CVE-2023-28997 is a cryptographic vulnerability classified under CWE-323, involving the reuse of a nonce and key pair in the encryption mechanism of the Nextcloud Desktop Client. This client software synchronizes files between a user's device and the Nextcloud Server, supporting end-to-end encryption to protect file confidentiality and integrity. Versions from 3.0.0 up to but not including 3.6.5 are affected. The flaw allows a malicious server administrator—who inherently has high privileges on the server—to exploit the nonce reuse to recover plaintext contents of encrypted files and modify them undetected. This breaks the fundamental guarantees of end-to-end encryption, exposing sensitive data and enabling tampering. The attack requires user interaction, likely in the form of synchronization actions, and leverages the server's ability to manipulate encrypted data streams due to the cryptographic misuse. The vulnerability was publicly disclosed on April 4, 2023, with no known exploits in the wild at the time. The CVSS 3.1 base score is 6.7, indicating a medium severity level, with an attack vector of 'physical or local' (AV:P), low attack complexity, high privileges required on the server (PR:H), and user interaction (UI:R). The scope is changed (S:C), meaning the vulnerability affects components beyond the initially vulnerable one, impacting confidentiality and integrity fully but not availability. The recommended remediation is upgrading the Nextcloud Desktop Client to version 3.6.5 or later, which contains the patch addressing nonce reuse. No alternative mitigations or workarounds are currently known.

For European organizations, this vulnerability poses a significant risk to the confidentiality and integrity of sensitive data synchronized via Nextcloud Desktop Client. Organizations relying on Nextcloud for secure file sharing, including government agencies, financial institutions, healthcare providers, and enterprises handling personal or regulated data, could face data breaches or undetected data tampering if a server administrator is malicious or if the server is compromised. The ability to recover and modify encrypted files undermines trust in the end-to-end encryption model, potentially exposing personal data protected under GDPR and other privacy regulations. This could lead to regulatory penalties, reputational damage, and operational disruptions. The attack requires high privileges on the server side, so insider threats or compromised server environments are the primary concern. Since no workarounds exist, failure to upgrade clients leaves organizations vulnerable. The impact is heightened in environments where Nextcloud is used extensively for collaboration and file synchronization across distributed teams.

The primary and only effective mitigation is to upgrade all Nextcloud Desktop Client installations to version 3.6.5 or later immediately. Organizations should implement an urgent patch management process to identify and update all affected clients. Additionally, organizations should audit server administrator activities and access controls to detect any suspicious behavior that could indicate exploitation attempts. Employing strict server access policies, multi-factor authentication for administrators, and monitoring for anomalous file modifications can reduce risk. Network segmentation and limiting server administrator privileges to only necessary functions can also help. Since no workarounds exist, educating users about the importance of updating the client and avoiding synchronization with untrusted servers is critical. Organizations should also review their incident response plans to address potential data integrity or confidentiality incidents related to this vulnerability.