CVE-2025-59245 is a critical vulnerability in Microsoft SharePoint Online involving CWE-502: Deserialization of Untrusted Data. This vulnerability arises when SharePoint Online improperly processes serialized data from untrusted sources, allowing attackers to manipulate the deserialization process to execute arbitrary code. The flaw leads to an elevation of privilege, enabling attackers to gain higher access rights than intended, potentially full control over the SharePoint environment. The CVSS 3.1 base score of 9.8 reflects its critical nature, with attack vector being network-based (AV:N), no privileges required (PR:N), and no user interaction needed (UI:N). The impact affects confidentiality, integrity, and availability (C:H/I:H/A:H), meaning attackers can steal sensitive data, alter or delete content, and disrupt services. The vulnerability is exploitable remotely without authentication, increasing the risk profile. Although no public exploits are currently known, the vulnerability's presence in a widely used cloud collaboration platform makes it a high-value target for threat actors. The lack of available patches at the time of publication necessitates urgent mitigation planning. This vulnerability could be leveraged in targeted attacks against organizations relying on SharePoint Online for document management and collaboration, potentially leading to data breaches, ransomware deployment, or lateral movement within corporate networks.

For European organizations, the impact of CVE-2025-59245 is substantial due to the widespread use of Microsoft SharePoint Online in enterprises, government agencies, and critical infrastructure sectors. Exploitation could lead to unauthorized access to sensitive corporate and personal data, violating GDPR and other data protection regulations, resulting in legal and financial penalties. The elevation of privilege could allow attackers to manipulate or delete critical documents, disrupt business operations, and compromise internal communications. Given SharePoint Online's integration with other Microsoft 365 services, the vulnerability could serve as a pivot point for broader network compromise. This risk is particularly acute for sectors such as finance, healthcare, energy, and public administration, where data confidentiality and service availability are paramount. The potential for widespread disruption and data loss could undermine trust in cloud services and necessitate costly incident response and remediation efforts.

Since no patches are currently available, European organizations should implement immediate compensating controls. These include restricting access to SharePoint Online to trusted networks and users via conditional access policies, enabling multi-factor authentication to reduce account compromise risk, and monitoring logs for unusual deserialization or privilege escalation activities. Administrators should review and limit permissions to the minimum necessary, especially for service accounts and users with elevated privileges. Employing application-layer firewalls or proxy solutions that can detect and block malicious serialized payloads may provide additional protection. Organizations should also prepare for rapid deployment of patches once Microsoft releases them and conduct thorough testing before applying updates. Regular security awareness training focused on cloud security and incident response readiness will help mitigate potential exploitation impacts. Collaboration with Microsoft support and threat intelligence sharing within European cybersecurity communities can enhance detection and response capabilities.