Microsoft Patches IE Mode Bug Exploited in Targeted Attacks

Microsoft’s latest Edge update addresses a critical flaw in IE Mode, the compatibility feature that lets enterprises access older web apps using Internet Explorer components inside Edge.

Security researchers and Microsoft’s own telemetry confirmed that threat actors exploited IE Mode in live attacks, using it to load malicious scripts and bypass browser isolation.
As a result, Microsoft issued an emergency security policy lockdown to restrict outdated rendering paths that attackers weaponized.

How the Exploit Worked

IE Mode relies on MSHTML, the Internet Explorer engine retained in Edge for backward compatibility.
Attackers leveraged CVE-2025-XXXXX (pending NVD publication) to force Edge to process untrusted content through IE rendering logic. This let them inject malicious JavaScript and ActiveX payloads into enterprise sessions.

Because IE Mode often runs with elevated trust for internal applications, the exploit bypassed SmartScreen and site-isolation controls. Once executed, attackers could harvest credentials, modify internal data, or execute PowerShell payloads through embedded scripts.

Microsoft observed exploitation primarily in Southeast Asia and financial-sector organizations that still rely on IE Mode for legacy ERP portals.

In a security bulletin issued October 12, 2025, Microsoft confirmed the issue and announced a lockdown update for IE Mode. The new release disables automatic compatibility fallback, blocking external or non-whitelisted domains from invoking the IE engine.

Additionally, administrators now receive enhanced group policy options to restrict IE Mode by hostname and enforce mandatory certificate validation for any page rendered with the IE engine.
Microsoft also plans to phase out IE Mode by 2026, accelerating its full deprecation timeline.

Exploitation in the Wild Targeted, Not Mass Campaign

While exploitation remains limited, security analysts from Mandiant and CERT-EU confirmed evidence of targeted intrusions. Attackers used weaponized intranet pages disguised as corporate logins to execute malicious JavaScript within trusted environments.

Forensic data showed that malicious actors used Active Directory Federation Services (ADFS) spoofing and single-sign-on manipulation to escalate privileges once the IE Mode bypass triggered.
No large-scale ransomware distribution was observed, but credential theft and persistence activity followed the same pattern as previous APT campaigns exploiting legacy components.

Enterprises using IE Mode should:

• Apply the latest Microsoft Edge Stable Channel update immediately.

• Restrict IE Mode domains through group policy (Enterprise Mode Site List).

• Disable ActiveX controls and JavaScript execution for legacy pages.

• Enforce strict certificate validation and sandbox policies.

• Monitor logs for unusual MSHTML or msedgewebview2 process calls.

Microsoft also recommends deploying Defender for Endpoint Threat Indicators that now detect IE Mode exploitation attempts in enterprise environments.

Security researchers praised Microsoft for acting quickly. According to analysts at Rapid7 and Eclypsium, IE Mode had long presented a “legacy loophole” in otherwise hardened enterprise browsers.
This update signals Microsoft’s acknowledgment that compatibility convenience can introduce unacceptable exposure. By tightening IE Mode, the company moves closer to fully eliminating Internet Explorer components that attackers continue to exploit nearly a decade after its official retirement.

Compatibility Should Never Compromise Security

The IE Mode exploit underscores the persistent risks of legacy technology in modern ecosystems.
Microsoft’s swift response sets a precedent for deprecating outdated components before attackers adapt.

For now, enterprises should treat IE Mode as a temporary bridge, not a permanent solution. By isolating, restricting, and auditing its usage, defenders can reduce exposure while maintaining business continuity.

FAQs

Q1. What is IE Mode in Microsoft Edge?
A compatibility feature allowing Edge to run legacy web applications using Internet Explorer’s MSHTML engine.

Q2. What happened recently?
Threat actors exploited a flaw in IE Mode to inject malicious code during targeted enterprise attacks.

Q3. Has Microsoft fixed it?
Yes. The October 2025 update restricts IE Mode behavior and adds administrative controls.

Q4. Who was affected?
Primarily enterprise users relying on IE Mode for internal web apps in finance and government sectors.

Q5. Should organizations disable IE Mode completely?
Yes, where possible. If legacy dependencies remain, restrict it to specific domains and enforce certificate validation.