CVE-2025-30381 is a high-severity vulnerability identified in Microsoft Office Online Server version 1.0.0, specifically an out-of-bounds read flaw classified under CWE-125. This vulnerability arises from improper handling of memory bounds within the Microsoft Office Excel component of the Office Online Server. An attacker can exploit this flaw to read memory outside the intended buffer boundaries, which can lead to local code execution. The vulnerability does not require privileges (PR:N) but does require user interaction (UI:R), such as opening a maliciously crafted Excel document via the Office Online Server interface. The attack vector is local (AV:L), meaning the attacker must have local access to the system or be able to induce the user to interact with the malicious content through the online server. The vulnerability impacts confidentiality, integrity, and availability (all rated high), indicating that successful exploitation could allow an attacker to execute arbitrary code with potentially full control over the affected system. The CVSS 3.1 base score is 7.8, reflecting the significant risk posed by this vulnerability. Although no known exploits are currently reported in the wild, the presence of this vulnerability in a widely deployed Microsoft product used for collaborative document editing and sharing makes it a critical concern for organizations relying on Office Online Server for productivity and document management.

For European organizations, the impact of CVE-2025-30381 could be substantial. Office Online Server is often deployed in enterprise environments to enable browser-based access to Office documents, facilitating collaboration across departments and with external partners. Exploitation of this vulnerability could allow attackers to execute arbitrary code on servers hosting Office Online Server, potentially leading to data breaches, disruption of business operations, and lateral movement within corporate networks. Given the high confidentiality and integrity impact, sensitive corporate data, including financial records, intellectual property, and personal data protected under GDPR, could be exposed or manipulated. The availability impact also means that critical document services could be disrupted, affecting productivity. The requirement for user interaction implies that phishing or social engineering campaigns targeting employees to open malicious Excel files via the online server could be a likely attack vector. This elevates the risk for organizations with large user bases and extensive document sharing practices. Additionally, the local attack vector suggests that attackers with some level of network access or compromised user accounts could leverage this vulnerability to escalate privileges or establish persistence.

To mitigate CVE-2025-30381, European organizations should prioritize the following actions: 1) Apply patches or updates from Microsoft as soon as they become available, even though no patch links are currently provided, monitoring official Microsoft security advisories closely. 2) Implement strict access controls and network segmentation to limit local access to Office Online Server instances, reducing the attack surface for local exploits. 3) Employ robust email and web filtering solutions to detect and block malicious Excel files or links that could trigger user interaction with the vulnerability. 4) Educate users about the risks of opening unsolicited or suspicious documents, emphasizing caution with Excel files accessed through the online server. 5) Monitor logs and network traffic for unusual activity related to Office Online Server, including unexpected file uploads or execution patterns. 6) Consider deploying application whitelisting and endpoint detection and response (EDR) tools on servers hosting Office Online Server to detect and prevent exploitation attempts. 7) Review and harden configurations of Office Online Server to minimize unnecessary features or services that could be leveraged by attackers. These targeted measures go beyond generic advice by focusing on the specific attack vector and environment of the vulnerability.