CVE-2026-81947 in Excel
Summary
by MITRE • 09/09/2026
Heap-based buffer overflow in Microsoft Office Excel allows an unauthorized attacker to execute code locally.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability described constitutes a critical security flaw within the Microsoft Office Excel application, specifically manifesting as a heap-based buffer overflow. This type of memory corruption error occurs when a program writes more data to a dynamically allocated block of memory on the heap than it was intended to hold. In the context of Excel, this typically arises during the processing of complex or malformed spreadsheet files that contain specially crafted formulas, embedded objects, or specific formatting structures. The application fails to adequately validate the size of input data against the bounds of the pre-allocated buffer before performing write operations. This lack of rigorous boundary checking allows an attacker to overwrite adjacent memory locations on the heap with arbitrary data. Such overwrites can corrupt internal data structures used by Excel for managing file content, potentially leading to unpredictable application behavior or, more critically, providing a mechanism for code execution.
From a technical perspective, this vulnerability aligns closely with Common Weakness Enumeration (CWE) identifiers such as CWE-120 Buffer Copy without Checking Size of Input and CWE-787 Out-of-bounds Write on Heap. The exploitation vector relies heavily on the attacker's ability to control the content of files opened by the victim using Excel. By crafting a malicious file that triggers this overflow, an unauthorized actor can manipulate the heap layout in memory. Advanced exploitation techniques may involve overwriting function pointers or exception handling structures within the process space. If successful, these manipulations allow the attacker to redirect the execution flow of the application away from its normal operations and toward shellcode injected into the buffer itself. This results in arbitrary code execution with the privileges of the currently logged-in user, which is classified under ATT&CK technique T1203 Exploitation for Client Execution when targeting end-user applications like office suites.
The operational impact of this vulnerability is severe due to its potential for remote or local exploitation depending on how the malicious file reaches the target system. If a user opens an infected document, either through direct download, email attachment, or network share access, the application may crash immediately, causing denial of service. However, in more sophisticated attacks where stability is maintained during exploitation, the attacker gains full control over the compromised host. This can lead to data theft, installation of additional malware such as ransomware or spyware, and lateral movement within a corporate network if the user has elevated privileges. The risk is particularly acute for organizations that rely on Excel for critical business operations involving external data sources, as it undermines the integrity and confidentiality of sensitive information processed through these files.
Mitigation strategies must focus on both immediate remediation and long-term defensive posture improvements. Organizations should immediately apply the latest security patches released by Microsoft to update their Office installations, ensuring that specific fixes addressing heap validation logic are deployed across all endpoints. Administrators should also enforce strict email filtering policies to block attachments from untrusted sources and disable automatic macro execution where possible. User awareness training is essential to prevent social engineering attacks that trick employees into opening malicious files. Additionally, implementing application whitelisting or controlled folder access can restrict Excel's ability to execute arbitrary code even if the vulnerability is exploited. Regular auditing of software versions and prompt patch management are critical components of a robust defense-in-depth strategy against such memory corruption vulnerabilities.