CVE-2026-27659 in Mattermost
Summary
by MITRE • 03/25/2026
Mattermost versions 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to properly validate CSRF tokens in the /api/v4/access_control_policies/{policy_id}/activate endpoint, which allows an attacker to trick an admin into changing access control policy active status via a crafted request.. Mattermost Advisory ID: MMSA-2026-00578
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Analysis
by VulDB Data Team • 04/01/2026
The vulnerability identified as CVE-2026-27659 represents a critical cross-site request forgery weakness in Mattermost server implementations across multiple version branches including 11.2.x through 11.2.2, 10.11.x through 10.11.10, 11.4.x through 11.4.0, and 11.3.x through 11.3.1. This flaw specifically affects the /api/v4/access_control_policies/{policy_id}/activate endpoint which governs the activation status of access control policies within the Mattermost platform. The vulnerability stems from inadequate validation of CSRF tokens, which are essential security mechanisms designed to prevent unauthorized commands from being executed on behalf of authenticated users. When CSRF token validation fails, malicious actors can exploit this weakness to perform unauthorized actions against the system.
The technical nature of this vulnerability places it squarely within the scope of CWE-352, which specifically addresses Cross-Site Request Forgery (CSRF) weaknesses in software applications. The flaw manifests when an attacker crafts a malicious request that targets the access control policy activation endpoint, potentially allowing them to modify critical security configurations without proper authorization. This represents a significant operational risk as access control policies govern user permissions, data access, and system security boundaries within Mattermost environments. The attack vector typically involves tricking an authenticated administrator into executing a malicious request through social engineering or by leveraging a compromised user session, where the attacker crafts a request that appears legitimate to the server but is actually designed to manipulate access control settings.
The operational impact of this vulnerability extends beyond simple privilege escalation as it directly affects the security posture of Mattermost installations. An attacker who successfully exploits this vulnerability could potentially activate or deactivate access control policies to gain unauthorized access to restricted resources, modify user permissions, or create backdoor access paths within the communication platform. This threat is particularly concerning given Mattermost's role as a collaboration platform where access control policies typically govern sensitive communications, document sharing, and team-based security configurations. The vulnerability also aligns with ATT&CK technique T1548.001 which covers privilege escalation through abuse of credentials, and T1071.004 which involves application layer protocol usage for command and control. Organizations using affected Mattermost versions face significant risk of unauthorized access to their communication infrastructure and potential data breaches.
Mitigation strategies for this vulnerability should prioritize immediate patching of affected Mattermost versions to the latest stable releases that contain proper CSRF token validation mechanisms. Organizations should also implement additional security controls such as network segmentation, monitoring for unusual access control policy modifications, and enhanced user session management practices. Security teams should conduct thorough audits of access control policies and monitor for any unauthorized changes to policy activation status. The implementation of additional authentication factors and enhanced logging around access control policy modifications would provide defense-in-depth measures against exploitation attempts. Organizations should also review their incident response procedures to ensure rapid detection and remediation of potential CSRF attacks targeting their Mattermost installations. Given the severity of this vulnerability, immediate action is required to protect organizational communication platforms from potential compromise.