CVE-2017-0883 in Server
Summary
by MITRE
Nextcloud Server before 9.0.55 and 10.0.2 suffers from a permission increase on re-sharing via OCS API issue. A permission related issue within the OCS sharing API allowed an authenticated adversary to reshare shared files with an increasing permission set. This may allow an attacker to edit files in a share despite having only a 'read' permission set. Note that this only affects folders and files that the adversary has at least read-only permissions for.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 08/25/2020
The vulnerability identified as CVE-2017-0883 represents a critical access control flaw within Nextcloud Server versions prior to 9.0.55 and 10.0.2, specifically affecting the OCS (Open Collaboration Services) sharing API implementation. This issue stems from improper permission validation during the re-sharing process, creating a privilege escalation vector that allows authenticated users to manipulate their access rights to shared resources. The flaw manifests when users attempt to re-share files or folders they have received through the sharing system, enabling them to incrementally increase their permission levels beyond the original authorization boundaries.
The technical root cause of this vulnerability lies in the insufficient validation of permission levels during the OCS API re-sharing operations. When an authenticated user attempts to share content they have received, the system fails to properly enforce the original permission constraints, allowing the re-sharing user to grant themselves additional privileges such as edit permissions even when they initially only possessed read-only access. This represents a classic case of insufficient authorization checks and improper privilege management within the sharing subsystem. The vulnerability specifically impacts the OCS sharing API which is used for interoperability with other applications and clients, making it a particularly concerning issue given the widespread adoption of this API standard.
The operational impact of CVE-2017-0883 extends beyond simple data exposure, as it enables unauthorized modification of content that users should not have the ability to alter. An attacker with read-only access to a shared folder or file could potentially escalate their privileges to edit or modify content, undermining the fundamental security model of the sharing system. This vulnerability particularly affects collaborative environments where users rely on granular permission controls to maintain data integrity and confidentiality. The issue is not limited to direct file access but extends to the broader sharing ecosystem, potentially allowing attackers to compromise entire sharing chains and create persistent backdoors within shared environments.
Security practitioners should note that this vulnerability aligns with CWE-284 (Improper Access Control) and can be mapped to ATT&CK technique T1078 (Valid Accounts) as it exploits legitimate user accounts to gain unauthorized access rights. The flaw demonstrates how seemingly benign sharing functionality can be weaponized to undermine security controls, particularly in environments where users frequently re-share content. Organizations using Nextcloud Server should implement immediate patching strategies and conduct thorough audits of shared content to identify potential privilege escalation that may have occurred. The vulnerability serves as a reminder of the critical importance of proper access control validation in collaborative platforms and highlights the need for comprehensive security testing of sharing and collaboration features. Remediation efforts should focus on ensuring that permission levels are properly enforced during re-sharing operations and that all access control decisions are validated against the original authorization boundaries to prevent unauthorized privilege escalation.