Heap-based Buffer Overflow in Microsoft Office Excel

Executive Summary

CVE-2026-55947 is a heap-based buffer overflow vulnerability in Microsoft Office Excel. This flaw allows an unauthorized attacker to execute arbitrary code on a victim's system.

A heap-based buffer overflow occurs when a program writes more data to a buffer than it can hold, corrupting or overwriting adjacent memory. In this case, the vulnerability exists in Excel, and exploitation could lead to local code execution.

The CVSS v3.1 score for this vulnerability is 7.8, indicating a high severity. The vector AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H means the attack requires local access, low attack complexity, no privileges, user interaction, and has high impacts on confidentiality, integrity, and availability.

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Detection Guidance

The provided context does not include specific detection methods or commands for identifying the presence of CVE-2026-55947 on a network or system. Detection typically involves checking for vulnerable versions of Microsoft Office Excel or using security tools that can identify heap-based buffer overflow vulnerabilities in Excel files.

For accurate detection, refer to Microsoft's official guidance or use their security tools, such as Microsoft Defender for Office 365 or the Microsoft Safety Scanner, which may include checks for this vulnerability.

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Impact Analysis

If exploited, this vulnerability could have severe consequences for affected users.

• An attacker could execute malicious code on your system, potentially gaining control over it.
• Sensitive data stored or processed in Excel files could be accessed, modified, or stolen.
• The attacker could install malware, create backdoors, or perform other malicious actions on your device.

Since the vulnerability requires user interaction (e.g., opening a malicious Excel file), the risk increases if you frequently handle untrusted Excel documents.

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Compliance Impact

This vulnerability could impact compliance with several standards and regulations, depending on the context of its exploitation.

• GDPR: If the vulnerability leads to unauthorized access or exfiltration of personal data, it could result in a data breach. Organizations may face fines or penalties for failing to protect personal data adequately.
• HIPAA: For organizations handling protected health information (PHI), exploitation of this vulnerability could lead to unauthorized access to PHI, violating HIPAA's security and privacy rules.
• Other standards like PCI DSS (for payment data) or industry-specific regulations may also be affected if sensitive data is compromised due to this vulnerability.

Organizations should assess their exposure to this vulnerability and apply patches or mitigations to maintain compliance with relevant regulations.

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Mitigation Strategies

To mitigate CVE-2026-55947, follow these immediate steps:

• Apply the latest security updates from Microsoft for Microsoft Office Excel. Refer to the Microsoft Update Guide for the specific patch related to this CVE.
• Avoid opening Excel files from untrusted or unknown sources, as this vulnerability can be exploited through malicious files.
• Use Microsoft Defender for Office 365 or other security solutions to scan for and block malicious Excel files.
• Enable attack surface reduction rules in Microsoft Defender to prevent Office apps from creating child processes, which can help mitigate exploitation attempts.
• Monitor Microsoft's official communications for additional guidance or workarounds if a patch is not immediately available.

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