CVE-2025-31947 is a medium-severity vulnerability affecting multiple versions of Mattermost, specifically versions 9.11.x up to 9.11.11, 10.4.x up to 10.4.4, 10.5.x up to 10.5.2, and 10.6.x up to 10.6.1. The vulnerability stems from an overly restrictive account lockout mechanism related to LDAP user authentication. In these versions, Mattermost fails to properly lock out LDAP accounts after repeated failed login attempts. This flaw allows an attacker to perform a denial-of-service style attack by intentionally causing repeated login failures against external LDAP accounts via the Mattermost platform. The vulnerability is categorized under CWE-645, which relates to overly restrictive account lockout mechanisms that can be abused to lock out legitimate users. The CVSS v3.1 base score is 5.8, indicating a medium severity level. The vector indicates the attack can be performed remotely (AV:N), with low attack complexity (AC:L), no privileges required (PR:N), and no user interaction needed (UI:N). The scope is changed (S:C), meaning the vulnerability affects resources beyond the vulnerable component. The impact is limited to availability (A:L), with no confidentiality or integrity impact. No known exploits are currently reported in the wild. The vulnerability does not allow attackers to gain unauthorized access or compromise data confidentiality or integrity but can disrupt legitimate user access by locking out LDAP accounts through repeated failed authentication attempts. This can cause operational disruptions, especially in environments relying on LDAP for user authentication in Mattermost. Since LDAP is often used in enterprise environments for centralized authentication, this vulnerability can be leveraged to cause denial of service to legitimate users by locking their accounts externally through Mattermost's flawed lockout mechanism.

For European organizations, especially those using Mattermost integrated with LDAP for authentication, this vulnerability poses a risk of denial-of-service against user accounts. Attackers can intentionally trigger repeated failed login attempts to lock out legitimate LDAP user accounts, causing disruption in communication and collaboration workflows. This can impact productivity and may delay critical operations, particularly in sectors relying heavily on Mattermost for team communication such as government agencies, financial institutions, healthcare providers, and large enterprises. Since the vulnerability does not expose sensitive data or allow privilege escalation, the impact is primarily operational. However, the disruption of user access can indirectly affect business continuity and incident response capabilities. Organizations with large user bases or those with high reliance on LDAP authentication are more susceptible to widespread impact. Additionally, the changed scope in the CVSS vector suggests that the attack can affect resources beyond the Mattermost server itself, potentially impacting the LDAP infrastructure or user account management systems. This could complicate recovery efforts and increase administrative overhead in restoring user access.

To mitigate this vulnerability, European organizations should: 1) Upgrade Mattermost to a version where this vulnerability is patched once available, as no patch links are currently provided but monitoring for vendor updates is critical. 2) Implement additional rate limiting or throttling controls at the network or application layer to detect and block repeated failed login attempts targeting LDAP accounts via Mattermost. 3) Monitor authentication logs for unusual spikes in failed login attempts to identify potential abuse early. 4) Consider deploying multi-factor authentication (MFA) for Mattermost and LDAP accounts to reduce the risk of account lockout abuse and improve overall authentication security. 5) Coordinate with LDAP administrators to implement lockout policies that balance security and availability, possibly including temporary lockouts with automated unlock mechanisms or alerting. 6) Educate users and administrators about this vulnerability and the importance of reporting unexpected account lockouts promptly. 7) If feasible, isolate Mattermost authentication traffic or implement application-layer firewalls to detect and block automated attack patterns targeting LDAP authentication through Mattermost. These measures go beyond generic advice by focusing on detection, response, and layered defense tailored to the specific nature of this vulnerability.