CVE-2018-16466 in Server
Summary
by MITRE
Improper revalidation of permissions in Nextcloud Server prior to 14.0.0, 13.0.6 and 12.0.11 lead to not accepting access restrictions by acess tokens.
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Analysis
by VulDB Data Team • 04/07/2020
The vulnerability identified as CVE-2018-16466 represents a critical authorization flaw in Nextcloud Server versions prior to 14.0.0, 13.0.6, and 12.0.11. This issue stems from improper revalidation of permissions within the application's access control mechanism, specifically affecting how the system handles access tokens and their associated restrictions. The flaw occurs at the core authentication and authorization layer where the system fails to properly validate or recheck user permissions when processing token-based access requests.
The technical implementation of this vulnerability allows malicious actors to bypass intended access controls by exploiting a weakness in the permission validation process. When Nextcloud processes access tokens, it should revalidate the user's current permissions against the token's granted scope, but this revalidation mechanism was not properly implemented or maintained. This creates a scenario where an attacker could potentially use a valid access token to access resources beyond what the token's original permissions should allow, effectively escalating their privileges without proper authorization checks.
The operational impact of this vulnerability is significant as it undermines the fundamental security model of Nextcloud's token-based access system. Attackers could exploit this flaw to gain unauthorized access to files, folders, and system resources that should be restricted to specific users or roles. The vulnerability affects the integrity and confidentiality of data stored within Nextcloud instances, as unauthorized users could potentially access sensitive information or perform operations they should not be permitted to execute. This issue particularly impacts organizations relying on Nextcloud for file sharing and collaboration, where proper access controls are essential for maintaining data security.
From a cybersecurity perspective, this vulnerability aligns with CWE-284 (Improper Access Control) and represents a failure in the principle of least privilege enforcement. The flaw demonstrates poor implementation of the authorization validation process and could be categorized under ATT&CK technique T1078 (Valid Accounts) as attackers might leverage compromised tokens to maintain persistent access. Organizations should implement immediate mitigations including upgrading to patched versions of Nextcloud, reviewing existing access token configurations, and monitoring for unauthorized access attempts. Additionally, security teams should consider implementing additional logging and monitoring around token usage and permission changes to detect potential exploitation attempts. The vulnerability highlights the importance of proper permission revalidation in web applications and the critical need for thorough security testing of authentication and authorization mechanisms.