CVE-2018-21263 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 4.7.0, 4.6.2, and 4.5.2. An attacker could authenticate to a different user's account via a crafted SAML response.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2018-21263 represents a critical authentication bypass flaw in Mattermost Server versions prior to 4.7.0, 4.6.2, and 4.5.2. This issue specifically affects systems utilizing Security Assertion Markup Language SAML authentication mechanisms, which are commonly deployed in enterprise environments to enable single sign-on functionality. The flaw allows malicious actors to exploit weaknesses in the SAML response processing logic, enabling them to impersonate legitimate users within the Mattermost collaboration platform.
The technical root cause of this vulnerability lies in insufficient validation of SAML response parameters during the authentication process. When Mattermost receives a SAML response from an identity provider, the server fails to properly verify the authenticity and integrity of the user identifier contained within the response. This weakness creates an opportunity for attackers to craft malicious SAML responses that contain forged user identifiers, effectively allowing them to authenticate as any user account within the system. The vulnerability specifically impacts the SAML assertion processing module where the system should validate the subject identifier against expected user attributes but instead accepts potentially manipulated values.
The operational impact of this vulnerability extends beyond simple unauthorized access, as it provides attackers with the ability to escalate privileges and gain persistent access to sensitive communication channels. Once authenticated as a different user, attackers can access private conversations, view confidential documents, send messages on behalf of legitimate users, and potentially exfiltrate sensitive data from the Mattermost environment. This authentication bypass can be particularly damaging in regulated environments where audit trails and user accountability are critical requirements, as the malicious activity would be logged under the compromised user's credentials rather than the attacker's identity.
Organizations utilizing Mattermost with SAML authentication should prioritize immediate remediation through the deployment of patched server versions. The vulnerability aligns with CWE-287 which addresses improper authentication issues in authentication protocols, and maps to ATT&CK technique T1078.001 for valid accounts used for lateral movement. Security teams should also implement additional monitoring controls to detect unusual authentication patterns and consider implementing multi-factor authentication as a compensating control. Network segmentation and access controls should be reviewed to limit the potential impact of successful exploitation, while SAML configuration parameters should be audited to ensure proper validation of assertion attributes. The vulnerability demonstrates the critical importance of proper input validation in authentication systems and highlights the need for comprehensive security testing of identity integration components.