CVE-2025-2527 is a security vulnerability identified in Mattermost versions 10.5.0 through 10.5.2 and 9.11.0 through 9.11.11. The issue stems from an incorrect authorization check (CWE-863) within the Mattermost platform's API when users attempt to access group information. Specifically, the software fails to properly verify whether a user has the necessary permissions to view certain groups. This flaw allows an attacker with at least some level of authenticated access (as indicated by the CVSS vector requiring privileges) to send crafted API requests and retrieve group information that should otherwise be restricted. The vulnerability does not require user interaction beyond making the API call and does not impact the integrity or availability of the system, but it does result in unauthorized disclosure of group data, which could include sensitive organizational or user group details. The CVSS 3.1 base score is 4.3, categorizing it as a medium severity vulnerability. No known exploits are currently reported in the wild, and no official patches have been linked yet, though the issue is publicly disclosed and tracked by Mattermost and CISA. The vulnerability affects network-exploitable API endpoints with low attack complexity but requires some level of user privileges, meaning it is not exploitable by unauthenticated attackers. The scope remains unchanged as the impact is limited to confidentiality of group information within the same security boundary.

For European organizations using Mattermost, this vulnerability could lead to unauthorized disclosure of internal group information, potentially exposing organizational structures, team compositions, or sensitive project groupings. Such information leakage can aid attackers in reconnaissance efforts, facilitating targeted social engineering or further attacks. While the vulnerability does not allow modification or disruption of data or services, the confidentiality breach could violate data protection regulations such as GDPR if personal or sensitive information is exposed. Organizations in regulated sectors (e.g., finance, healthcare, government) may face compliance risks and reputational damage if group data is leaked. Since Mattermost is often used for internal collaboration, the exposure of group metadata could undermine trust and operational security within teams. The medium severity rating suggests that while the risk is not critical, it is significant enough to warrant prompt attention, especially in environments where group information is sensitive or where insider threat risks are elevated.

European organizations should immediately assess their Mattermost deployments to identify if they are running affected versions (10.5.0 to 10.5.2 or 9.11.0 to 9.11.11). Until patches are available, organizations should restrict API access to trusted users only, enforce strict access controls, and monitor API usage logs for unusual or unauthorized group information requests. Implementing network segmentation and limiting Mattermost API exposure to internal networks can reduce exploitation risk. Organizations should also review user privilege assignments to ensure minimal necessary permissions are granted, reducing the pool of users who could exploit this flaw. Once Mattermost releases official patches or updates, organizations must prioritize timely patching. Additionally, conducting internal audits of group information access and educating users about the sensitivity of group data can help mitigate potential insider threats. Finally, integrating Mattermost logs with SIEM solutions to detect anomalous API calls can provide early warning of exploitation attempts.