Browser Security

Microsoft Edge Locks Down IE Mode After Exploitation; JavaScript Risks Persist

Nishant Sharma

— 3 min read

Microsoft Edge Hardens IE Mode Security After Exploitation

Vulnerability Overview

  • CVE ID(s): Undisclosed (leveraging an unknown IE vulnerability)
  • Severity: High (Functional impact: RCE/Backdoor)
  • Vulnerability Type: Undisclosed (used for malware delivery and persistence)
  • Affected Component: Internet Explorer Mode in Microsoft Edge
  • Affected Versions: Microsoft Edge versions prior to recent October 2025 updates
  • Platforms: Windows
  • Exploitation Status: Active exploitation observed in targeted attacks

Technical Details

Microsoft has reinforced the security of its Edge browser's Internet Explorer (IE) Mode in response to attacks observed in August 2025, where threat actors successfully leveraged an undisclosed Internet Explorer vulnerability. These attackers exploited the legacy rendering feature to install malware and establish persistence on targeted systems, effectively turning IE Mode into a backdoor. The exploitation chain allowed for initial access and subsequent command-and-control capabilities, bypassing traditional security measures.

To mitigate this threat, Microsoft deployed security updates that restrict key functionalities within IE Mode. Specifically, the ability to launch local files directly from the internet zone within IE Mode has been significantly curtailed. This measure aims to disrupt a common attack vector used by malicious actors who often trick users into opening seemingly benign files that then execute malicious code via the compromised IE Mode environment. This hardening is particularly crucial for enterprise environments that still rely on IE Mode for compatibility with legacy web applications.

Patch Information

  • Fixed Version: Addressed through the latest Microsoft Edge security updates (as of October 2025)
  • Release Date: Ongoing rollout; effective with recent browser updates
  • Rollout Status: Immediate availability via standard Edge update channels
  • Update Method: Automatic updates for Microsoft Edge should be enabled; users are advised to manually check for and apply pending updates.

The Pervasive Threat of Unmonitored Client-Side JavaScript

Overview

Beyond browser core vulnerabilities, the security landscape continues to be significantly impacted by the risks associated with unmonitored client-side JavaScript. As highlighted by security researchers, the pervasive nature of JavaScript on modern websites, particularly e-commerce platforms, presents a substantial attack surface. This threat becomes even more pronounced during high-traffic periods like the holiday shopping season, when malicious actors intensify efforts to exploit client-side vulnerabilities for data theft.

Attack Vectors & Techniques

The primary attack vectors involve the injection of malicious JavaScript code directly into websites. This can occur through compromised first-party infrastructure or, more commonly, via vulnerable or manipulated third-party scripts. These third-party dependencies, which include analytics tools, payment gateways, live chat widgets, and ad networks, often operate with significant privileges within the user's browser, making them attractive targets for attackers.

Common attack techniques include:

  • Magecart Attacks: Digital skimming campaigns that inject malicious code to steal credit card information directly from payment forms.
  • Formjacking: Similar to Magecart, this involves hijacking web forms to exfiltrate personally identifiable information (PII), login credentials, and other sensitive data entered by users.
  • Supply Chain Attacks: Compromising a legitimate third-party JavaScript provider to distribute malicious code to all websites using that provider's service.

Once injected, these scripts run silently in the user's browser, exfiltrating sensitive data to attacker-controlled servers without detection by server-side security solutions, which often lack visibility into client-side execution.

Implications & Mitigation

The implications of unmonitored JavaScript are severe, leading to data breaches, financial fraud, and reputational damage for affected organizations. For users, it means the risk of sensitive information being compromised during routine web interactions.

To counter these threats, organizations must implement comprehensive client-side security strategies:

  • Client-Side Security Solutions: Deploy specialized tools that provide real-time visibility and monitoring of all JavaScript executing in the browser, regardless of its origin.
  • Content Security Policy (CSP): Implement a robust CSP to define trusted sources for content and scripts, effectively blocking unauthorized script execution.
  • Subresource Integrity (SRI): Utilize SRI for critical third-party scripts to ensure that content delivered by external servers has not been tampered with.
  • Regular Audits: Conduct frequent security audits of all third-party JavaScript dependencies and their configurations.
  • Least Privilege: Ensure that third-party scripts operate with the minimum necessary permissions.

References

  1. Microsoft Locks Down IE Mode After Hackers Turned Legacy Feature Into Backdoor - The Hacker News
  2. Microsoft revamps Internet Explorer Mode in Edge after August attacks - Security Affairs
  3. Why Unmonitored JavaScript Is Your Biggest Holiday Security Risk - The Hacker News

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