CVE-2026-20956 is a vulnerability classified under CWE-822 (Untrusted Pointer Dereference) found in Microsoft 365 Apps for Enterprise, specifically in Microsoft Excel version 16.0.1. This flaw arises when Excel improperly handles pointers that reference memory locations without adequate validation, allowing an attacker to dereference untrusted pointers. Such dereferencing can lead to arbitrary code execution within the context of the current user. The vulnerability requires local access and user interaction, such as opening a malicious Excel file, but does not require any prior privileges, making it accessible to low-privileged attackers who can trick users into opening crafted documents. The CVSS v3.1 score is 7.8 (high), reflecting the significant impact on confidentiality, integrity, and availability, as successful exploitation can lead to full system compromise. The vulnerability is currently published but lacks an official patch or known exploits in the wild. The flaw's exploitation vector is local, meaning remote exploitation is not feasible without additional access. The vulnerability's root cause is improper validation of pointer references, which can cause the program to execute attacker-controlled code. This type of vulnerability is critical in environments where users frequently open Excel files from external or untrusted sources. Given Microsoft 365's widespread use in enterprises, this vulnerability poses a substantial risk if exploited.

For European organizations, the impact of CVE-2026-20956 is significant due to the widespread use of Microsoft 365 Apps for Enterprise across industries including finance, government, healthcare, and critical infrastructure. Successful exploitation can lead to unauthorized code execution, resulting in data breaches, disruption of business operations, and potential lateral movement within networks. Confidential information could be exfiltrated, integrity of data compromised, and availability of systems disrupted. The requirement for local access and user interaction somewhat limits the attack surface but does not eliminate risk, especially in environments with high user interaction with Excel files, such as finance and administration departments. The lack of a patch increases exposure time, necessitating immediate mitigation efforts. European organizations with remote or hybrid workforces may face additional challenges controlling local access and user behavior, increasing the likelihood of exploitation. The vulnerability could also be leveraged as part of multi-stage attacks targeting sensitive European entities, amplifying its impact.

1. Enforce strict local access controls to limit who can execute or open files on endpoints running Microsoft 365 Apps for Enterprise. 2. Implement application whitelisting and restrict execution of untrusted or unsigned macros and scripts within Excel. 3. Educate users on the risks of opening Excel files from unknown or untrusted sources, emphasizing phishing awareness. 4. Use endpoint detection and response (EDR) tools to monitor for suspicious activities related to Excel processes and pointer dereferencing anomalies. 5. Apply the principle of least privilege to user accounts to minimize the impact of local code execution. 6. Disable or restrict features in Excel that allow automatic execution of embedded content where feasible. 7. Prepare for rapid deployment of official patches from Microsoft once released, including testing and validation in controlled environments. 8. Employ network segmentation to contain potential lateral movement following exploitation. 9. Regularly back up critical data and verify recovery processes to mitigate availability impacts. 10. Monitor threat intelligence feeds for updates on exploit development or active campaigns targeting this vulnerability.