execution (RCE).

Following the initial compromise, a secondary exploit was executed to escalate privileges beyond the browser’s sandbox. This two-stage attack granted adversaries full control of the victim’s system. 

Once elevated, attackers could install malware, move laterally within corporate networks, or exfiltrate sensitive data. The exploitation of both execution and privilege escalation pathways underscored the sophistication and planning behind the campaign.

Microsoft’s response

Upon identifying this threat, Microsoft’s Edge security team acted swiftly to contain the damage. 

The company received credible intelligence confirming that the attack was active and widespread. 

In response, Microsoft removed several high-risk entry points that allowed easy access to IE mode, including toolbar buttons, context menu options, and hamburger menu shortcuts. 

For enterprise users requiring IE compatibility, Microsoft retained policy-based controls to ensure business continuity. 

Non-commercial users can still access IE mode, but the process now requires multiple manual steps. Users must navigate to Settings > Default Browser, enable “Allow sites to be reloaded in Internet Explorer mode,” and manually add approved sites to a trusted list. 

This procedural friction increases user awareness and limits automated or deceptive transitions into IE mode.

Balancing innovation with backward compatibility

This campaign serves as a stark reminder that backward compatibility introduces ongoing security risks. 

Internet Explorer, officially retired in June 2022, was not designed with the modern principles of defense-in-depth, sandboxing, or site isolation that are standard in contemporary browsers. 

By maintaining partial functionality through IE mode, Microsoft inadvertently preserved an attack surface that adversaries could exploit.

The compromise of IE mode represents a broader challenge in cybersecurity: balancing innovation and legacy support. 

Many organizations rely on outdated web applications for critical operations, yet maintaining compatibility without introducing vulnerabilities remains a complex task.  

Recommendations for strengthening browser security

Microsoft strongly advises users and administrators to transition away from legacy technologies dependent on Internet Explorer. 

Modern browsers such as Edge, Chrome, and Firefox offer advanced protections including process isolation, memory safety mechanisms, and integrated phishing defenses. 

Organizations should audit their web dependencies, update or replace outdated systems, and restrict access to IE mode unless absolutely necessary.

End users should also exercise caution when interacting with unexpected prompts or website notifications. 

Threat actors often rely on social engineering to bypass technical defenses, emphasizing the need for user education alongside technical safeguards.

The exploitation of Microsoft Edge’s Internet Explorer mode underscores the evolving sophistication of cyber threats and the enduring danger of legacy software components. 

By exploiting IE mode’s compatibility features, attackers successfully bridged the gap between modern and outdated browser technologies, achieving full system compromise through a combination of social engineering and technical exploitation. 

While Microsoft’s swift response has mitigated immediate risks, the incident underscores a fundamental reality: legacy technologies will always remain prime targets for exploitation. 

Transitioning to modern, secure platforms is imperative to maintain resilience in today’s complex cyber landscape.