CVE-2025-34521 is a reflected Cross-Site Scripting (XSS) vulnerability identified in the web interface of Arcserve Unified Data Protection (UDP) versions prior to 10.2. This vulnerability arises due to improper neutralization of user-supplied input during web page generation, specifically where unsanitized input is reflected in HTTP responses. An attacker with low privileges can craft malicious URLs containing JavaScript payloads that, when clicked by another user, execute arbitrary scripts within the victim’s browser context. This can lead to session hijacking, credential theft, or other client-side impacts such as unauthorized actions performed on behalf of the victim. The vulnerability requires user interaction, meaning the victim must click on a malicious link, and it occurs within a shared browser context, which may increase the risk in multi-user environments. Arcserve UDP versions 8.0 through 10.1 are affected and supported, requiring patching or upgrading to version 10.2, which contains the fix. Versions 7.x and earlier are unsupported and must be upgraded to 10.2 to remediate the issue. The CVSS 4.0 base score is 4.8 (medium severity), reflecting network attack vector, low attack complexity, no authentication required, user interaction needed, and limited impact on confidentiality and integrity. No known exploits are currently reported in the wild.

For European organizations, this vulnerability poses a moderate risk primarily to the confidentiality and integrity of user sessions and credentials. Arcserve UDP is a data protection and backup solution widely used in enterprise environments, including in Europe, to safeguard critical data. Exploitation could allow attackers to hijack sessions of administrators or users accessing the UDP web interface, potentially leading to unauthorized access to backup configurations or sensitive operational data. This could disrupt backup operations or expose sensitive data, impacting business continuity and compliance with data protection regulations such as GDPR. The requirement for user interaction limits automated widespread exploitation, but targeted phishing campaigns could leverage this vulnerability to compromise key personnel. The shared browser context aspect may increase risk in environments where multiple users access UDP from shared terminals. Although no active exploits are known, the presence of this vulnerability in supported versions means European organizations using affected UDP versions remain exposed until patched or upgraded.

European organizations should prioritize upgrading Arcserve UDP installations to version 10.2, which contains the necessary security patches. If immediate upgrade is not feasible, applying any available security patches addressing this vulnerability is critical. Additionally, organizations should implement strict input validation and output encoding controls on any custom integrations with the UDP web interface to reduce XSS risks. User awareness training to recognize and avoid clicking suspicious links can reduce the risk of exploitation via phishing. Network-level protections such as Web Application Firewalls (WAFs) configured to detect and block reflected XSS payloads targeting UDP interfaces can provide an additional layer of defense. Monitoring web server logs for unusual URL patterns and failed authentication attempts may help detect exploitation attempts. Finally, restricting access to the UDP web interface to trusted networks or via VPN can reduce exposure to external attackers.