CVE-2016-9464 in Nextcloud Server
Summary
by MITRE
Nextcloud Server before 9.0.54 and 10.0.0 suffers from an improper authorization check on removing shares. The Sharing Backend as implemented in Nextcloud does differentiate between shares to users and groups. In case of a received group share, users should be able to unshare the file to themselves but not to the whole group. The previous API implementation simply unshared the file to all users in the group.
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Analysis
by VulDB Data Team • 11/22/2022
The vulnerability described in CVE-2016-9464 represents a critical authorization flaw within the Nextcloud server software that affects versions prior to 9.0.54 and 10.0.0. This issue stems from an improper implementation of the sharing backend functionality that governs how files and folders are shared between users and groups within the Nextcloud ecosystem. The vulnerability specifically targets the authorization mechanisms that should enforce granular access controls when users attempt to remove shared content, creating a scenario where individual users can inadvertently or maliciously remove access for entire groups rather than just themselves.
The technical flaw manifests in the sharing backend's failure to properly distinguish between different types of shares when processing removal requests. When a user receives a file share through a group membership, the system should only permit that individual user to revoke their own access to the shared content while maintaining the group's access permissions. However, the vulnerable implementation incorrectly processes share removal requests by applying the unshare operation to all members of the group rather than to the specific user who initiated the request. This fundamental misconfiguration violates core principles of access control and privilege management, allowing unauthorized actions that can lead to complete loss of access for group members.
The operational impact of this vulnerability extends beyond simple access control breaches and can result in significant data exposure and access disruption within Nextcloud environments. An attacker who exploits this vulnerability can effectively remove shared content access for entire groups of users simultaneously, potentially causing data unavailability for business operations or exposing sensitive information to unauthorized parties. The flaw particularly affects organizations that rely heavily on group-based sharing mechanisms, where the removal of a single user's access could inadvertently impact dozens or hundreds of group members. This creates a cascading effect that can compromise data integrity and availability across shared workspaces.
This vulnerability aligns with CWE-284, which describes improper access control, and represents a classic example of insufficient authorization checks in web applications. The flaw demonstrates the importance of implementing proper privilege separation and user-specific access controls, particularly in collaborative environments where multiple users interact with shared resources. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques that could enable adversaries to manipulate shared resource access controls and potentially gain broader system access through compromised sharing mechanisms.
Organizations should immediately implement the available security patches for Nextcloud versions 9.0.54 and 10.0.0 to address this vulnerability. Additionally, administrators should conduct thorough reviews of existing group share configurations and implement monitoring for unusual share removal activities. The mitigation strategy should include verifying that individual user access controls are properly enforced and that group share removal operations are limited to the specific user initiating the request rather than affecting all group members. Regular security audits of sharing configurations and access control mechanisms should be implemented to prevent similar authorization flaws from occurring in other system components.