Chakra Engine Zero-Day Enables RCE via IE Mode in Microsoft Edge

In August 2025, Microsoft’s Edge security team identified a serious exploitation campaign targeting Internet Explorer (IE) mode within Microsoft Edge. This campaign highlighted how legacy compatibility features, when combined with modern browsers, can inadvertently reopen old attack surfaces. To mitigate this, Microsoft has introduced restrictive changes to how IE mode can be accessed, prioritizing security over convenience while maintaining limited enterprise compatibility.

Severity: Moderate

Threat Details

1. Initial Access: Social engineering (prompting users to reload in IE mode).
2. Vulnerability Type: 0-day in Chakra JavaScript engine (IE’s scripting engine).
3. Execution Chain:
   • User visits spoofed site.
   • Page requests reload in IE mode.
   • Chakra exploit triggers RCE.
   • Secondary exploit escalates privileges.
4. Affected Environment: Systems using Microsoft Edge with IE mode enabled for legacy sites or enterprise apps.
5. Exploit Complexity: Moderate relies on user interaction but uses 0-days for execution.
6. Potential Impact
   If exploited, this attack could:
   • Allow attackers to install malware or create backdoors.
   • Enable credential theft and data exfiltration.
   • Facilitate lateral movement across corporate networks.
   • Expose organizations to regulatory and reputational risk.

Recommendations

1. Disable Internet Explorer Mode entirely for all users unless absolutely necessary for legacy business applications.
2. For organizations that must use it, enforce strict site whitelisting under Edge Settings → Default Browser → Internet Explorer Mode Pages List.
3. Ensure all endpoints are running the latest version of Microsoft Edge with October 2025 security updates applied.
4. Conduct targeted awareness campaigns explaining:
   • The risks of reloading pages in IE mode.
   • How social engineering prompts can appear legitimate but trigger malicious actions.
5. Microsoft strongly advises users to migrate away from legacy web technologies as soon as possible to benefit from enhanced security, greater reliability, and improved performance offered by modern browsers.

Source:

• https://microsoftedge.github.io/edgevr/posts/Changes-to-Internet-Explorer-Mode-in-Microsoft-Edge/

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