CVE-2026-40362: Microsoft Excel Heap-based Buffer Overflow Vulnerability

CVE-2026-40362 is a heap-based buffer overflow vulnerability affecting Microsoft Office Excel. This vulnerability allows an attacker with local access to execute arbitrary code. An unauthenticated attacker could exploit this vulnerability by crafting a malicious Excel file. User interaction is required, as the user must open the specially crafted file. Successful exploitation could lead to arbitrary code execution in the context of the current user. Defenders should prioritize patching this vulnerability to prevent potential exploitation.

Attack Chain

1. Attacker crafts a malicious Excel file (.xls or .xlsx) designed to trigger the heap-based buffer overflow.
2. The attacker delivers the malicious file to the target user. This could be via a shared network drive, removable media, or social engineering.
3. The target user opens the malicious Excel file with Microsoft Office Excel.
4. Excel parses the malicious file, triggering the heap-based buffer overflow when processing a specific data structure within the file.
5. The overflow allows the attacker to overwrite adjacent memory regions on the heap, potentially gaining control of program execution.
6. The attacker leverages the memory corruption to inject and execute malicious code within the Excel process.
7. The attacker’s code executes with the privileges of the user who opened the file, allowing for local code execution.
8. The attacker performs malicious actions such as installing malware, exfiltrating data, or further compromising the system.

Impact

Successful exploitation of CVE-2026-40362 allows an attacker to execute arbitrary code on the victim’s machine. Due to the local nature of the attack, the impact is limited to the compromised system. An attacker can leverage this vulnerability to gain a foothold on the system, potentially leading to data theft, malware installation, or further lateral movement within the network, depending on the user’s privileges.

Recommendation

• Apply the security update provided by Microsoft to patch CVE-2026-40362 in Microsoft Office Excel.
• Deploy the Sigma rule “Detect Suspicious Excel File Execution” to identify potential exploitation attempts based on process creation events.
• Educate users about the risks of opening untrusted or unsolicited Excel files.
• Monitor process execution for unusual or unexpected child processes spawned by Excel, as detected by “Detect Suspicious Excel Spawning”.