CVE-2026-20957 is an integer underflow vulnerability classified under CWE-191 affecting Microsoft Office Online Server's Excel component, version 16.0.0.0. The vulnerability arises from an integer wrap or wraparound condition, which can cause incorrect calculations or memory handling errors leading to potential code execution. An attacker with local access and the ability to induce user interaction can exploit this flaw to execute arbitrary code with the privileges of the user running the Office Online Server. The vulnerability does not require prior authentication or elevated privileges, increasing its risk profile. The CVSS 3.1 score of 7.8 indicates a high severity, with the vector showing local attack vector (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 rated high, meaning successful exploitation could lead to full system compromise. Although no public exploits are currently known, the vulnerability's nature suggests that once exploited, it could allow attackers to manipulate or disrupt Excel processing within Office Online Server, potentially affecting document integrity and server stability. The lack of available patches at the time of publication necessitates immediate attention to mitigation strategies. This vulnerability is particularly relevant for organizations deploying Office Online Server in environments where local user access is possible, such as internal networks or shared hosting environments.

For European organizations, the impact of CVE-2026-20957 could be significant, especially for enterprises relying on Microsoft Office Online Server for collaborative document editing and processing. Successful exploitation could lead to unauthorized code execution on servers handling sensitive documents, risking data confidentiality breaches and potential data manipulation. This could disrupt business operations, cause data loss, or lead to lateral movement within the network. Given the high integration of Microsoft products in European corporate and governmental environments, the vulnerability could affect critical infrastructure and services. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk in environments where multiple users have access to the server or where attackers have gained footholds through other means. The absence of known exploits currently provides a window for proactive defense, but the high severity score demands urgent mitigation to prevent future attacks.

1. Monitor Microsoft security advisories closely and apply official patches or updates as soon as they become available for Office Online Server version 16.0.0.0. 2. Restrict local access to Office Online Server machines strictly to trusted administrators and users to reduce the attack surface. 3. Implement strict user privilege management, ensuring that users do not have unnecessary permissions that could facilitate exploitation. 4. Employ application whitelisting and endpoint protection solutions to detect and block suspicious code execution attempts on servers. 5. Conduct regular security audits and vulnerability scans focusing on Office Online Server deployments. 6. Educate users about the risks of interacting with untrusted content or executing unknown files, as user interaction is required for exploitation. 7. Use network segmentation to isolate Office Online Server infrastructure from less trusted network zones. 8. Enable comprehensive logging and monitoring to detect anomalous behavior indicative of exploitation attempts.