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 buffers on the heap, which can be exploited by an unauthorized attacker to execute arbitrary code locally. The flaw is classified under CWE-122, indicating a classic heap-based buffer overflow scenario. The vulnerability affects version 1.0.0 of Office Online Server and was published on May 13, 2025. According to the CVSS 3.1 scoring system, it has a score of 7.8, categorizing it as a high-severity issue. The vector string (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C) indicates that the attack requires local access (AV:L), low attack complexity (AC:L), no privileges (PR:N), but user interaction (UI:R) is necessary. The scope remains unchanged (S:U), and the impact on confidentiality, integrity, and availability is high (C:H/I:H/A:H). No known exploits are currently in the wild, and no patches have been linked yet. The vulnerability allows an attacker who can trick a user into opening a malicious Excel file or interacting with a compromised Office Online Server instance to execute arbitrary code with the privileges of the user, potentially leading to full system compromise. This local code execution risk is significant because Office Online Server is often deployed in enterprise environments to provide web-based Office functionality, and exploitation could lead to lateral movement or data exfiltration within an organization.

For European organizations, the impact of CVE-2025-29979 could be substantial. Many enterprises and public sector institutions across Europe rely on Microsoft Office Online Server to enable collaborative document editing and sharing. Successful exploitation could lead to unauthorized code execution on servers or client machines, potentially compromising sensitive corporate or governmental data. The high confidentiality, integrity, and availability impacts mean attackers could steal, alter, or destroy critical information. Given the requirement for local access and user interaction, phishing or social engineering campaigns could be used to deliver malicious Excel files. This could be particularly damaging in sectors such as finance, healthcare, and government, where data sensitivity and regulatory compliance (e.g., GDPR) are paramount. Furthermore, disruption of Office Online Server services could impact business continuity and productivity. The lack of known exploits in the wild currently provides a window for mitigation, but the high severity score necessitates prompt action to prevent future attacks.

Organizations should implement a multi-layered mitigation strategy specific to this vulnerability. First, restrict local access to systems running Office Online Server by enforcing strict access controls and network segmentation to limit exposure. Employ application whitelisting to prevent execution of unauthorized code. Educate users about the risks of opening unsolicited or suspicious Excel files, emphasizing the importance of verifying file sources to reduce the likelihood of social engineering attacks. Monitor logs and network traffic for unusual activity indicative of exploitation attempts. Since no official patches are currently available, consider deploying virtual patching via web application firewalls (WAFs) or endpoint detection and response (EDR) solutions that can detect and block exploitation patterns. Regularly check for updates from Microsoft and apply patches immediately upon release. Additionally, conduct thorough security assessments and penetration testing focused on Office Online Server deployments to identify and remediate any related weaknesses.