CVE-2025-66284 is a stored cross-site scripting (XSS) vulnerability identified in Japan Total System Co., Ltd.'s GroupSession collaboration software products, specifically the Free edition, byCloud, and ZION versions prior to 5.7.1. The vulnerability allows a logged-in user to inject malicious JavaScript code into the application by preparing a crafted page or URL. When another user accesses this malicious content, the embedded script executes within their browser context. This can lead to unauthorized actions such as session hijacking, credential theft, or manipulation of displayed data, compromising confidentiality and integrity of user interactions. The vulnerability requires the attacker to have authenticated access and involves user interaction (the victim must access the malicious page or URL). 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 affects resources beyond the initially vulnerable component. No public exploits have been reported yet, but the presence of stored XSS in collaboration tools is concerning due to the potential for lateral movement and data exposure within organizations. The vulnerability was published on December 12, 2025, with the vendor having reserved the CVE on November 27, 2025. No patch links were provided in the data, but upgrading to version 5.7.1 or later is implied as the remediation.

For European organizations, this vulnerability poses a moderate risk primarily to confidentiality and integrity of data within GroupSession collaboration environments. Attackers with valid credentials can craft malicious content that, when accessed by other users, can execute scripts capable of stealing session tokens, manipulating displayed information, or performing unauthorized actions on behalf of users. This can lead to data leakage, unauthorized access to sensitive information, and potential internal phishing or social engineering attacks leveraging the compromised platform. While availability is not directly impacted, the trustworthiness of collaboration tools can be undermined, affecting productivity and compliance with data protection regulations such as GDPR. Organizations relying on GroupSession for internal communication and document sharing are particularly at risk, especially if they do not enforce strict input validation or content security policies. The lack of known exploits in the wild reduces immediate risk but does not eliminate the threat, especially as attackers often weaponize such vulnerabilities after disclosure.

1. Upgrade all affected GroupSession products (Free edition, byCloud, ZION) to version 5.7.1 or later as soon as possible to apply the official fix. 2. Implement strict input validation and sanitization on all user-supplied content to prevent injection of malicious scripts. 3. Deploy Content Security Policy (CSP) headers to restrict execution of unauthorized scripts within the application context. 4. Educate users about the risks of clicking on suspicious links or pages within the collaboration environment. 5. Monitor logs for unusual activity indicative of XSS exploitation attempts, such as unexpected script execution or anomalous user behavior. 6. Restrict privileges to only necessary users to minimize the number of accounts capable of injecting malicious content. 7. Conduct regular security assessments and penetration testing focused on web application vulnerabilities, including XSS. 8. Consider implementing web application firewalls (WAF) with rules targeting XSS attack patterns specific to GroupSession's context.