CVE-2025-41395 is a medium-severity vulnerability affecting Mattermost versions 9.11.x up to 9.11.10, 10.4.x up to 10.4.2, and 10.5.x up to 10.5.0. The issue arises from improper validation of the properties (props) used by the RetrospectivePost custom post type within the Playbooks plugin. Specifically, the application fails to correctly validate the type and content of input props, allowing an attacker to craft malicious posts with specially crafted props. When such a post is processed by the Mattermost web application, it can trigger a denial of service (DoS) condition that impacts the availability of the web app for all users. The vulnerability is categorized under CWE-1287, which relates to improper validation of specified input types, indicating that the root cause is insufficient input sanitization or type checking. This flaw does not require authentication or user interaction to exploit, as an attacker can create a malicious post that is then rendered by the application, causing the DoS. No known exploits are currently reported in the wild, and no official patches have been linked yet. The vulnerability was publicly disclosed on April 24, 2025, and has been enriched by CISA, indicating recognition by US cybersecurity authorities. The attack vector targets the Playbooks plugin, which is used for workflow automation and team collaboration, making the vulnerability relevant to organizations relying on Mattermost for internal communication and operational coordination.

The primary impact of this vulnerability is a denial of service affecting the Mattermost web application, rendering it unavailable to all users. For European organizations, this can disrupt internal communications, incident response coordination, and collaborative workflows, especially in sectors where Mattermost is integrated into critical operational processes such as IT, healthcare, finance, and government. The DoS could halt business operations, delay decision-making, and reduce productivity. Since Mattermost is often deployed in enterprise and public sector environments for secure messaging, the disruption could also indirectly affect incident response and crisis management capabilities. Although the vulnerability does not directly compromise confidentiality or integrity, the loss of availability can have significant operational and reputational consequences. The lack of authentication requirement for exploitation increases the risk, as external attackers or malicious insiders could trigger the DoS without elevated privileges. The scope of affected systems includes all Mattermost instances running the vulnerable versions with the Playbooks plugin enabled, which may be widespread in organizations using Mattermost for agile project management and team collaboration across Europe.

Organizations should immediately audit their Mattermost deployments to identify if they are running affected versions (9.11.0 to 9.11.10, 10.4.0 to 10.4.2, or 10.5.0). Until official patches are released, administrators should consider disabling or restricting access to the Playbooks plugin, especially the RetrospectivePost custom post type, to prevent exploitation. Implementing strict input validation and sanitization at the application or proxy level could help mitigate malformed posts. Monitoring logs for unusual post creation activity or malformed payloads targeting the Playbooks plugin is recommended to detect potential exploitation attempts. Network-level controls such as web application firewalls (WAFs) can be tuned to detect and block suspicious requests containing malformed props. Organizations should also prepare incident response plans to quickly restore service availability in case of an attack. Regular backups and high-availability configurations for Mattermost servers can reduce downtime. Finally, organizations should stay alert for official patches or updates from Mattermost and apply them promptly once available.