CVE-2025-29979 is a heap-based buffer overflow vulnerability identified in Microsoft Office Online Server, specifically affecting the Excel component. This vulnerability arises due to improper handling of memory allocation on the heap, which can be exploited when processing specially crafted Excel files. An unauthorized attacker can leverage this flaw to execute arbitrary code locally on the affected system. The vulnerability requires local access (Attack Vector: Local) and does not require privileges (Privileges Required: None), but it does require user interaction (UI: Required), such as opening or interacting with a malicious Excel file through Office Online Server. The vulnerability impacts confidentiality, integrity, and availability, as successful exploitation allows full code execution, potentially leading to data theft, system compromise, or denial of service. The CVSS v3.1 base score is 7.8 (High), reflecting the significant risk posed by this flaw. Although no known exploits are currently reported in the wild, the presence of this vulnerability in a widely used Microsoft product necessitates prompt attention. The affected version is Office Online Server 1.0.0, indicating that this is a newly discovered issue in the initial release or early versions of the product. The vulnerability is classified under CWE-122, which corresponds to heap-based buffer overflows, a common and dangerous class of memory corruption bugs that can lead to arbitrary code execution.

For European organizations, the impact of CVE-2025-29979 can be substantial, especially for enterprises and public sector entities relying on Microsoft Office Online Server for collaborative document editing and sharing. Exploitation could allow attackers to execute arbitrary code on servers hosting Office Online Server, potentially leading to unauthorized access to sensitive documents, disruption of business operations, and lateral movement within internal networks. Given the high confidentiality, integrity, and availability impact, organizations could face data breaches, intellectual property theft, and operational downtime. This is particularly critical for sectors such as finance, government, healthcare, and critical infrastructure, where document confidentiality and service availability are paramount. The requirement for local access and user interaction somewhat limits remote exploitation but does not eliminate risk, as attackers may use social engineering or phishing to trick users into opening malicious files. Additionally, Office Online Server often integrates with broader Microsoft ecosystems, so compromise could have cascading effects on other services and data.

To mitigate this vulnerability, European organizations should prioritize the following actions: 1) Apply patches or updates from Microsoft as soon as they become available, as no patch links are currently provided but are expected given the vulnerability's severity. 2) Implement strict access controls and monitoring on servers running Office Online Server to limit local access only to trusted administrators and users. 3) Educate users about the risks of opening untrusted or unexpected Excel files, especially those received via email or external sources, to reduce the likelihood of user interaction exploitation. 4) Employ application whitelisting and endpoint protection solutions capable of detecting and blocking exploitation attempts targeting heap-based buffer overflows. 5) Monitor logs and network traffic for unusual activity indicative of exploitation attempts or post-exploitation behavior. 6) Consider isolating Office Online Server environments from critical infrastructure to contain potential breaches. 7) Regularly review and update incident response plans to include scenarios involving Office Online Server compromise.