CVE-2025-54478 is a high-severity vulnerability identified in the Mattermost Confluence Plugin versions prior to 1.5.0. The vulnerability is classified under CWE-306, which relates to missing authentication for a critical function. Specifically, the plugin fails to enforce user authentication when accessing the API endpoint responsible for editing channel subscriptions within a Mattermost instance. This flaw allows unauthenticated attackers to modify channel subscription settings via direct API calls without any verification of user identity or privileges. The vulnerability has a CVSS v3.1 base score of 7.2, reflecting its network exploitability (AV:N), low attack complexity (AC:L), no privileges required (PR:N), and no user interaction needed (UI:N). The impact scope is changed (S:C), indicating that the vulnerability affects resources beyond the vulnerable component itself. The confidentiality and integrity impacts are rated as low (C:L, I:L), while availability is unaffected (A:N). Although no known exploits are currently reported in the wild, the lack of authentication on a critical function exposes organizations to unauthorized modifications of communication channels, potentially leading to information leakage, manipulation of collaboration workflows, or disruption of team communications. The vulnerability arises from improper access control in the plugin's API design, which should enforce authentication and authorization checks before allowing modifications to channel subscriptions. Since Mattermost is widely used for team collaboration and integrates with Confluence for knowledge management, this vulnerability could be leveraged to alter subscription settings, potentially exposing sensitive information or disrupting notification mechanisms.

For European organizations, the impact of CVE-2025-54478 can be significant, especially for those relying on Mattermost integrated with Confluence for internal communications and knowledge sharing. Unauthorized editing of channel subscriptions can lead to unintended disclosure of sensitive project or organizational information if attackers subscribe themselves or others to channels they should not access. It can also disrupt communication flows by unsubscribing legitimate users or altering notification settings, thereby impacting operational efficiency and incident response capabilities. Given the collaborative nature of these platforms, such unauthorized changes could facilitate further social engineering or lateral movement within the organization. Additionally, organizations subject to strict data protection regulations such as GDPR may face compliance risks if unauthorized access leads to personal data exposure. The vulnerability's network accessibility and lack of required authentication make it easier for remote attackers to exploit without insider access, increasing the threat surface. Although availability is not directly impacted, the integrity and confidentiality concerns are sufficient to warrant urgent remediation in environments where Mattermost and Confluence are critical to business operations.

To mitigate CVE-2025-54478, European organizations should immediately upgrade the Mattermost Confluence Plugin to version 1.5.0 or later, where proper authentication enforcement has been implemented. Until the patch is applied, organizations should restrict network access to the Mattermost API endpoints by implementing firewall rules or API gateway controls that limit access to trusted users and IP ranges. Monitoring and logging API calls related to channel subscription edits should be enhanced to detect any unauthorized or anomalous activity promptly. Organizations should also review and tighten Mattermost instance permissions and audit existing channel subscriptions for unauthorized changes. Employing multi-factor authentication (MFA) on Mattermost user accounts can reduce the risk of compromised credentials being used to exploit related vulnerabilities. Additionally, security teams should conduct penetration testing focused on API endpoints to identify any other missing authentication or authorization controls. Finally, raising user awareness about potential phishing or social engineering attempts that could leverage altered subscription settings will help mitigate secondary risks.