CVE-2014-1776 in Internet Explorer
Summary
by MITRE
Use-after-free vulnerability in Microsoft Internet Explorer 6 through 11 allows remote attackers to execute arbitrary code or cause a denial of service (memory corruption) via vectors related to the CMarkup::IsConnectedToPrimaryMarkup function, as exploited in the wild in April 2014. NOTE: this issue originally emphasized VGX.DLL, but Microsoft clarified that "VGX.DLL does not contain the vulnerable code leveraged in this exploit. Disabling VGX.DLL is an exploit-specific workaround that provides an immediate, effective workaround to help block known attacks."
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Analysis
by VulDB Data Team • 04/22/2026
This vulnerability represents a critical use-after-free flaw in Microsoft Internet Explorer spanning versions 6 through 11, demonstrating the persistent security challenges inherent in complex browser software architectures. The vulnerability specifically resides within the CMarkup::IsConnectedToPrimaryMarkup function, which operates within the browser's HTML parsing and rendering engine. This function handles markup connection logic that occurs during document processing, creating a scenario where freed memory objects can be accessed and manipulated by malicious code. The exploitation technique leverages memory corruption principles that allow attackers to execute arbitrary code with the privileges of the affected user, making it particularly dangerous in enterprise environments where users may have elevated system access. The vulnerability's exploitation in the wild during April 2014 highlighted the sophisticated nature of modern attack vectors targeting browser components.
The technical implementation of this vulnerability involves a classic use-after-free condition where the CMarkup::IsConnectedToPrimaryMarkup function fails to properly validate memory references after object deallocation. When Internet Explorer processes certain HTML elements that trigger this function, it can lead to memory that has been freed being accessed again, potentially allowing an attacker to control the execution flow of the application. This type of vulnerability falls under CWE-416, which specifically addresses use-after-free conditions, and aligns with the broader category of memory safety issues that plague modern software systems. The exploitation mechanism typically involves crafting malicious HTML content that, when rendered by the browser, triggers the vulnerable code path and subsequently corrupts memory structures to achieve code execution. The fact that this vulnerability affected multiple versions of Internet Explorer demonstrates how deeply embedded flaws can persist across product lifecycles, particularly in legacy software components.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to encompass full system compromise capabilities, making it a significant threat vector for attackers. Remote code execution vulnerabilities of this nature provide adversaries with the ability to install malware, steal sensitive data, or establish persistent access to compromised systems. The memory corruption aspect creates opportunities for attackers to manipulate heap structures, potentially leading to privilege escalation or information disclosure. The vulnerability's exploitation in the wild during April 2014 indicates that it was actively targeted by threat actors, suggesting that exploit code was readily available and effective against the affected browser versions. Organizations running these older browser versions faced heightened risk of successful exploitation, particularly in environments where users had administrative privileges or where the browser was used to access untrusted web content.
Microsoft's clarification regarding VGX.DLL highlights the complexity of vulnerability analysis and the importance of accurate technical assessment in security response. While the initial focus was on VGX.DLL as the vulnerable component, the actual flaw was identified within the core Internet Explorer rendering engine's markup processing functionality. This distinction is crucial for security professionals as it demonstrates how initial vulnerability assessments may require refinement and how exploit chains can involve multiple components. The recommended mitigation approach of disabling VGX.DLL served as a temporary workaround that provided immediate protection against known attack patterns, though it did not address the root cause of the vulnerability. Security teams should implement layered defense strategies that include browser updates, network segmentation, and user education to effectively address such vulnerabilities. The incident underscores the importance of maintaining up-to-date security patches and the need for organizations to have robust vulnerability management processes in place to respond quickly to emerging threats. This vulnerability also aligns with ATT&CK framework techniques related to exploitation of software vulnerabilities and privilege escalation, demonstrating how such flaws can be leveraged in comprehensive attack campaigns.