CVE-2026-40364: Microsoft Office Word Type Confusion Vulnerability

CVE-2026-40364 is a type confusion vulnerability in Microsoft Office Word that can lead to arbitrary code execution. An attacker could potentially exploit this vulnerability to execute code locally with the privileges of the current user. The vulnerability arises from improper handling of object types within Word, leading to memory corruption when processing specially crafted documents. While the specifics of exploitation are not detailed in the advisory, the high CVSS score and potential for local code execution make this a significant threat for systems running affected versions of Microsoft Office Word. Defenders should prioritize patching and consider implementing proactive detection measures to identify potential exploitation attempts.

Attack Chain

1. An attacker crafts a malicious Word document containing a payload designed to trigger the type confusion vulnerability (CVE-2026-40364).
2. The attacker delivers the crafted document to a target user via email, shared drive, or other means.
3. The user opens the malicious document in Microsoft Office Word.
4. Word attempts to process the document, triggering the type confusion vulnerability due to the incompatible object types.
5. The type confusion error leads to memory corruption within the Word process.
6. The attacker’s payload leverages the memory corruption to overwrite critical data structures.
7. The overwritten data structures are manipulated to redirect execution flow to attacker-controlled code.
8. The attacker achieves local code execution with the privileges of the user, potentially leading to further malicious actions.

Impact

Successful exploitation of CVE-2026-40364 allows an attacker to execute arbitrary code locally on a vulnerable system. The impact is significant, potentially allowing an attacker to install malware, steal sensitive data, or perform other malicious actions with the privileges of the logged-on user. Given the widespread use of Microsoft Office Word, this vulnerability poses a substantial risk to a large number of users and organizations.

Recommendation

• Apply the security update released by Microsoft to patch CVE-2026-40364 as soon as possible (reference: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-40364).
• Deploy the Sigma rule “Detect Suspicious Process Creation from Winword.exe” to identify potential exploitation attempts.
• Enable and review Microsoft Office’s Protected View settings to mitigate the risk of malicious documents.
• Educate users about the risks of opening unsolicited or suspicious documents.