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 visited by other users within the same browser context, execute arbitrary scripts. The exploitation requires user interaction, such as clicking on a malicious link, and can lead to client-side impacts including session hijacking, credential theft, or other malicious actions performed in the victim’s browser. The vulnerability affects all supported UDP versions from 8.0 through 10.1, with versions 7.x and earlier being unsupported and also vulnerable. Arcserve UDP 10.2 includes patches that fully remediate this issue. The CVSS 4.0 base score is 4.8, indicating a medium severity level, reflecting the network attack vector, low attack complexity, no required authentication, but requiring user interaction and limited impact on confidentiality and integrity. No known exploits are currently reported in the wild. This vulnerability is classified under CWE-79, which is a common web application security flaw involving improper input sanitization leading to XSS attacks.

For European organizations using Arcserve UDP for backup and data protection, this vulnerability presents a moderate risk. Successful exploitation could allow attackers to execute malicious scripts in the context of legitimate users, potentially leading to session hijacking and credential theft. This could compromise the security of backup management consoles, enabling unauthorized access or manipulation of backup configurations and data. Given that Arcserve UDP is often used in enterprise environments to protect critical data, any compromise could disrupt backup operations, leading to data loss or delayed recovery in case of incidents. The requirement for user interaction limits the attack scope somewhat, but phishing or social engineering campaigns could be used to lure users into clicking malicious links. The vulnerability's presence in supported versions means many organizations may still be exposed if patches or upgrades are not applied promptly. The impact on confidentiality and integrity of backup data and management credentials is significant, though availability impact is limited. European organizations with regulatory obligations around data protection (e.g., GDPR) must consider the risk of data exposure or unauthorized access resulting from this vulnerability.

European organizations should prioritize upgrading Arcserve UDP installations to version 10.2, which contains the necessary patches to remediate this XSS vulnerability. If immediate upgrade is not feasible, organizations should apply any available patches or hotfixes provided by Arcserve for versions 8.0 through 10.1. Additionally, organizations should implement strict input validation and output encoding on the web interface to prevent injection of malicious scripts. Employing web application firewalls (WAFs) with rules targeting reflected XSS patterns can provide interim protection. User awareness training to recognize phishing attempts and suspicious links is critical to reduce the risk of user interaction exploitation. Monitoring web server logs for unusual URL patterns and failed login attempts can help detect exploitation attempts. Restricting access to the UDP web interface to trusted networks or via VPN can reduce exposure. Finally, organizations should review and enforce least privilege principles for UDP users to limit the impact of compromised accounts.