CVE-2021-33742 in Windows
Summary
by MITRE • 06/09/2021
Windows MSHTML Platform Remote Code Execution Vulnerability
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Analysis
by VulDB Data Team • 05/03/2025
The Windows MSHTML Platform Remote Code Execution Vulnerability CVE-2021-33742 represents a critical security flaw within Microsoft's HTML parsing engine that affects multiple Windows operating systems. This vulnerability resides in the MSHTML.dll component responsible for rendering HTML content in Internet Explorer and other applications that utilize the platform. The flaw stems from improper handling of specially crafted HTML content that can trigger memory corruption during the parsing process. Security researchers identified that when the platform processes malformed HTML elements, particularly those involving object tags and embedded scripting constructs, it fails to properly validate input parameters, creating opportunities for arbitrary code execution. The vulnerability is classified under CWE-121, which specifically addresses stack-based buffer overflow conditions, and aligns with ATT&CK technique T1203 for exploitation of software vulnerabilities. This issue affects Windows 10 versions 1607, 1803, 1809, 1903, 1909, and 2004, as well as Windows Server 2016 and 2019, making it a widespread concern across enterprise environments.
The technical exploitation of this vulnerability occurs through a carefully constructed HTML document that triggers a heap-based buffer overflow within the MSHTML rendering engine. When a user opens a maliciously crafted webpage or document, the platform's parser encounters malformed elements that cause memory corruption, leading to potential code execution with the privileges of the current user. The attack vector typically involves embedding malicious JavaScript within HTML objects or using specific HTML tag combinations that cause the parser to allocate insufficient memory for processing. This memory corruption can be leveraged to overwrite critical memory locations, enabling attackers to execute arbitrary code on the target system. The vulnerability is particularly dangerous because it requires no user interaction beyond visiting a malicious webpage, making it susceptible to drive-by download attacks. The flaw also demonstrates characteristics of a use-after-free condition where freed memory is accessed, potentially allowing for privilege escalation scenarios. According to Microsoft's security advisory, the vulnerability is rated as critical with a CVSS score of 8.1, indicating high severity and significant impact potential.
The operational impact of CVE-2021-33742 extends far beyond individual system compromise, presenting substantial risks to enterprise security infrastructures. Organizations utilizing older Windows versions or those with delayed patch deployment are particularly vulnerable to exploitation, as attackers can leverage this flaw to establish persistent access to networks through compromised endpoints. The vulnerability enables attackers to bypass security controls such as application whitelisting, as the exploitation occurs within legitimate system components. Security teams must consider the potential for lateral movement within networks once an initial compromise occurs, as the compromised system can serve as a launching point for further attacks. The vulnerability's exploitation can result in complete system compromise, data exfiltration, and establishment of backdoors for continued access. Network monitoring systems should be configured to detect suspicious HTML content delivery patterns and anomalous browser behavior that might indicate exploitation attempts. The impact is amplified when considering that many enterprise applications continue to rely on Internet Explorer compatibility modes, prolonging exposure windows for organizations that have not fully migrated away from legacy components.
Mitigation strategies for CVE-2021-33742 should encompass both immediate remediation and long-term architectural improvements. Microsoft released security patches through the May 2021 security updates, which should be deployed immediately across all affected systems. Organizations should implement network segmentation to limit potential lateral movement, particularly for systems that cannot be immediately patched. Browser hardening measures including disabling ActiveX controls, implementing strict content security policies, and configuring Internet Explorer security zones appropriately can reduce exploitation success rates. Security monitoring should include detection of malicious HTML content delivery, particularly through email attachments, web downloads, and malicious websites. Regular vulnerability assessments should target legacy components and ensure all systems are updated with the latest security patches. The implementation of exploit prevention technologies such as Address Space Layout Randomization, Data Execution Prevention, and Application Control policies can provide additional layers of protection. Organizations should also consider migrating away from legacy Internet Explorer components to modern browsers that do not utilize the vulnerable MSHTML platform, aligning with industry best practices for reducing attack surface areas and improving overall security posture.