CVE-2025-65120 is a reflected cross-site scripting (XSS) vulnerability identified in Japan Total System Co., Ltd.'s GroupSession product line, including the Free edition, byCloud, and ZION versions prior to 5.7.1. Reflected XSS occurs when untrusted user input is immediately returned by a web application without proper sanitization, allowing attackers to inject malicious scripts into web pages viewed by other users. In this case, an attacker can craft a malicious URL or web page that, when accessed by a user, executes arbitrary JavaScript code in the victim's browser context. This can lead to various malicious outcomes such as session hijacking, credential theft, or performing unauthorized actions on behalf of the user. The vulnerability is remotely exploitable over the network without requiring authentication, but it does require user interaction (clicking or visiting a crafted link). The CVSS 3.0 base score of 6.1 reflects a medium severity level, with the vector indicating network attack vector, low attack complexity, no privileges required, user interaction required, and a scope change affecting confidentiality and integrity but not availability. The vulnerability affects versions prior to 5.7.1, and no official patches or exploit code are currently publicly available. The vulnerability was published on December 12, 2025, and assigned by JPCERT. Given the nature of GroupSession as a collaborative groupware platform, exploitation could compromise sensitive business communications and data integrity.

For European organizations using GroupSession products, this vulnerability poses a risk to the confidentiality and integrity of sensitive information exchanged via the platform. Attackers exploiting this XSS flaw could steal session cookies or authentication tokens, enabling unauthorized access to user accounts and potentially sensitive corporate data. This could lead to data breaches, unauthorized data manipulation, or lateral movement within the network if the compromised accounts have elevated privileges. The reflected XSS nature requires user interaction, so phishing or social engineering campaigns may be used to lure victims. The impact is particularly significant for organizations with external-facing GroupSession portals accessible by partners, clients, or remote employees. Disruption of trust and potential regulatory consequences under GDPR could arise if personal or sensitive data is exposed. However, availability is not impacted, and no known exploits are currently active in the wild, which somewhat limits immediate risk.

The primary mitigation is to upgrade all affected GroupSession installations to version 5.7.1 or later, where the vulnerability has been addressed. In the absence of immediate upgrade capability, organizations should implement strict input validation and output encoding on all user-supplied data to prevent script injection. Deploying Content Security Policy (CSP) headers can help restrict the execution of unauthorized scripts. Additionally, organizations should educate users about the risks of clicking on suspicious links and implement email filtering to reduce phishing attempts. Monitoring web server logs for unusual URL patterns or repeated attempts to exploit XSS can aid early detection. Network segmentation and limiting access to GroupSession interfaces to trusted IP ranges can reduce exposure. Finally, regular security assessments and penetration testing focused on web application security should be conducted to identify and remediate similar vulnerabilities proactively.