CVE-2026-20955 is a vulnerability classified under CWE-822 (Untrusted Pointer Dereference) affecting Microsoft Office Online Server version 16.0.0.0. The issue arises from improper handling of pointers within Microsoft Office Excel components, leading to dereferencing of untrusted pointers. This flaw enables an unauthorized attacker to execute arbitrary code locally on the target system. The attack vector requires local access (AV:L), low attack complexity (AC:L), no privileges (PR:N), and user interaction (UI:R), with unchanged scope (S:U). The vulnerability impacts confidentiality, integrity, and availability at a high level (C:H/I:H/A:H). Although no public exploits are known, the vulnerability's nature allows an attacker to craft malicious Excel files that, when opened by a user, could trigger code execution. This could lead to full system compromise, data theft, or disruption of services hosted on Office Online Server. The vulnerability was reserved in December 2025 and published in January 2026, with no patches currently listed, indicating that mitigation relies on interim controls until official updates are released.

For European organizations, the impact of CVE-2026-20955 could be substantial. Microsoft Office Online Server is widely used in enterprises and public sector institutions for collaborative document editing and sharing. Successful exploitation could lead to local code execution on servers or client machines, resulting in unauthorized data access, data corruption, or service disruption. This is particularly critical for organizations handling sensitive or regulated data under GDPR and other compliance frameworks. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, especially in environments where users frequently open Excel files from external or untrusted sources. The vulnerability could be leveraged in targeted attacks against government agencies, financial institutions, and large enterprises, potentially leading to espionage, data breaches, or operational outages.

Given the absence of an official patch at this time, European organizations should implement several specific mitigations: 1) Enforce strict file validation and scanning policies for Excel files before opening, using advanced endpoint protection solutions capable of detecting malicious payloads. 2) Limit user privileges to prevent unauthorized local code execution and restrict the ability to open untrusted documents. 3) Employ application whitelisting and sandboxing for Office Online Server and client environments to contain potential exploitation. 4) Educate users about the risks of opening unsolicited or suspicious Excel files, emphasizing the need for caution with email attachments and shared documents. 5) Monitor logs and system behavior for unusual activity related to Office Online Server and Excel processes. 6) Prepare for rapid deployment of patches once Microsoft releases updates by maintaining an effective vulnerability management process. 7) Consider network segmentation to isolate Office Online Server from critical infrastructure to reduce lateral movement risk.