CVE-2025-53857 is a security vulnerability identified in the Mattermost Confluence Plugin versions prior to 1.5.0. The vulnerability is classified under CWE-862, which corresponds to Missing Authorization. Specifically, the plugin fails to verify whether a user has the appropriate access rights to a Mattermost channel when processing API requests to the GET autocomplete/GetChannelSubscriptions endpoint. This flaw allows an unauthenticated attacker to retrieve channel subscription details without proper authorization. The vulnerability is exploitable remotely over the network without requiring any user interaction or prior authentication, although it has a relatively high attack complexity. The vulnerability impacts confidentiality by exposing potentially sensitive subscription information related to Mattermost channels but does not affect integrity or availability. The CVSS v3.1 base score is 3.7, indicating a low severity level. No known exploits are currently reported in the wild, and no patches have been explicitly linked yet. The issue arises because the plugin does not enforce access control checks on the API endpoint, allowing unauthorized data disclosure. This could potentially aid attackers in reconnaissance activities by revealing user subscription details and channel metadata, which might be leveraged in further targeted attacks or social engineering campaigns.

For European organizations using Mattermost with the Confluence Plugin, this vulnerability could lead to unauthorized disclosure of channel subscription information. While the direct impact on confidentiality is limited to subscription metadata rather than message content or sensitive documents, this information could still be valuable for attackers to map internal communication structures and identify key personnel or groups. This reconnaissance capability might facilitate more sophisticated attacks such as phishing or lateral movement within the network. Organizations in sectors with strict data privacy regulations, such as GDPR, should be cautious as unauthorized exposure of user-related information could have compliance implications. However, since the vulnerability does not allow modification or deletion of data, nor does it disrupt service availability, the overall operational impact is limited. The low CVSS score reflects this constrained impact, but the risk remains relevant for organizations relying heavily on Mattermost for internal collaboration and communication.

To mitigate this vulnerability, European organizations should prioritize upgrading the Mattermost Confluence Plugin to version 1.5.0 or later where the authorization checks are properly enforced. Until an official patch is available, organizations should consider restricting access to the vulnerable API endpoints via network-level controls such as firewall rules or API gateways to limit exposure to trusted users only. Monitoring API usage logs for unusual or unauthorized access attempts to the GetChannelSubscriptions endpoint can help detect potential exploitation attempts. Additionally, organizations should review and tighten Mattermost channel subscription policies to minimize unnecessary subscriptions and reduce the potential information exposure. Implementing strict role-based access controls (RBAC) within Mattermost and Confluence integrations can further reduce the attack surface. Finally, educating users about phishing and social engineering risks that could leverage such reconnaissance information is advisable.