CVE-2019-20855 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.16.1, 5.15.2, 5.14.5, and 5.9.6. It allows attackers to obtain sensitive information (local files) during legacy attachment migration.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20855 represents a critical information disclosure flaw within the Mattermost server software ecosystem. This security weakness affects multiple versions of the Mattermost platform including releases before 5.16.1, 5.15.2, 5.14.5, and 5.9.6, creating a significant risk for organizations relying on this communication platform for enterprise collaboration. The flaw specifically manifests during the legacy attachment migration process, which occurs when older versions of Mattermost are upgraded to newer releases that support enhanced file handling capabilities.
The technical implementation of this vulnerability stems from inadequate access controls and input validation during the migration of legacy attachments from older storage formats to newer ones. Attackers can exploit this weakness to gain unauthorized access to local files that should remain restricted within the server environment. This occurs because the migration process fails to properly validate or sanitize file paths and access permissions, allowing malicious actors to traverse the file system and retrieve sensitive data. The vulnerability essentially creates a path traversal condition that bypasses normal security boundaries designed to protect server resources.
The operational impact of CVE-2019-20855 extends beyond simple information disclosure, as it can potentially expose sensitive organizational data including configuration files, user credentials, system logs, and proprietary documents. Organizations using Mattermost for internal communications, compliance management, or secure collaboration may face significant risks when this vulnerability is exploited. The attack vector is particularly concerning because it can be executed during routine system maintenance activities such as version upgrades, making it difficult to detect and prevent. The vulnerability aligns with CWE-22 Path Traversal and CWE-200 Information Disclosure, both of which are classified as high-risk issues in the Common Weakness Enumeration catalog.
From a threat actor perspective, this vulnerability maps to several ATT&CK techniques including T1005 Local Data Staging and T1074 Data Staged, as attackers can leverage the migration process to access and exfiltrate sensitive information. The exploitation requires minimal privileges and can be automated, making it attractive for both internal threat actors and external attackers targeting organizations using vulnerable Mattermost installations. Security teams should note that this vulnerability may not be immediately apparent during routine security scanning, as it only manifests during specific migration operations. Organizations should implement comprehensive monitoring of file access patterns during upgrade procedures and establish proper access controls for migration processes.
The recommended mitigations for CVE-2019-20855 involve immediate patching of affected Mattermost server installations to versions 5.16.1, 5.15.2, 5.14.5, or 5.9.6, depending on the organization's current deployment. Additionally, administrators should implement network segmentation to limit access to the Mattermost server during migration activities, establish proper file system permissions, and conduct thorough access reviews of legacy attachment migration processes. Organizations should also consider implementing automated monitoring solutions that can detect anomalous file access patterns during upgrade operations, which would help identify potential exploitation attempts. The vulnerability demonstrates the importance of proper input validation and access control implementation during system upgrade processes, particularly when dealing with legacy data migration scenarios.