CVE-2023-35628: Windows MSHTML Platform RCE Vulnerability

CVE-2023-35628 Overview

CVE-2023-35628 is a critical remote code execution vulnerability affecting the Windows MSHTML Platform, a core component used for rendering HTML content across Microsoft Windows operating systems. This Use After Free (CWE-416) vulnerability allows attackers to execute arbitrary code remotely on affected systems without requiring any user interaction or prior authentication.

The MSHTML platform (also known as Trident) is the legacy browser engine that powers Internet Explorer and is still used by numerous Windows applications for HTML rendering. Due to its deep integration into the Windows ecosystem, vulnerabilities in MSHTML can have widespread implications across enterprise and consumer environments.

Critical Impact

This vulnerability enables network-based remote code execution with no user interaction required, potentially allowing attackers to gain complete control of affected Windows systems.

Affected Products

• Microsoft Windows 10 (versions 1507, 1607, 1809, 21H2, 22H2)
• Microsoft Windows 11 (versions 21H2, 22H2, 23H2)
• Microsoft Windows Server 2008 R2 SP1, 2012, 2012 R2, 2016, 2019, 2022, and 2022 23H2

Discovery Timeline

• December 12, 2023 - CVE-2023-35628 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-35628

Vulnerability Analysis

This vulnerability stems from a Use After Free condition within the Windows MSHTML Platform. Use After Free vulnerabilities occur when a program continues to reference memory after it has been freed, leading to memory corruption that attackers can exploit to achieve code execution. In this case, the MSHTML rendering engine improperly handles memory during HTML content processing, creating an exploitable condition.

The attack can be initiated remotely over the network, though it requires specific conditions to be met for successful exploitation. No user credentials or authentication are needed, and notably, no user interaction is required—making this a particularly dangerous zero-click attack vector. The vulnerability affects confidentiality, integrity, and availability, meaning a successful exploit could allow attackers to read sensitive data, modify system content, and disrupt normal operations.

With an EPSS (Exploit Prediction Scoring System) score of 17.104% placing it in the 94.8th percentile, this vulnerability is predicted to have a significantly higher likelihood of exploitation compared to most other CVEs, indicating elevated real-world risk.

Root Cause

The root cause of CVE-2023-35628 is a Use After Free (CWE-416) memory corruption issue in the MSHTML component. During HTML parsing and rendering operations, the platform deallocates memory objects prematurely while retaining references to them. Subsequent operations then attempt to access or manipulate this freed memory, leading to undefined behavior that attackers can leverage to hijack program execution flow.

The MSHTML platform's complex memory management during DOM manipulation and rendering creates conditions where object lifetimes are not properly tracked, allowing dangling pointers to persist and be dereferenced.

Attack Vector

The vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker could craft a malicious HTML document or web content that triggers the Use After Free condition when processed by the MSHTML engine. Potential attack scenarios include:

• Delivering malicious content via email that triggers MSHTML rendering in the Outlook preview pane
• Hosting exploit code on a website that victims navigate to
• Embedding malicious HTML content in documents or applications that use MSHTML for rendering

The complexity of successfully exploiting this vulnerability is considered high, as it requires precise memory layout manipulation and timing. However, the zero-click nature and lack of authentication requirements make it an attractive target for sophisticated threat actors.

Technical details regarding specific exploitation techniques can be found in the Microsoft Security Update Guide for CVE-2023-35628.

Detection Methods for CVE-2023-35628

Indicators of Compromise

• Unexpected crashes or abnormal behavior in applications using MSHTML (mshtml.dll)
• Suspicious network connections originating from processes utilizing the MSHTML platform
• Anomalous memory access patterns or heap corruption events in Windows Event logs
• Unexpected child process spawning from Outlook, Internet Explorer, or other MSHTML-dependent applications

Detection Strategies

• Monitor for exploitation attempts by deploying behavioral detection rules targeting abnormal MSHTML process activity
• Implement memory protection monitoring to detect heap spray techniques and Use After Free exploitation patterns
• Deploy endpoint detection and response (EDR) solutions capable of identifying memory corruption attacks
• Enable Windows Defender Exploit Guard and configure Attack Surface Reduction rules for Office applications

Monitoring Recommendations

• Configure SIEM alerts for unusual process chains involving mshtml.dll or iexplore.exe
• Monitor for suspicious email attachments or links that may deliver MSHTML exploit payloads
• Track patch compliance across all Windows systems to identify vulnerable endpoints
• Implement network traffic analysis to detect potential exploit delivery mechanisms

How to Mitigate CVE-2023-35628

Immediate Actions Required

• Apply the December 2023 security updates from Microsoft immediately across all affected systems
• Prioritize patching internet-facing systems and email servers due to the network-based attack vector
• Restrict MSHTML access where possible through Group Policy or application controls
• Enable Windows Defender and ensure definitions are current

Patch Information

Microsoft has released security patches addressing this vulnerability as part of the December 2023 Patch Tuesday updates. Detailed patch information and download links are available in the Microsoft Security Update Guide for CVE-2023-35628.

Organizations should verify patch deployment across all supported Windows versions listed in the affected products section. For Windows Server 2008 R2 SP1 and Windows Server 2012/2012 R2, organizations may need Extended Security Update (ESU) subscriptions to receive patches.

Workarounds

• Disable the MSHTML component through registry modifications where application compatibility allows
• Block potentially malicious HTML content at the email gateway to prevent exploitation via email vectors
• Implement network segmentation to limit exposure of unpatched systems
• Configure Microsoft Office applications to disable the Outlook preview pane until patches are applied

# Disable MSHTML ActiveX controls via registry (administrator privileges required)
reg add "HKLM\SOFTWARE\Microsoft\Internet Explorer\ActiveX Compatibility\{25336920-03F9-11CF-8FD0-00AA00686F13}" /v "Compatibility Flags" /t REG_DWORD /d 0x00000400 /f