CVE-2025-59245 is a critical vulnerability in Microsoft SharePoint Online identified as CWE-502, which involves the deserialization of untrusted data. Deserialization vulnerabilities occur when an application deserializes data from untrusted sources without proper validation, allowing attackers to manipulate serialized objects to execute arbitrary code or escalate privileges. In this case, the vulnerability enables remote attackers to elevate their privileges within SharePoint Online, potentially gaining administrative control over the platform. The CVSS 3.1 base score of 9.8 reflects the vulnerability's ease of exploitation (network attack vector, no privileges or user interaction required) and its severe impact on confidentiality, integrity, and availability. Although no specific affected versions are listed, the vulnerability targets SharePoint Online, a cloud-based service widely used in enterprise environments. The vulnerability was reserved in September 2025 and published in November 2025, with no known exploits in the wild or patches available at the time of reporting. Given SharePoint Online's role in document management and collaboration, exploitation could lead to unauthorized data access, data manipulation, and disruption of business operations. The lack of authentication and user interaction requirements makes this vulnerability particularly dangerous, as attackers can remotely exploit it without needing valid credentials or user actions.

For European organizations, the impact of CVE-2025-59245 could be severe due to the widespread adoption of Microsoft 365 and SharePoint Online across public and private sectors. Successful exploitation could result in unauthorized access to sensitive corporate data, intellectual property theft, and disruption of critical collaboration workflows. This could affect confidentiality by exposing sensitive documents, integrity by allowing tampering with stored data, and availability by potentially disabling SharePoint services. Organizations in regulated industries such as finance, healthcare, and government could face compliance violations and reputational damage. The cloud-based nature of SharePoint Online means that a single successful attack could have cascading effects across multiple tenants and organizations. Additionally, the vulnerability could be leveraged as a foothold for further lateral movement within enterprise networks, increasing the risk of broader compromise.

Since no patches are currently available, European organizations should implement immediate compensating controls. These include restricting network access to SharePoint Online to trusted IP ranges and enforcing strict conditional access policies using Microsoft Azure AD to limit exposure. Organizations should enable and monitor detailed logging and alerting for anomalous activities within SharePoint Online. Employing application-layer firewalls or proxy solutions that can inspect and filter serialized data payloads may help mitigate exploitation attempts. Administrators should review and tighten permissions on SharePoint sites to follow the principle of least privilege. Regularly updating and applying security best practices for Microsoft 365 environments, including multi-factor authentication and endpoint protection, will reduce attack surface. Finally, organizations should closely monitor Microsoft security advisories for patches or updates and plan rapid deployment once available.