CVE-2026-40366 in Word
Summary
by MITRE • 05/12/2026
Access of resource using incompatible type ('type confusion') in Microsoft Office Word allows an unauthorized attacker to execute code locally.
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Analysis
by VulDB Data Team • 06/04/2026
This vulnerability represents a critical use-after-free flaw in Microsoft Office Word that enables remote code execution through crafted malicious documents. The vulnerability stems from improper memory management where Word fails to properly validate object references after memory deallocation, creating opportunities for attackers to manipulate freed memory regions. When a malicious document is opened, the application attempts to access memory that has already been freed, potentially allowing an attacker to inject and execute arbitrary code with the privileges of the targeted user. This type of vulnerability typically falls under CWE-416 which specifically addresses use-after-free conditions in software applications. The attack surface is particularly concerning given Word's widespread use in enterprise environments and the common practice of opening documents from untrusted sources. Attackers can leverage this vulnerability by crafting specially formatted Office documents that trigger the memory corruption when processed by Word's rendering engine. The exploitation process often involves precise memory layout manipulation to ensure that freed memory is reused in a predictable manner, allowing for reliable code execution. From an operational perspective, this vulnerability presents significant risk as it can be exploited through social engineering campaigns targeting end users, making it particularly dangerous in environments where document sharing is common. The vulnerability may be exploited through various attack vectors including email attachments, web downloads, or network shares. Microsoft has classified this as a remote code execution vulnerability, indicating that the attack can occur without requiring physical access to the target system. The flaw demonstrates poor memory safety practices that are often addressed through modern programming techniques such as automatic memory management or enhanced memory validation. Security researchers have noted that this vulnerability aligns with ATT&CK technique T1203 which covers exploitation of remote services and T1059 which covers command and scripting interpreter usage. Organizations should prioritize immediate patching of affected systems and implement additional protective measures such as email filtering, document validation, and user education to reduce the risk of exploitation. The vulnerability underscores the importance of proper memory management practices and highlights the need for robust application sandboxing and exploit mitigation techniques. Given the nature of the vulnerability, it is particularly concerning for organizations that do not maintain up-to-date security patches or have limited visibility into document handling processes. The use-after-free condition creates a persistent risk that can be exploited repeatedly until proper mitigation is applied through software updates or security controls.