CVE-2018-21251 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.2 and 5.1.1. Authorization could be bypassed if the channel name were not the same in the params and the body.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2018-21251 represents a critical authorization bypass flaw within the Mattermost Server platform that affected versions prior to 5.2 and 5.1.1. This issue stems from a fundamental inconsistency in how the server validates channel names during API requests, creating a pathway for unauthorized users to access channels they should not have permission to view or interact with. The flaw specifically manifests when there is a discrepancy between channel identifiers provided in the request parameters versus those included in the request body, allowing malicious actors to exploit this inconsistency for privilege escalation.
The technical implementation of this vulnerability resides in the server's channel access control mechanism, where the authorization logic fails to properly synchronize channel name validation between different parts of the HTTP request. When a user attempts to access a channel through the Mattermost API, the system should verify that the channel identifier matches across all relevant request components to prevent unauthorized access. However, the flawed validation process allows attackers to manipulate either the parameters or body of the request to bypass these checks, effectively creating a condition where channel membership verification becomes unreliable. This type of authorization bypass vulnerability aligns with CWE-285, which specifically addresses improper authorization issues in software systems, and can be categorized under ATT&CK technique T1078 for valid accounts used for unauthorized access.
The operational impact of this vulnerability extends beyond simple unauthorized channel access, as it fundamentally undermines the security model of the Mattermost collaboration platform. Organizations relying on Mattermost for secure communication and collaboration could experience data leakage, unauthorized access to sensitive discussions, and potential compromise of confidential information shared within protected channels. The vulnerability affects the core integrity of the platform's access control system, potentially allowing attackers to escalate privileges and access channels containing sensitive business information, user communications, or proprietary data. Security administrators would be unable to rely on the channel-based access controls to protect their organization's communication channels, creating a significant risk for enterprises that depend on Mattermost for secure team collaboration and enterprise communication.
Mitigation strategies for CVE-2018-21251 require immediate patching of affected Mattermost Server installations to versions 5.2 or 5.1.1, which contain the necessary fixes for the authorization bypass issue. Organizations should also implement network-level monitoring to detect unusual API access patterns that might indicate exploitation attempts, particularly focusing on requests where channel identifiers show inconsistencies between parameters and request bodies. Security teams should conduct comprehensive audits of channel access controls and user permissions to identify any potential unauthorized access that may have occurred during the vulnerability window. Additionally, implementing proper input validation and request consistency checks at the application level can help prevent similar issues in other components of the system, while regular security assessments and penetration testing should be conducted to identify and remediate other potential authorization bypass vulnerabilities within the platform's architecture.