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. The vulnerability allows an authenticated user to inject malicious scripts into pages or URLs within the application. When other users access these crafted pages, the embedded scripts execute in their browsers under the context of the vulnerable web application. This can lead to unauthorized actions such as session hijacking, theft of sensitive information, or manipulation of the user interface, thereby compromising confidentiality and integrity. The attack vector is network-based, requiring the attacker to have valid credentials (low privilege) and some user interaction (victim must access the malicious content). The vulnerability does not impact availability. The CVSS 3.0 score of 5.4 reflects these factors: network attack vector (AV:N), low attack complexity (AC:L), privileges required (PR:L), user interaction required (UI:R), scope changed (S:C), and partial impacts on confidentiality (C:L) and integrity (I:L). No public exploits are currently known, but the vulnerability is published and should be addressed promptly. The lack of patch links suggests users should consult the vendor for updates or apply available version upgrades to 5.3.0 or later.

For European organizations, this vulnerability poses a moderate risk primarily to confidentiality and integrity of data within GroupSession environments. Attackers with valid user credentials can embed malicious scripts that execute in other users' browsers, potentially leading to session hijacking, unauthorized data access, or manipulation of displayed information. This can undermine trust in collaboration platforms and expose sensitive corporate or personal data. Although availability is not directly impacted, the indirect effects of data leakage or unauthorized actions could disrupt business operations or compliance with data protection regulations such as GDPR. Organizations in sectors relying heavily on collaborative tools for sensitive communications—such as finance, healthcare, and government—may face heightened risks. The requirement for authenticated access and user interaction limits the attack surface but does not eliminate the threat, especially in environments with many users or insufficient user training. The absence of known exploits reduces immediate risk but should not lead to complacency.

European organizations using GroupSession products should immediately verify their software versions and upgrade to the fixed releases: Free edition version 5.3.0 or later, byCloud version 5.3.3 or later, and ZION version 5.3.2 or later. If immediate upgrades are not feasible, implement strict input validation and output encoding on all user-generated content within the application to prevent script injection. Employ Content Security Policy (CSP) headers to restrict script execution sources and reduce the impact of any injected scripts. Conduct user awareness training emphasizing the risks of clicking on suspicious links or pages within the collaboration platform. Monitor logs for unusual user activity or attempts to inject scripts. Limit user privileges to the minimum necessary to reduce the number of users capable of injecting malicious content. Engage with the vendor for official patches and security advisories. Regularly review and update web application firewall (WAF) rules to detect and block XSS payloads targeting GroupSession URLs. Finally, implement multi-factor authentication (MFA) to reduce the risk of compromised credentials being used to exploit this vulnerability.