A newly discovered zero-day vulnerability (CVE-2025-30397) in Microsoft's legacy MSHTML scripting engine is being actively exploited in the wild, putting millions of Windows users at risk of remote code execution. This critical flaw affects all supported Windows versions through Internet Explorer mode in Edge and applications using the WebBrowser control.

The Anatomy of CVE-2025-30397

The vulnerability stems from a type confusion bug in how the legacy MSHTML engine processes specially crafted web content. When exploited, it leads to memory corruption that attackers can leverage to execute arbitrary code with the same privileges as the current user. Microsoft has confirmed the vulnerability is being exploited in limited, targeted attacks before a patch was available.

Affected Systems and Attack Vectors

  • All Windows versions from Windows 10 20H2 through Windows 11 24H2
  • Internet Explorer mode in Microsoft Edge (despite IE's retirement)
  • Applications using WebBrowser control (common in enterprise software)
  • Office documents with embedded web content
  • Attack vectors: Malicious websites, phishing emails with Office attachments, compromised ads

Technical Deep Dive

Security researchers have identified that the exploit takes advantage of how MSHTML handles:
1. Legacy ActiveX controls
2. Certain DOM manipulation sequences
3. Memory allocation for script objects

The type confusion occurs when specially crafted JavaScript interacts with deprecated COM interfaces that weren't properly hardened when IE was discontinued. This creates an exploitable condition where attackers can manipulate memory pointers.

Mitigation Strategies (Pre-Patch)

Microsoft has released these temporary workarounds:

  1. Disable IE mode in Edge through Group Policy:
    Computer Configuration > Administrative Templates > Microsoft Edge > Enable IE Integration = Disabled
  2. Restrict ActiveX execution via registry:
    HKLM\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{CLSID}\Compatibility Flags = 0x400 (for known vulnerable controls)
  3. Enable Enhanced Security Configuration for Office applications
  4. Apply EMET-style mitigations through Defender Exploit Protection

Enterprise Impact and Response

For organizations, this vulnerability presents particular challenges:

  • Legacy web apps: Many enterprises still rely on IE-mode for internal portals
  • Patch latency: Large organizations often need weeks to test and deploy updates
  • Detection difficulty: Exploits leave minimal traces in standard logging

Recommended enterprise actions:

  1. Prioritize patching all public-facing systems
  2. Monitor for unusual mshtml.dll memory patterns
  3. Temporarily block Office files from untrusted sources
  4. Update web application firewalls to detect exploit patterns

The Bigger Picture: Legacy Code Risks

This incident highlights ongoing challenges with:

  • Backward compatibility vs security tradeoffs
  • The hidden attack surface of supposedly retired components
  • Enterprise dependency on deprecated technologies

Microsoft reports this is the 4th critical MSHTML vulnerability since IE's official retirement, suggesting systemic issues with maintaining legacy web engines.

Patch Timeline and Expectations

The fix is scheduled for May 2025 Patch Tuesday, but may arrive earlier as an out-of-band update if exploitation escalates. The patch is expected to:

  • Add proper type checking in the scripting engine
  • Remove several deprecated COM interfaces
  • Introduce memory randomization for MSHTML heaps

Long-Term Recommendations

  1. Accelerate migration from IE-dependent applications
  2. Implement application allowlisting to prevent unexpected MSHTML usage
  3. Monitor for post-patch regression testing - similar flaws often reoccur
  4. Review all software using WebBrowser controls - many third-party apps bundle vulnerable versions

Detection and Monitoring

Signs of potential exploitation:

  • Unexpected mshtml.dll memory usage patterns
  • Office applications spawning unusual child processes
  • Web requests to newly registered domains with Office file extensions

Free detection tools available:

  • Microsoft's MSHTML Exploit Detection Toolkit (available through Security Center)
  • Sysmon configurations for MSHTML anomaly detection
  • YARA rules for exploit pattern matching

The Future of Windows Legacy Components

This incident renews questions about:

  • How long Microsoft should maintain legacy web engines
  • Whether enterprises need better tools for legacy app modernization
  • If component retirement timelines should be accelerated despite business impact

As attackers increasingly target these legacy pathways, organizations must weigh compatibility needs against growing security risks.