CVE-2018-16467 in Server
Summary
by MITRE
A missing check in Nextcloud Server prior to 14.0.0 could give unauthorized access to the previews of single file password protected shares.
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Analysis
by VulDB Data Team • 04/07/2020
The vulnerability identified as CVE-2018-16467 represents a critical access control flaw in Nextcloud Server versions prior to 14.0.0 that undermines the security of password protected file shares. This issue stems from a missing validation mechanism that should have enforced proper authentication checks before granting access to file previews within protected shares. The flaw specifically affects the preview generation functionality where users could potentially access previews of files contained within password protected shares without proper authorization. This vulnerability operates at the intersection of access control and preview generation services, creating an unexpected information disclosure channel that bypasses the intended security boundaries of share protection mechanisms.
The technical implementation of this vulnerability manifests when Nextcloud processes requests for file previews within password protected shares. The system fails to validate whether the requesting user has proper authentication credentials for the specific share before serving preview content. This missing authorization check allows malicious actors to exploit the preview service endpoint to retrieve visual representations of files that should remain protected by share-level passwords. The flaw essentially creates a pathway where preview functionality becomes a vector for unauthorized information access, even when the underlying file access is properly restricted through password protection mechanisms. This type of vulnerability is classified under CWE-284 Access Control Issues, specifically representing inadequate access control enforcement where the system fails to properly verify user permissions before granting access to protected resources.
The operational impact of this vulnerability extends beyond simple information disclosure to potentially enable more sophisticated attacks within compromised environments. An attacker could leverage this flaw to gather intelligence about file types, sizes, and potentially content characteristics without needing to bypass the actual password protection mechanisms. This reconnaissance capability could significantly aid in planning more targeted attacks against the shared files or the broader system. The vulnerability affects organizations using Nextcloud Server versions before 14.0.0 where password protected shares are actively utilized, potentially exposing sensitive documents, media files, or other content that should remain restricted to authorized users only. From an attacker's perspective, this represents a low-effort, high-impact vector that could be automated to systematically gather information from multiple protected shares across an organization.
Organizations should immediately upgrade to Nextcloud Server version 14.0.0 or later to remediate this vulnerability, as no effective workarounds exist for the underlying access control flaw. The security patch addresses the missing authorization check in the preview service by ensuring that proper authentication validation occurs before any preview content is served. System administrators should conduct thorough audits of existing password protected shares to identify any potential exploitation attempts and implement additional monitoring for unusual preview access patterns. The vulnerability demonstrates the importance of comprehensive security testing across all service endpoints, particularly those that provide auxiliary functionality like previews that may not be immediately obvious as security-critical components. This flaw serves as a reminder that even seemingly benign services can become attack vectors when proper access controls are not consistently enforced throughout the application architecture.