A Disturbing New Attack: Microsoft Edge’s IE Mode Exploited by Threat Actors

The cybersecurity landscape has documented a concerning escalation: threat actors have discovered and actively leveraged a novel attack vector targeting Microsoft Edge’s Internet Explorer mode functionality. This sophisticated campaign, first identified in August 2025, exploits inherent security weaknesses of legacy browser technology, enabling unauthorized access to unsuspecting users’ devices. This development signals a significant evolution in threat actor tactics, demonstrating their persistent innovation in finding and exploiting vulnerabilities within modern systems.

Understanding the Attack Vector: IE Mode’s Double-Edged Sword

Microsoft Edge’s Internet Explorer (IE) mode was designed to provide backward compatibility for organizations that rely on older web applications and internal tools built for Internet Explorer. While intended to facilitate a smoother transition away from a deprecated browser, this feature inadvertently reintroduces legacy security risks. Threat actors are exploiting this bridging mechanism, essentially using Edge as a conduit to bypass modern security controls and tap into the vulnerabilities associated with the older IE rendering engine.

The core of the attack lies in malicious actors crafting specific web content that, when rendered within Edge’s IE mode, can execute arbitrary code or exfiltrate sensitive data. This isn’t a direct vulnerability in Microsoft Edge itself, but rather an exploitation of the compatibility layer that allows older, less secure browser functionalities to operate within a contemporary browser environment.

The Evolution of Threat Actor Tactics

This campaign underscores a critical shift in how adversaries operate. Instead of focusing solely on zero-day vulnerabilities in current browsers, they are now adept at identifying and weaponizing features designed for backward compatibility. This approach is particularly insidious because it targets a seemingly benign functionality, frequently used in enterprise environments, making detection and mitigation more challenging. The August 2025 discovery highlights the continuous cat-and-mouse game between security researchers and malicious actors, where even legacy features can become potent weapons.

Potential Impact and Risks

The exploitation of IE mode can lead to several severe consequences:

• Data Exfiltration: Threat actors can access and steal sensitive data, including credentials, financial information, and proprietary business intelligence.
• Malware Injection: Compromised devices can be infected with various forms of malware, including ransomware, keyloggers, and spyware.
• Privilege Escalation: Successful exploitation could allow attackers to gain higher levels of access on a compromised system.
• Network Penetration: An infected device can serve as a beachhead for further attacks within a corporate network.

Remediation Actions and Best Practices

Addressing this specific threat requires a multi-faceted approach focusing on configuration, user education, and proactive monitoring.

• Limit IE Mode Usage: Enterprises should review their need for IE mode. If possible, restrict its use to only essential, verified internal applications. Utilize group policies to control which sites can open in IE mode.
• Patch Management: While the core issue isn’t a direct Edge vulnerability, maintaining all software, including the operating system and Edge browser, up to date with the latest security patches is always crucial.
• Enhanced Security Configurations: Implement robust security settings within Edge and across the operating system (e.g., Application Guard for Edge, Exploit Protection).
• Endpoint Detection and Response (EDR): Leverage EDR solutions to monitor for suspicious activity on endpoints, which can help detect and respond to exploitation attempts.
• User Awareness Training: Educate users about the risks of clicking on unfamiliar links, especially those that might trigger legacy browser modes.
• Network Segmentation: Isolate systems that rely heavily on legacy applications and IE mode to minimize potential lateral movement in case of a breach.

Tools for Detection and Mitigation

Implementing various tools can significantly enhance an organization’s ability to defend against such attacks.

Tool Name	Purpose	Link
Microsoft Defender for Endpoint	Endpoint Detection and Response (EDR) for threat detection and response.	Learn more
Group Policy Management Console	Configuring and enforcing IE mode policies across an organization.	Learn more
Browser Guard Solutions (e.g., Menlo Security)	Isolating web browsing sessions to prevent malware execution on endpoints.	Menlo Security
Network Intrusion Detection/Prevention Systems (NIDS/NIPS)	Monitoring network traffic for indicators of compromise (IoCs) and blocking malicious activity.	N/A (Vendor-specific)

Key Takeaways for a Secure Future

The exploitation of Microsoft Edge’s Internet Explorer mode serves as a stark reminder that legacy compatibility features can be double-edged swords, inadvertently reopening doors to older vulnerabilities. Organizations must continuously assess their reliance on such features, prioritize strict configuration management, enforce robust security policies, and commit to ongoing user education. Proactive threat intelligence and a defense-in-depth strategy are paramount in navigating an ever-evolving threat landscape where malicious actors are increasingly sophisticated in their attack vectors.