CVE-2025-59235 is an out-of-bounds read vulnerability classified under CWE-125 affecting Microsoft Office Online Server, specifically its Excel processing component. This flaw arises when the software improperly reads memory outside the bounds of allocated buffers during Excel file handling. An unauthorized attacker with local access and requiring user interaction can exploit this vulnerability to disclose sensitive information from memory, potentially exposing confidential data. The vulnerability does not require privileges but does require local access, limiting remote exploitation. The CVSS v3.1 score is 7.1, reflecting high severity due to the high impact on confidentiality and availability, ease of attack complexity, and no required privileges. The affected product version is 16.0.0.0, and as of the publication date, no patches have been released, nor are there known exploits in the wild. The vulnerability could also cause application instability or denial of service due to memory corruption. This flaw is significant because Office Online Server is widely used in enterprise environments to provide web-based Office functionality, meaning that sensitive corporate data processed through Excel files could be exposed if exploited. The vulnerability's requirement for local access and user interaction somewhat limits the attack surface but does not eliminate risk, especially in environments where multiple users share systems or where attackers can gain local footholds. The absence of patches necessitates immediate mitigation strategies to reduce exposure until official fixes are available.

For European organizations, the primary impact of CVE-2025-59235 is the potential unauthorized disclosure of sensitive information processed by Microsoft Office Online Server, particularly Excel files. This can lead to breaches of confidentiality, exposing intellectual property, personal data, or financial information. Additionally, the vulnerability may cause denial of service conditions, disrupting business operations reliant on Office Online Server. Sectors such as finance, government, healthcare, and critical infrastructure that heavily utilize Office Online Server for collaborative document editing and sharing are at heightened risk. The requirement for local access and user interaction reduces the likelihood of remote mass exploitation but does not prevent insider threats or attackers who have gained initial access through other means. The impact on availability could affect service continuity, leading to operational delays and potential regulatory non-compliance under GDPR if personal data is compromised. Overall, the vulnerability poses a significant risk to confidentiality and availability within European enterprises using the affected software version.

1. Monitor Microsoft’s official channels closely for the release of security patches addressing CVE-2025-59235 and apply them immediately upon availability. 2. Restrict local access to servers running Office Online Server to trusted personnel only, using strict access controls and network segmentation to limit exposure. 3. Implement application whitelisting and endpoint protection to detect and prevent exploitation attempts requiring user interaction. 4. Conduct regular audits of user activity and system logs to identify suspicious behavior indicative of exploitation attempts. 5. Educate users about the risks of interacting with untrusted Excel files or prompts within Office Online Server environments. 6. Consider deploying virtualized or sandboxed environments for processing untrusted Excel documents to contain potential exploitation. 7. Review and harden system configurations, disabling unnecessary services and limiting the attack surface on Office Online Server hosts. 8. Employ data loss prevention (DLP) solutions to monitor and control sensitive data flows that could be exposed through this vulnerability. These targeted measures go beyond generic patching advice by focusing on access control, monitoring, and user awareness to mitigate risk until patches are available.