CVE-2025-29979 is a heap-based buffer overflow vulnerability identified in Microsoft Office Excel, part of the Microsoft 365 Apps for Enterprise suite, specifically version 16.0.1. The vulnerability arises from improper handling of memory buffers during Excel file processing, which can lead to memory corruption. An attacker can exploit this flaw by crafting a malicious Excel file that, when opened by a user, triggers the overflow and allows execution of arbitrary code with the privileges of the user. This vulnerability does not require any prior authentication or elevated privileges but does require user interaction, such as opening a malicious document. The CVSS 3.1 base score is 7.8, indicating high severity, with attack vector local (AV:L), low attack complexity (AC:L), no privileges required (PR:N), and user interaction required (UI:R). The impact covers confidentiality, integrity, and availability, meaning an attacker could potentially steal sensitive data, modify or corrupt files, or disrupt system operations. Although no known exploits have been reported in the wild yet, the vulnerability is publicly disclosed and could be targeted by threat actors. The lack of an available patch at the time of disclosure increases the urgency for organizations to implement interim mitigations. Given Microsoft 365's extensive deployment in enterprise environments, this vulnerability poses a significant risk to organizations relying on Excel for business-critical operations.

For European organizations, the impact of CVE-2025-29979 is substantial due to the widespread use of Microsoft 365 Apps for Enterprise across public and private sectors. Successful exploitation could lead to unauthorized code execution on user machines, potentially enabling lateral movement within networks, data exfiltration, or disruption of business processes. Confidentiality breaches could expose sensitive corporate or personal data, violating GDPR requirements and leading to regulatory penalties. Integrity compromises could corrupt financial or operational data, affecting decision-making and operational continuity. Availability impacts could result in denial of service or system instability, disrupting productivity. The requirement for user interaction means phishing or social engineering campaigns could be leveraged to deliver malicious Excel files, increasing the attack surface. The absence of known exploits currently provides a window for proactive defense, but the high severity score and potential for rapid weaponization necessitate urgent attention. Organizations in sectors such as finance, government, healthcare, and critical infrastructure are particularly at risk due to the sensitivity of their data and the critical nature of their operations.

1. Monitor Microsoft security advisories closely and apply official patches immediately upon release to remediate the vulnerability. 2. Until patches are available, implement application whitelisting and restrict execution of untrusted Excel macros or files. 3. Employ email filtering and attachment sandboxing to detect and block malicious Excel files before reaching end users. 4. Educate users on the risks of opening unsolicited or unexpected Excel documents, emphasizing caution with email attachments and links. 5. Use endpoint detection and response (EDR) tools to monitor for suspicious behaviors indicative of exploitation attempts. 6. Configure Microsoft Defender for Office 365 and other security solutions to scan and block malicious content. 7. Limit user privileges to the minimum necessary to reduce the impact of potential code execution. 8. Consider disabling legacy or unnecessary Excel features that could be exploited. 9. Maintain regular backups of critical data to enable recovery in case of compromise. 10. Conduct simulated phishing exercises to raise awareness and test user readiness against social engineering attacks.