CVE-2018-16464 in Server
Summary
by MITRE
A missing access check in Nextcloud Server prior to 14.0.0 could lead to continued access to password protected link shares when the owner had changed the password.
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Analysis
by VulDB Data Team • 04/07/2020
The vulnerability identified as CVE-2018-16464 represents a critical access control flaw within Nextcloud Server versions prior to 14.0.0 that fundamentally undermines the security of password protected link shares. This issue arises from a missing access check mechanism that fails to properly validate user credentials when accessing shared resources, creating a persistent security gap that allows unauthorized users to maintain access even after the original share owner has updated the password protection. The flaw specifically affects the authentication flow for link shares, where the system should verify that the current password matches the one stored in the system's database before granting access to the shared content. When this verification step is omitted or improperly implemented, it creates a scenario where previously accessed shares remain accessible regardless of password changes, effectively bypassing the intended security controls.
The technical implementation of this vulnerability stems from inadequate session management and authentication validation within the Nextcloud server architecture. When a user accesses a password protected link share, the system should perform a fresh authentication check against the current password stored in the database. However, due to the missing access check, the system fails to validate the password against the most recent version, allowing access to remain valid even when the share owner has updated their password. This represents a classic case of improper input validation and access control enforcement, aligning with CWE-284 which addresses "Improper Access Control" in software systems. The vulnerability essentially creates a time window where access tokens or session identifiers become permanently valid, regardless of password modifications, fundamentally weakening the authentication model.
The operational impact of CVE-2018-16464 extends beyond simple unauthorized access to potentially expose sensitive data and compromise the integrity of shared resources within Nextcloud environments. Organizations relying on Nextcloud for file sharing and collaboration may experience data leakage when share owners change passwords, as the system fails to invalidate existing access permissions. This vulnerability directly conflicts with the principle of least privilege and can lead to unauthorized data exposure, particularly in environments where sensitive business information, personal data, or confidential documents are shared through link shares. The impact is amplified in scenarios where multiple users access the same shared resources, as the system becomes vulnerable to prolonged unauthorized access periods that could go unnoticed for extended durations.
Security professionals should consider this vulnerability in the context of the ATT&CK framework, specifically under the T1078 technique for Valid Accounts, as it enables persistent access to systems through compromised authentication mechanisms. The vulnerability also relates to T1566 which covers Social Engineering, as it can be exploited by attackers who gain initial access through legitimate means and then maintain access even after password changes. Organizations should implement immediate mitigations including upgrading to Nextcloud Server version 14.0.0 or later, where the access control mechanisms have been properly implemented. Additionally, administrators should review existing link shares and revoke access to any shares that may have been compromised, while implementing monitoring solutions that can detect anomalous access patterns or unauthorized access attempts to shared resources. The vulnerability highlights the critical importance of proper access control implementation and demonstrates how a single missing validation step can create persistent security weaknesses in file sharing systems.