CVE-2019-20890 in Mattermost Server
Summary
by MITRE
An issue was discovered in Mattermost Server before 5.7. It allows a bypass of e-mail address discovery restrictions.
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Analysis
by VulDB Data Team • 10/25/2020
The vulnerability identified as CVE-2019-20890 represents a significant security flaw in Mattermost Server versions prior to 5.7, specifically targeting the email address discovery mechanisms that are fundamental to user identification and communication within the platform. This issue falls under the category of information disclosure vulnerabilities, where unauthorized parties can potentially access email addresses that should otherwise be restricted or protected. The flaw exploits weaknesses in the server's access control and authorization logic, allowing attackers to bypass established restrictions that typically prevent users from discovering email addresses of other users without proper authentication or authorization. This type of vulnerability can be particularly dangerous in enterprise environments where user privacy and data protection are paramount considerations.
The technical implementation of this vulnerability stems from insufficient validation and access control checks within the Mattermost Server's email discovery functionality. Attackers can exploit this flaw to enumerate user email addresses through various API endpoints or direct requests that should normally require proper authentication or administrative privileges. The vulnerability likely exists in the server's response handling logic where it fails to properly verify user permissions before returning email address information. This flaw may be related to improper input validation, lack of role-based access control enforcement, or insufficient session management that allows unauthenticated or unauthorized users to access restricted email address data. The vulnerability can be classified under CWE-200 (Information Exposure) and potentially CWE-284 (Improper Access Control) depending on the specific implementation details.
The operational impact of CVE-2019-20890 extends beyond simple information disclosure, as email addresses are often considered sensitive user data that can be leveraged for social engineering attacks, credential stuffing, or phishing campaigns. When attackers can bypass email address discovery restrictions, they gain the ability to build comprehensive user directories that can be used for targeted attacks against specific individuals within the organization. This vulnerability particularly affects collaborative platforms where user identification is crucial for communication and access control, potentially enabling attackers to escalate privileges or conduct more sophisticated attacks. The exposure of email addresses can also violate privacy regulations and data protection standards such as GDPR or HIPAA, depending on the nature of the organization using Mattermost. From an attack chain perspective, this vulnerability aligns with ATT&CK technique T1589 (Steal Web Session ID) and T1078 (Valid Accounts) as it enables unauthorized access to user information that can be used for further exploitation.
Organizations using Mattermost Server versions prior to 5.7 should implement immediate mitigations including upgrading to version 5.7 or later where the vulnerability has been patched. Additional defensive measures should include implementing network-level restrictions on API endpoints that handle email discovery, enabling enhanced logging and monitoring for suspicious email address enumeration attempts, and reviewing existing access control policies to ensure proper enforcement of user permissions. The vulnerability demonstrates the importance of proper access control implementation and the need for comprehensive security testing of authentication mechanisms in collaborative platforms. Security teams should also consider implementing rate limiting and IP-based restrictions on email discovery API calls to prevent automated enumeration attacks. Regular security assessments of communication platforms should include testing for similar access control bypass vulnerabilities to prevent similar issues from emerging in other components of the system.