CVE-2017-0884 in Server
Summary
by MITRE
Nextcloud Server before 9.0.55 and 10.0.2 suffers from a creation of folders in read-only folders despite lacking permissions issue. Due to a logical error in the file caching layer an authenticated adversary is able to create empty folders inside a shared folder. Note that this only affects folders and files that the adversary has at least read-only permissions for.
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Analysis
by VulDB Data Team • 08/25/2020
The vulnerability identified as CVE-2017-0884 represents a critical access control flaw in Nextcloud Server versions prior to 9.0.55 and 10.0.2. This issue stems from a fundamental logical error within the file caching layer that undermines the integrity of permission enforcement mechanisms. The flaw allows authenticated adversaries to bypass intended read-only restrictions and create new folders within shared directories where they should only have read access. This vulnerability directly impacts the core security model of Nextcloud by enabling unauthorized modification of shared resources, effectively creating a privilege escalation vector through legitimate user authentication.
The technical root cause of this vulnerability lies in the file caching implementation where the system fails to properly validate write permissions during folder creation operations. When an authenticated user attempts to create a folder within a shared directory, the caching layer incorrectly processes the permission checks, allowing creation of empty directories despite the user lacking write permissions. This logical error occurs specifically within the file system abstraction layer that manages cached metadata and access control states. The vulnerability is categorized under CWE-284, which addresses improper access control, and represents a classic case of inadequate privilege validation in file system operations. The flaw demonstrates how caching mechanisms can introduce security weaknesses when permission checks are not consistently enforced across all system layers.
Operationally, this vulnerability enables adversaries to manipulate shared folder structures in ways that can compromise data integrity and potentially facilitate further attacks. An attacker with read-only access to a shared folder can exploit this flaw to create new directory structures that may be used for data exfiltration, steganography, or to establish persistent access points within the shared environment. The impact extends beyond simple folder creation as it undermines the trust model of shared resources, potentially allowing attackers to disrupt normal workflow patterns or create confusion in collaborative environments. This vulnerability affects organizations using Nextcloud for document sharing and collaboration, where unauthorized modifications could lead to compliance violations or data leakage. The attack requires only authentication credentials, making it particularly dangerous as it can be exploited by insiders or compromised accounts.
Mitigation strategies for CVE-2017-0884 primarily involve upgrading to Nextcloud Server versions 9.0.55 or 10.0.2, which contain the necessary patches to correct the permission validation logic in the file caching layer. Organizations should also implement additional monitoring controls to detect unauthorized folder creation activities within shared directories, particularly focusing on the creation of empty folders that may indicate exploitation attempts. Network segmentation and access control reviews should be conducted to minimize potential impact from compromised accounts. Security teams should also consider implementing automated compliance checks that verify folder permissions and detect inconsistencies in shared resource access patterns. The vulnerability demonstrates the importance of thorough testing of caching mechanisms and permission enforcement in collaborative platforms, aligning with ATT&CK technique T1078 for valid accounts and T1566 for credential harvesting as potential attack vectors that could be amplified by this flaw.