CVE-2010-3962

What is Internet Explorer and its CSS Engine?

Microsoft Internet Explorer (IE) was the dominant web browser through the mid-2000s, maintaining roughly 60% market share by 2010. IE's layout and rendering engine (Trident/MSHTML) handled HTML parsing, CSS styling, JavaScript execution, and DOM manipulation. The CSS implementation in IE 6-8 was particularly complex — supporting years of proprietary CSS extensions alongside evolving standards — making the CSS processing code an ongoing source of memory safety vulnerabilities. IE 8, the current version in 2010, lacked many of the modern exploit mitigations that would later appear in IE 9 and beyond, making CSS-related use-after-free vulnerabilities reliably exploitable.

Overview

CVE-2010-3962 is a high-severity use-after-free vulnerability (CWE-416, CVSS 8.1) in Microsoft Internet Explorer. When JavaScript modifies the DOM in specific ways that interact with the CSS layout engine, IE accesses a memory object after it has been freed, resulting in memory corruption. A specially crafted web page can exploit this condition to achieve remote code execution. This vulnerability was actively exploited as a zero-day before Microsoft issued the patch in Security Bulletin MS10-090 (December 2010). CISA added to KEV in October 2025, indicating confirmed exploitation of legacy IE installations more than 15 years after the original patch.

Affected Versions

Note: Internet Explorer has reached end-of-life. Microsoft retired IE 11 on June 15, 2022, and no supported version of IE exists. The impacted product should be discontinued.

Technical Details

The use-after-free vulnerability (CWE-416) exists in Internet Explorer's handling of certain CSS property interactions with dynamically modified DOM elements. When JavaScript modifies or removes DOM elements that have CSS properties applied, IE's CSS cascading and layout logic may retain a pointer to a DOM element or CSS object that has been freed from memory. When IE subsequently accesses this stale pointer to process a layout update, it reads from (and potentially writes to) freed memory.

The High Attack Complexity (AC:H) in the CVSS score reflects that reliable exploitation required bypassing IE's DEP (Data Execution Prevention) and ASLR memory protections on Vista and Windows 7 — requiring heap spray or ROP chain techniques to achieve reliable code execution. On Windows XP (without ASLR), exploitation was significantly more straightforward.

A public proof-of-concept exploit was released in late October 2010, demonstrating code execution on IE 6 and IE 7 on Windows XP. The approximately 47-day window between zero-day discovery and the December Patch Tuesday patch gave attackers substantial time to weaponize the exploit and target unpatched systems.

Discovery

Reported by security researchers who identified active exploitation in the wild. Microsoft issued Security Advisory 2458511 on October 28, 2010, acknowledging the zero-day and providing a workaround (setting Internet Zone security to High, and deploying Enhanced Mitigation Experience Toolkit). The patch was held for the December 2010 Patch Tuesday cycle rather than receiving an emergency out-of-band release, a decision that extended the exposure window.

Exploitation Context

Internet Explorer zero-days in 2010 were among the most valuable offensive tools:

• Zero-day targeted attacks: The pre-patch exploitation suggests sophisticated actors — IE 8 on Windows XP and Vista was the dominant browser configuration in 2010, making a reliable IE zero-day accessible to virtually any corporate user's workstation.
• Rapid criminal adoption: After the public proof-of-concept release, the exploit was incorporated into criminal exploit kits. Drive-by downloads from compromised websites silently exploited IE users who had not applied the patch.
• Windows XP targeting: Windows XP's lack of ASLR made exploitation more reliable than on Vista or Windows 7. The large installed base of XP systems — particularly in corporate environments — provided abundant targets.
• 47-day exposure window: The gap between zero-day disclosure (October 28) and patch release (December 14) left organizations relying on Microsoft's standard Patch Tuesday cycle exposed for six weeks.
• 2025 KEV addition context: CISA's October 2025 KEV addition reflects continued exploitation of legacy IE installations — likely embedded in industrial control system HMIs, legacy enterprise applications requiring IE, or IT environments that have not completed browser migration. Internet Explorer's architectural presence in Windows (MSHTML/WebBrowser control) means IE vulnerabilities may affect non-browser applications that embed the IE rendering engine.

Remediation

1. Remove Internet Explorer: IE has been retired by Microsoft (June 2022). Uninstall or disable IE on all systems.
2. Migrate IE-dependent applications: Identify applications using IE-mode or the WebBrowser control and migrate them to use Edge (which includes IE mode for legacy compatibility but with modern security mitigations).
3. Apply MS10-090: For historical systems, MS10-090 (December 2010 Patch Tuesday) addressed this vulnerability.
4. Deploy Edge: Microsoft Edge replaces IE and provides modern security mitigations. Deploy Edge with Microsoft Defender SmartScreen and Enhanced Security Configuration.
5. Isolate legacy IE systems: Systems that cannot be updated (industrial HMIs, embedded kiosks) running IE should be isolated from direct internet access behind network controls.