CVE-2019-20863 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.13.0. Incoming webhook creation is not properly restricted.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20863 represents a critical access control flaw in Mattermost Server versions prior to 5.13.0, specifically affecting the incoming webhook creation functionality. This issue stems from insufficient authorization checks during the webhook creation process, allowing unauthorized users to create webhooks that can subsequently be used to inject malicious data into the system. The flaw exists within the server's permission model where the validation mechanisms fail to properly verify user credentials and privileges before permitting webhook creation operations. This vulnerability directly impacts the integrity and security posture of Mattermost deployments by enabling potential attackers to bypass normal access controls and establish communication channels with the server.
The technical implementation of this vulnerability resides in the server-side webhook creation endpoint where proper authentication and authorization checks are either missing or inadequately enforced. When users attempt to create incoming webhooks, the system should validate that the requesting user possesses the necessary permissions to perform this action, typically requiring administrative privileges or specific roles within the Mattermost organization. However, in affected versions, the validation logic fails to properly authenticate the user context or verify that the user has the appropriate permissions to create webhooks, creating a path for privilege escalation. This flaw can be exploited through various attack vectors including social engineering, credential compromise, or by leveraging other existing vulnerabilities to gain access to user accounts with sufficient privileges to create webhooks.
The operational impact of CVE-2019-20863 extends beyond simple unauthorized webhook creation, as it can be leveraged for more sophisticated attacks within the Mattermost environment. An attacker who successfully exploits this vulnerability can create malicious webhooks that forward data to external systems, potentially enabling data exfiltration or command execution. The threat model for this vulnerability aligns with ATT&CK technique T1078.004 which covers valid accounts used for lateral movement and persistence, as the attacker can use created webhooks to maintain access and establish communication channels. Additionally, this vulnerability can be classified under CWE-285 which addresses improper authorization in software systems, specifically focusing on the failure to properly enforce access controls for privileged operations. The risk is particularly elevated in environments where Mattermost serves as a communication platform for sensitive information sharing, as the created webhooks can be used to intercept or manipulate communications between team members.
Organizations using Mattermost Server versions prior to 5.13.0 should immediately implement mitigations including upgrading to the patched version where proper authorization controls have been restored. The upgrade process addresses the root cause by implementing comprehensive validation checks that ensure only authorized users can create incoming webhooks within the system. Additional defensive measures include monitoring webhook creation activities through audit logs and implementing network-level restrictions to prevent unauthorized access to webhook endpoints. Security teams should also review existing webhook configurations to identify any potentially malicious webhooks that may have been created by compromised accounts. The vulnerability demonstrates the critical importance of proper access control implementation in collaborative platforms where multiple users require different levels of system access, as inadequate authorization checks can lead to significant security breaches and data compromise across the entire communication ecosystem.