CVE-2025-66402 is a vulnerability classified under CWE-862 (Missing Authorization) found in the Misskey open-source federated social media platform. This flaw exists in versions starting from 13.0.0-beta.16 up to but excluding 2025.12.0. The vulnerability allows an unauthorized actor—one who lacks permission to view favorites or clips—to export posts and view their contents. This means that access control checks are improperly implemented or missing in the export functionality, enabling data exposure without proper authorization. The vulnerability can be exploited remotely over the network without requiring user interaction or elevated privileges, making it relatively easy to exploit. The impact is primarily on confidentiality, as unauthorized users can access potentially sensitive or private user-generated content. The issue was addressed and fixed in version 2025.12.0 of Misskey. There are no known exploits in the wild at the time of publication, but the high CVSS 4.0 score of 7.1 indicates a serious risk if left unpatched. The vulnerability affects the integrity of access control mechanisms, undermining trust in the platform's data privacy guarantees. Given Misskey's role as a federated social media platform, the exposure of favorites and clips can lead to privacy violations and potential reputational damage for organizations and users relying on the platform for secure communications.

For European organizations, this vulnerability poses a significant risk to the confidentiality of user data hosted on Misskey instances. Organizations using Misskey to manage internal or community social media interactions could experience unauthorized data disclosure, potentially exposing sensitive or private posts. This can lead to privacy violations, regulatory non-compliance (e.g., GDPR breaches), and reputational damage. The federated nature of Misskey means that a compromised instance could affect trust across interconnected nodes, amplifying the impact. Since the vulnerability requires no user interaction and can be exploited remotely, attackers can automate data extraction at scale. This is particularly concerning for organizations in sectors with strict data privacy requirements such as healthcare, education, and government. The lack of known exploits in the wild currently reduces immediate risk but does not eliminate the threat, especially as threat actors may develop exploits rapidly once the vulnerability is public. Failure to patch promptly could also expose organizations to targeted attacks leveraging this flaw for espionage or data harvesting.

The primary mitigation is to upgrade all affected Misskey instances to version 2025.12.0 or later, where the authorization checks have been properly implemented and the vulnerability fixed. Organizations should audit their current Misskey deployments to identify affected versions and prioritize patching. In addition to patching, administrators should review and tighten access control policies related to favorites, clips, and export functionalities to ensure only authorized users have access. Implement network-level protections such as IP whitelisting or VPN access for administrative interfaces to reduce exposure. Monitoring and logging export activities can help detect anomalous behavior indicative of exploitation attempts. Given the federated architecture, organizations should also verify the security posture of federated peers to limit risk propagation. Finally, educating users about the importance of updating software and reporting suspicious activity can enhance overall security posture.