CVE-2025-53523 is a stored cross-site scripting (XSS) vulnerability identified in Japan Total System Co., Ltd.'s GroupSession collaboration software, specifically affecting the Free edition prior to version 5.3.0, byCloud prior to 5.3.3, and ZION prior to 5.3.2. Stored XSS occurs when malicious scripts are permanently stored on the target server, such as in a database, message forum, or comment field, and then served to other users. In this case, a logged-in user with legitimate access can craft a malicious page or URL containing a script payload. When another user accesses this crafted content, the script executes in their browser context, potentially allowing the attacker to steal session cookies, perform actions on behalf of the victim, or manipulate the displayed content. The vulnerability requires the attacker to be authenticated (privileged user) and for the victim to interact with the malicious content (user interaction). The CVSS v3.0 score of 5.4 reflects a medium severity, with network attack vector, low attack complexity, privileges required, and user interaction necessary. The scope is changed (S:C), indicating that the vulnerability can affect resources beyond the initially vulnerable component. No known exploits are currently reported in the wild, but the presence of stored XSS in collaboration software is concerning due to the potential for lateral movement and data exfiltration within organizations. The vulnerability affects multiple GroupSession editions, which are used for groupware and collaboration, increasing the risk of sensitive information exposure or manipulation if exploited.

For European organizations, this vulnerability poses a risk primarily to confidentiality and integrity of data within collaboration environments. Exploitation could allow attackers to hijack user sessions, steal credentials, or perform unauthorized actions within the GroupSession platform, potentially leading to data leakage or manipulation of collaborative documents and communications. Given that GroupSession is used for internal communication and project management, successful exploitation could disrupt workflows and damage trust in the platform. The requirement for authenticated access limits exposure to internal or trusted users, but insider threats or compromised accounts could leverage this vulnerability. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as stored XSS can be a stepping stone for more complex attacks. European organizations in sectors such as finance, government, and manufacturing, which rely heavily on secure collaboration tools, may face increased risk if GroupSession is deployed. The medium severity suggests moderate urgency but should not be ignored, especially in environments with sensitive or regulated data.

1. Apply official patches from Japan Total System Co., Ltd. as soon as they are released for all affected GroupSession editions to remediate the vulnerability. 2. Until patches are available, restrict the ability of users to input or upload content that can contain executable scripts, including sanitizing and validating all user inputs rigorously. 3. Implement Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts in browsers accessing GroupSession. 4. Limit user privileges to the minimum necessary, reducing the number of users who can create or modify content that might contain malicious scripts. 5. Monitor logs and user activity for unusual behavior or attempts to inject scripts, especially from authenticated users. 6. Educate users about the risks of clicking on suspicious links or content within the collaboration platform. 7. Consider deploying web application firewalls (WAF) with rules designed to detect and block XSS payloads targeting GroupSession. 8. Regularly review and update security configurations and conduct penetration testing focused on XSS vulnerabilities within collaboration tools.