CVE-2025-27921 is a reflected Cross-Site Scripting (XSS) vulnerability identified in Output Messenger versions prior to 2.0.63. Reflected XSS occurs when an application takes user-supplied input and immediately includes it in the response webpage without proper sanitization or encoding, allowing attackers to inject malicious scripts that execute in the context of the victim's browser. This vulnerability specifically affects Output Messenger, a communication platform, where unsanitized input is reflected back to users. The CVSS v3.1 score of 6.1 indicates a medium severity level, with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), but requiring user interaction (UI:R). The scope is changed (S:C), meaning the vulnerability can affect resources beyond the vulnerable component. The impact affects confidentiality and integrity partially (C:L/I:L), but not availability (A:N). Exploitation could allow attackers to steal session tokens, perform actions on behalf of users, or redirect users to malicious sites. No known exploits are currently reported in the wild, and no patches are linked yet, suggesting organizations should monitor for updates. The vulnerability is classified under CWE-79, which is the standard category for XSS issues. Given the nature of Output Messenger as a communication tool, exploitation could lead to unauthorized access to sensitive communications or credential theft if users are tricked into executing malicious scripts.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on Output Messenger for internal or external communications. Successful exploitation could lead to session hijacking, unauthorized disclosure of confidential messages, or manipulation of communication content, undermining trust and potentially violating data protection regulations such as GDPR. The reflected XSS could also be used as a vector for phishing attacks targeting employees, increasing the risk of credential compromise or malware deployment. Since the vulnerability requires user interaction, social engineering tactics could be employed to maximize impact. Organizations in sectors with high confidentiality requirements—such as finance, healthcare, and government—may face reputational damage, regulatory penalties, and operational disruptions if this vulnerability is exploited. Additionally, the scope change in the CVSS vector suggests that the impact could extend beyond the immediate application, potentially affecting integrated systems or services.

To mitigate this vulnerability, European organizations should prioritize the following actions: 1) Monitor for and apply official patches or updates from Output Messenger as soon as they become available, as no patch links are currently provided. 2) Implement Web Application Firewall (WAF) rules that detect and block reflected XSS attack patterns targeting Output Messenger endpoints. 3) Conduct input validation and output encoding on all user-supplied data within the application environment, if customization or internal development is possible. 4) Educate users about the risks of clicking on suspicious links or interacting with unexpected messages, reducing the likelihood of successful social engineering. 5) Employ Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts in browsers accessing Output Messenger. 6) Regularly audit and monitor logs for unusual activity or attempted exploitation attempts. 7) Consider network segmentation to limit access to Output Messenger to trusted users and devices, reducing exposure. These measures collectively reduce the attack surface and the likelihood of successful exploitation.