CVE-2023-48732 is a medium-severity vulnerability identified in Mattermost, an open-source collaboration and messaging platform widely used for team communication. The vulnerability stems from improper scoping of WebSocket responses related to user notifications. Specifically, when a post notification is sent within a Mattermost channel, the WebSocket response that indicates which users were notified is broadcasted to all members of the channel rather than being scoped individually to each notified user. This results in unauthorized exposure of sensitive information about notification recipients to other channel members who should not have access to this data. The vulnerability is classified under CWE-200, which involves the exposure of sensitive information to unauthorized actors. The CVSS 3.1 base score is 4.3, reflecting a medium severity level. The vector indicates that the attack can be performed remotely over the network (AV:N) with low attack complexity (AC:L), requires privileges (PR:L), does not require user interaction (UI:N), and impacts confidentiality only (C:L), with no impact on integrity or availability. No known exploits are currently reported in the wild, and no patches have been linked yet. The vulnerability affects all versions of Mattermost as indicated by the affectedVersions field. The root cause is a design or implementation flaw in the WebSocket notification mechanism, which fails to isolate notification data per user, thereby leaking information about who was notified to all channel participants. This could allow an attacker or unauthorized user within a channel to infer sensitive information about user activity and notification status, potentially aiding further targeted attacks or privacy violations.

For European organizations using Mattermost for internal communication, this vulnerability could lead to unauthorized disclosure of user notification information within channels. While the exposed data is limited to notification recipients and does not include message content or credentials, it still represents a breach of confidentiality and privacy. This could undermine trust in communication platforms, especially in sectors with strict data protection regulations such as GDPR. Organizations handling sensitive or regulated data (e.g., finance, healthcare, government) may face compliance risks if such information leakage is exploited or leads to further data exposure. Additionally, knowledge of who was notified about specific posts could be leveraged by malicious insiders or external attackers with access to channels to map user activity patterns or identify key personnel involved in sensitive discussions. Although the vulnerability does not affect system integrity or availability, the confidentiality impact and potential privacy violations warrant attention. The lack of known exploits reduces immediate risk, but the presence of the vulnerability in all versions means that unpatched or unmitigated deployments remain exposed.

To mitigate CVE-2023-48732, European organizations should: 1) Monitor Mattermost vendor communications closely for official patches or updates addressing this vulnerability and apply them promptly once available. 2) In the interim, restrict channel membership to trusted users only, minimizing exposure to unauthorized actors within channels. 3) Review and tighten access controls and permissions on Mattermost channels to limit unnecessary user inclusion, especially in sensitive or confidential channels. 4) Consider disabling or limiting WebSocket notifications if configurable, or implement custom filtering at the application or network layer to prevent broadcast of notification metadata. 5) Conduct internal audits and monitoring to detect unusual access patterns or attempts to exploit notification data. 6) Educate users about the sensitivity of notification information and encourage cautious sharing of channel membership details. 7) If feasible, deploy network segmentation and zero-trust principles to isolate Mattermost servers and restrict lateral movement in case of compromise. These steps go beyond generic advice by focusing on access control, monitoring, and interim configuration changes pending official patches.