CVE-2026-20957 is an integer underflow vulnerability classified under CWE-191 affecting Microsoft Office Online Server, specifically the Excel component in version 16.0.0.0. An integer underflow occurs when an arithmetic operation causes a value to wrap around below its minimum representable value, leading to unexpected behavior such as memory corruption. In this case, the vulnerability allows an unauthorized attacker to execute arbitrary code locally by exploiting the wraparound condition. The vulnerability does not require prior privileges (PR:N) but does require user interaction (UI:R), indicating that the attacker must trick a user into triggering the exploit locally. The CVSS 3.1 base score is 7.8, reflecting high impact on confidentiality, integrity, and availability (C:H/I:H/A:H), with low attack complexity (AC:L) and local attack vector (AV:L). The vulnerability was reserved in December 2025 and published in January 2026, with no known exploits in the wild yet. The lack of patch links suggests that a fix may be forthcoming or pending deployment. The vulnerability could be leveraged to execute arbitrary code, potentially allowing an attacker to gain control over the affected system or escalate privileges. Given the local attack vector, the threat is primarily to users with access to the vulnerable Office Online Server environment, emphasizing the need for strict access controls and user awareness. The vulnerability's presence in a widely deployed enterprise product like Microsoft Office Online Server makes it a significant concern for organizations relying on this platform for document collaboration and processing.

For European organizations, this vulnerability poses a significant risk to the confidentiality, integrity, and availability of sensitive documents and internal systems. Successful exploitation could lead to local code execution, enabling attackers to install malware, exfiltrate data, or disrupt services. Organizations using Office Online Server for collaborative document editing and processing could face operational disruptions and data breaches. The local attack vector means insider threats or compromised user accounts could be leveraged to exploit this vulnerability. Given the high adoption rate of Microsoft Office products across Europe, especially in government, finance, healthcare, and critical infrastructure sectors, the impact could be widespread. Additionally, the potential for privilege escalation and lateral movement within networks increases the risk of broader compromise. The absence of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, as attackers may develop exploits once patches are released. Failure to address this vulnerability promptly could result in regulatory non-compliance under GDPR due to potential data breaches.

1. Monitor Microsoft’s official channels closely for the release of security patches addressing CVE-2026-20957 and apply them immediately upon availability. 2. Restrict local access to Office Online Server environments to trusted personnel only, minimizing the attack surface. 3. Implement strict user privilege management to ensure users operate with the least privileges necessary, reducing the impact of potential exploitation. 4. Employ application whitelisting and endpoint protection solutions capable of detecting anomalous behavior indicative of exploitation attempts. 5. Conduct user awareness training to prevent social engineering attempts that could trigger the required user interaction for exploitation. 6. Regularly audit and monitor logs for unusual activity related to Office Online Server, focusing on local execution attempts and memory corruption indicators. 7. Consider network segmentation to isolate Office Online Server from critical systems, limiting lateral movement opportunities. 8. Utilize vulnerability scanning tools to identify instances of the vulnerable Office Online Server version 16.0.0.0 within the environment. 9. Prepare incident response plans specifically addressing local code execution scenarios in Office Online Server contexts.