CVE-2026-20946 is an out-of-bounds read vulnerability classified under CWE-125 affecting Microsoft Office Excel 2019, specifically version 19.0.0. This vulnerability arises when Excel improperly handles memory bounds during processing of certain data structures, leading to reading memory outside the intended buffer. Such out-of-bounds reads can cause unpredictable behavior, including memory corruption that attackers can leverage to execute arbitrary code locally. The vulnerability does not require any privileges or elevated permissions but does require user interaction, such as opening a malicious Excel file. The CVSS 3.1 score of 7.8 indicates a high severity, with attack vector local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), and user interaction required (UI:R). The impact on confidentiality, integrity, and availability is high, meaning an attacker can fully compromise the affected system under the context of the logged-in user. Currently, no known exploits are publicly available, and no patches have been released, increasing the risk window. The vulnerability was reserved in December 2025 and published in January 2026. Given Microsoft Office's widespread use, this vulnerability poses a significant threat to organizations relying on Excel for critical operations.

For European organizations, this vulnerability presents a significant risk due to the widespread use of Microsoft Office 2019 across enterprises, government agencies, and critical infrastructure sectors. Successful exploitation could lead to local code execution, allowing attackers to escalate privileges, deploy malware, or exfiltrate sensitive data. This is particularly concerning for sectors handling sensitive personal data under GDPR, financial data, or national security information. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments where users may open untrusted files or where endpoint security is weak. The absence of a patch increases the window of exposure, potentially inviting targeted attacks or insider threats. Disruption to availability and integrity of critical documents and systems could have cascading effects on business continuity and regulatory compliance.

Until an official patch is released, European organizations should implement the following mitigations: 1) Enforce strict endpoint security policies to limit local access to trusted users only. 2) Educate users on the risks of opening untrusted Excel files, especially from unknown sources. 3) Employ application whitelisting and sandboxing to restrict execution of unauthorized code. 4) Use Microsoft Office Protected View and disable macros or other active content where possible. 5) Monitor systems for unusual behavior indicative of exploitation attempts, such as unexpected process launches or memory anomalies. 6) Maintain up-to-date backups to enable recovery in case of compromise. 7) Prepare for rapid deployment of patches once Microsoft releases a fix. 8) Consider network segmentation to limit lateral movement from compromised endpoints. These targeted measures go beyond generic advice by focusing on local access control, user behavior, and proactive monitoring tailored to this vulnerability's characteristics.