CVE-2025-34521 is a reflected cross-site scripting (XSS) vulnerability classified under CWE-79, affecting the web interface of Arcserve Unified Data Protection (UDP) software. The flaw arises from improper neutralization of user-supplied input during web page generation, where unsanitized input is reflected in HTTP responses. This allows an attacker with low privileges to craft malicious URLs that, when visited by another user, execute arbitrary JavaScript code in the victim’s browser context. The vulnerability requires user interaction, specifically the victim clicking on a malicious link, and occurs within a shared browser context, which could be a multi-user environment or shared session. Potential impacts include session hijacking, credential theft, and other client-side attacks that compromise confidentiality and integrity of user sessions. The vulnerability affects all UDP versions prior to 10.2; version 10.2 includes necessary patches and is not vulnerable. Supported versions 8.0 through 10.1 require patching or upgrading to 10.2, while unsupported versions 7.x and earlier must be upgraded to 10.2 to remediate the issue. The CVSS 4.0 score of 4.8 reflects medium severity, with network attack vector, low attack complexity, no privileges required, but user interaction necessary. No known exploits have been reported in the wild as of the publication date.

For European organizations using Arcserve UDP versions prior to 10.2, this vulnerability poses a moderate risk primarily to the confidentiality and integrity of user sessions. Successful exploitation could lead to session hijacking and credential theft, potentially allowing attackers to escalate privileges or move laterally within the network. Given that Arcserve UDP is used for backup and data protection, compromise of the management interface could disrupt backup operations or expose sensitive backup data indirectly. The requirement for user interaction limits the ease of exploitation but does not eliminate risk, especially in environments where phishing or social engineering attacks are common. Organizations with shared or multi-user access to the UDP web interface are at higher risk. The lack of known exploits in the wild reduces immediate threat but does not preclude future exploitation. Overall, the impact could affect operational continuity and data security, particularly in sectors with stringent data protection requirements such as finance, healthcare, and critical infrastructure in Europe.

European organizations should prioritize upgrading Arcserve UDP installations to version 10.2, which contains the necessary patches to fully remediate CVE-2025-34521. For environments where immediate upgrade is not feasible, organizations should implement strict access controls to limit web interface exposure, including network segmentation and firewall rules restricting access to trusted IPs only. Employ web application firewalls (WAFs) with rules to detect and block reflected XSS payloads targeting the UDP interface. Conduct user awareness training to reduce the risk of phishing and malicious link clicks, emphasizing caution with unsolicited URLs. Regularly monitor logs for unusual access patterns or suspicious HTTP requests to the UDP web interface. Disable or restrict browser features that can exacerbate XSS impact, such as shared sessions or persistent authentication cookies. Finally, maintain an up-to-date inventory of UDP versions deployed across the organization to ensure timely patch management and compliance.