CVE-2021-39905 in Community Edition
Summary
by MITRE • 11/05/2021
An information disclosure vulnerability in the GitLab CE/EE API since version 8.9.6 allows a user to see basic information on private groups that a public project has been shared with
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Analysis
by VulDB Data Team • 11/09/2021
The information disclosure vulnerability identified as CVE-2021-39905 affects GitLab Community Edition and Enterprise Edition versions 8.9.6 and later, representing a significant security flaw in the platform's access control mechanisms. This vulnerability stems from improper authorization checks within the GitLab API that governs group membership and project sharing permissions. The flaw allows authenticated users to exploit the system's API endpoints to discover metadata about private groups that contain public projects they have access to, effectively bypassing intended security boundaries that should prevent such information leakage.
The technical implementation of this vulnerability occurs through the GitLab API's response handling for project-related queries where the system fails to properly validate whether the requesting user has legitimate access to view the group information. When a user accesses a public project's API endpoint, the system inadvertently returns information about private groups that have been granted access to that project, including group names, identifiers, and potentially other metadata that should remain confidential. This occurs because the API response logic does not adequately filter or restrict the information returned based on the user's actual authorization level for each group in question.
The operational impact of this vulnerability extends beyond simple information disclosure, creating potential risks for organizations that rely heavily on GitLab for code repository management and collaboration. Attackers could leverage this vulnerability to map out the organizational structure of private groups, identify sensitive project relationships, and potentially discover additional attack vectors through the mapping of group memberships. The vulnerability particularly affects environments where public projects are shared with private groups containing sensitive or restricted information, as it allows unauthorized users to gain insights into the sharing relationships and organizational access patterns.
This vulnerability aligns with CWE-200, which categorizes information disclosure flaws in software systems, and demonstrates characteristics consistent with the ATT&CK technique T1087.001 for account discovery and T1566.001 for malicious file execution through information gathering. Organizations using GitLab should consider implementing immediate mitigations including API rate limiting, enhanced access control logging, and comprehensive monitoring of API endpoint access patterns for unusual query behavior. The vulnerability also highlights the importance of proper input validation and output filtering in API implementations, particularly when dealing with multi-tenant environments where different user groups have varying levels of access permissions. Security teams should conduct thorough audits of their GitLab installations to identify other potential API access control issues and ensure that all group membership information is properly protected through robust authorization mechanisms that prevent unauthorized access to private group metadata.