CVE-2025-59896 is a persistent Cross-Site Scripting (XSS) vulnerability classified under CWE-79, found in Flexense Sync Breeze Enterprise Server version 10.4.18. The vulnerability arises from improper neutralization of user-supplied input during web page generation, specifically in the 'command_name' parameter of the '/add_command?sid=' endpoint. This parameter fails to adequately sanitize or encode input, allowing an attacker with authenticated access to inject malicious JavaScript code that is stored persistently on the server. When other authenticated users access the affected functionality, the malicious script executes in their browsers within the security context of the Sync Breeze Enterprise Server application. This can lead to session hijacking, theft of sensitive information, or unauthorized actions performed on behalf of the victim user. The CVSS 4.0 score is 5.1 (medium severity), reflecting that the attack vector is network-based, requires low attack complexity, no privileges beyond authentication, and user interaction. The vulnerability does not impact confidentiality, integrity, or availability directly beyond the session compromise risk. No public exploits have been reported yet, but the presence of persistent XSS in an enterprise server product poses a significant risk if exploited. The vulnerability was reserved in September 2025 and published in January 2026. The lack of a patch link suggests that a fix may not yet be available, emphasizing the need for interim mitigations.

For European organizations, this vulnerability poses a risk primarily to the confidentiality of user sessions and potentially sensitive data accessible via the Sync Breeze Enterprise Server interface. Attackers exploiting this flaw can hijack authenticated sessions, leading to unauthorized access to enterprise data, configuration settings, or operational commands. This can result in data leakage, unauthorized changes, or lateral movement within the network. The requirement for authentication limits the attack surface to internal or trusted users, but insider threats or compromised credentials could be leveraged. The persistent nature of the XSS increases the risk as malicious scripts remain active until removed. Given the use of Sync Breeze Enterprise Server in file synchronization and monitoring in enterprise environments, disruption or data theft could impact business continuity and compliance with data protection regulations such as GDPR. The medium severity rating indicates a moderate but actionable threat, especially in sectors handling sensitive or regulated data.

1. Apply official patches or updates from Flexense as soon as they become available to address the input validation flaw. 2. Implement strict input validation and output encoding on the 'command_name' parameter to neutralize malicious scripts. 3. Restrict access to the '/add_command?sid=' endpoint to only trusted and necessary users, employing network segmentation and access controls. 4. Employ Web Application Firewalls (WAFs) with rules to detect and block XSS payloads targeting this endpoint. 5. Conduct regular security audits and penetration testing focusing on authenticated user input vectors. 6. Educate users about the risks of interacting with suspicious content within the application. 7. Monitor logs for unusual activity related to command additions or script injections. 8. Consider implementing Content Security Policy (CSP) headers to reduce the impact of XSS attacks by restricting script execution sources. 9. Use multi-factor authentication to reduce the risk of credential compromise that could facilitate exploitation.