CVE-2019-20876 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.9.0, 5.8.1, 5.7.3, and 4.10.8. Users can deactivate themselves, bypassing a policy.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20876 represents a critical authorization bypass flaw in Mattermost Server versions prior to 5.9.0, 5.8.1, 5.7.3, and 4.10.8. This issue allows authenticated users to deactivate their own accounts without proper administrative approval or policy enforcement mechanisms. The flaw stems from insufficient access control validation within the user deactivation workflow, where the system fails to properly verify whether account deactivation requests comply with organizational security policies. This vulnerability specifically affects the privilege escalation and user management components of the Mattermost platform, creating a scenario where individual users can circumvent administrative controls designed to prevent unauthorized account modifications.
The technical implementation of this vulnerability lies in the lack of proper authorization checks during the user deactivation process. When a user attempts to deactivate their account, the system should validate against configured policies that may restrict such actions or require administrative approval. However, the vulnerable versions fail to enforce these checks, allowing any authenticated user to execute the deactivation function regardless of organizational security requirements. This design flaw creates a path for both malicious insiders and accidental policy violations, as users can bypass controls that would normally require administrator intervention or approval workflows. The vulnerability manifests through the application programming interface or web interface where user deactivation functions are exposed, with the system accepting the deactivation request without proper validation of user permissions or organizational policies.
The operational impact of this vulnerability extends beyond simple account management concerns, potentially enabling broader security risks within organizations that rely on Mattermost for communication and collaboration. When users can bypass deactivation policies, it creates opportunities for unauthorized access to sensitive information, especially in environments where account deactivation is required upon employee termination or security incidents. The vulnerability undermines the principle of least privilege by allowing users to modify their own access status without proper oversight, potentially enabling data exfiltration or continued access to restricted resources after appropriate deactivation should have occurred. This weakness particularly affects organizations implementing zero-trust security models where strict access controls and audit trails are essential for maintaining security boundaries and compliance requirements.
Organizations should implement immediate mitigations including upgrading to Mattermost Server versions 5.9.0, 5.8.1, 5.7.3, or 4.10.8 where the vulnerability has been patched. The patch addresses the authorization bypass by implementing proper access control validation before allowing account deactivation requests to proceed. Security teams should also review existing user deactivation policies and implement additional monitoring controls to detect unauthorized account deactivation attempts. The vulnerability aligns with CWE-284, which describes improper access control in software systems, and maps to ATT&CK technique T1078 for valid accounts and T1566 for credential access. Organizations should conduct comprehensive security assessments of their Mattermost implementations to identify any other potential bypass vulnerabilities and ensure that all user management functions properly enforce organizational security policies. The remediation process should include verifying that administrative approval workflows are properly implemented and that audit logs capture all account deactivation activities for compliance and forensic purposes.