The vulnerability identified as CVE-2025-64766 pertains to the NixOS module for the OnlyOffice document server distributed via nixpkgs. In affected versions (22.11 up to 25.05 and unstable versions before 25.11), a hard-coded secret was embedded in the software to protect the file cache. This secret acts as a static credential that does not change per deployment or user, violating secure credential management best practices (CWE-798). An attacker who can obtain a valid revision ID of a document stored in the OnlyOffice server's cache can leverage this hard-coded secret to retrieve the document contents without proper authorization. The difficulty lies in acquiring a valid revision ID, which is not trivially guessable or publicly exposed under normal circumstances. The vulnerability impacts confidentiality by potentially exposing documents that should no longer be accessible, such as those from users with expired access rights. There is no impact on data integrity or system availability, and exploitation does not require authentication or user interaction, making it remotely exploitable over the network. The issue was addressed by removing or replacing the hard-coded secret in NixOS unstable 25.11 and stable 25.05 releases. No public exploits have been reported, but the presence of hard-coded credentials represents a significant security weakness that could be leveraged in targeted attacks.

For European organizations, the primary impact of CVE-2025-64766 is unauthorized disclosure of sensitive documents managed via OnlyOffice on NixOS systems. This could lead to leakage of confidential business information, intellectual property, or personal data, especially if revision IDs are exposed through other means such as insider threats, misconfigurations, or indirect information leaks. The vulnerability does not affect data integrity or system availability, limiting the scope of damage. However, the ease of exploitation without authentication and user interaction increases the risk profile, particularly for organizations with publicly accessible OnlyOffice deployments or weak access controls. Organizations in sectors with stringent data protection requirements, such as finance, healthcare, and government, may face compliance risks under GDPR if sensitive data is exposed. The lack of known exploits suggests limited active targeting currently, but the vulnerability should be treated proactively to prevent future abuse.

European organizations should immediately assess their NixOS deployments to identify if affected versions of nixpkgs with OnlyOffice are in use. The primary mitigation is to upgrade to NixOS stable version 25.05 or unstable 25.11 and later, where the hard-coded secret has been removed. If upgrading is not immediately feasible, organizations should implement network-level access controls to restrict access to OnlyOffice servers, ensuring they are not exposed to untrusted networks. Monitoring and logging access to OnlyOffice document caches can help detect suspicious activities involving revision IDs. Additionally, organizations should audit their document management policies to minimize exposure of revision IDs and ensure that expired user access is properly revoked and enforced. Employing application-layer security measures such as web application firewalls (WAFs) and intrusion detection systems (IDS) can provide additional layers of defense. Finally, educating administrators and users about the risks of sharing document revision links can reduce the likelihood of accidental exposure.