CVE-2025-59900 is a persistent Cross-Site Scripting (XSS) vulnerability classified under CWE-79, found in Flexense Sync Breeze Enterprise Server version 10.4.18 and Disk Pulse Enterprise v10.4.18. The vulnerability exists due to insufficient sanitization and neutralization of user-supplied input in the web interface endpoint '/server_options?sid='. Specifically, the parameters 'tasks_logs_dir', 'errors_logs_dir', 'error_notifications_address', 'status_notifications_address', and 'status_reports_address' do not properly validate or encode input, allowing an authenticated attacker to inject malicious JavaScript code. This malicious code is stored persistently and executed in the browsers of other authenticated users who access the affected pages, enabling session hijacking, credential theft, or unauthorized actions within the application context. The attack vector requires the attacker to have at least low privileges (authenticated user) and some user interaction (victim visiting the maliciously crafted page). The vulnerability does not affect confidentiality, integrity, or availability directly but compromises user session confidentiality and trust. The CVSS 4.0 base score of 5.1 reflects medium severity, considering the network attack vector, low attack complexity, no privileges required beyond authentication, and user interaction needed. No public exploits have been reported, but the vulnerability poses a risk in environments where multiple users access the server interface. The lack of available patches at the time of reporting necessitates immediate mitigation steps to reduce exposure.

For European organizations, this vulnerability could lead to unauthorized disclosure of session tokens and sensitive user information within the Sync Breeze Enterprise Server environment. Since the attack requires authenticated access, insider threats or compromised credentials could be leveraged to inject malicious scripts, potentially escalating to broader network compromise or data exfiltration. Organizations relying on this software for file synchronization and monitoring may face operational disruptions if attackers exploit this vulnerability to manipulate server settings or intercept administrative sessions. The persistent nature of the XSS increases the risk of repeated exploitation and lateral movement within the network. Confidentiality of user sessions and integrity of administrative operations are at risk, which could undermine trust in the affected systems and lead to compliance issues under GDPR if personal data is exposed. The medium severity suggests a moderate but significant risk, especially in sectors with sensitive data such as finance, healthcare, and critical infrastructure within Europe.

European organizations should immediately restrict access to the Sync Breeze Enterprise Server management interface to trusted administrators only, ideally via VPN or segmented network zones. Implement strict input validation and output encoding on all user-supplied data in the affected parameters, either by applying vendor patches when available or by deploying web application firewalls (WAFs) with custom rules to detect and block malicious payloads targeting '/server_options?sid='. Conduct thorough credential audits and enforce multi-factor authentication (MFA) to reduce the risk of unauthorized authenticated access. Regularly monitor server logs for unusual parameter values or repeated access attempts to the vulnerable endpoint. Educate administrators about the risks of XSS and the importance of not clicking on suspicious links or content within the server interface. If patching is not immediately possible, consider disabling or limiting the functionality of the affected parameters temporarily. Finally, maintain an incident response plan ready to address potential session hijacking or data leakage incidents stemming from this vulnerability.