CVE-2018-16465 in Server
Summary
by MITRE
Missing state in Nextcloud Server prior to 14.0.0 would not enforce the use of a second factor at login if the the provider of the second factor failed to load.
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Analysis
by VulDB Data Team • 04/07/2020
The vulnerability identified as CVE-2018-16465 represents a critical authentication weakness in Nextcloud Server versions prior to 14.0.0 that fundamentally undermines multi-factor authentication security controls. This flaw stems from inadequate state management within the authentication framework, specifically failing to properly validate the availability and functionality of secondary authentication factors during the login process. The vulnerability manifests when the system encounters issues loading the second factor provider, yet continues to grant access without requiring the additional authentication step, effectively bypassing the intended security mechanism.
This security gap directly relates to CWE-305 Authentication Bypass Through User-Controllable Key, as it allows attackers to circumvent the multi-factor authentication requirements through manipulation of the authentication state. The flaw operates at the application layer within the authentication subsystem, specifically affecting how the system handles failed provider loading scenarios. When a secondary factor provider fails to initialize or becomes unavailable due to network issues, configuration errors, or service disruptions, the Nextcloud server fails to enforce the mandatory second factor requirement, leaving accounts vulnerable to unauthorized access.
The operational impact of this vulnerability is significant as it creates a window of opportunity for attackers to exploit the authentication system during provider failure conditions. An attacker could potentially leverage this weakness by either intentionally disrupting the second factor service or by exploiting timing conditions where the provider fails to load properly. This creates a scenario where legitimate users might be granted access without proper authentication, while malicious actors could exploit the same condition to bypass security controls entirely. The vulnerability affects the core principle of defense in depth, as it removes an essential layer of security that should be mandatory regardless of provider availability.
Mitigation strategies for this vulnerability require immediate implementation of Nextcloud Server version 14.0.0 or later, which includes proper state management for authentication factors. Organizations should also implement monitoring for authentication failures and provider loading issues to detect potential exploitation attempts. The fix addresses this through improved error handling that ensures authentication factors are properly validated before granting access, implementing a fail-closed approach rather than fail-open. Security teams should conduct comprehensive testing of authentication workflows to ensure that all secondary factor providers are properly initialized and that the system correctly enforces multi-factor requirements even during partial service disruptions. Additionally, implementing robust logging and alerting mechanisms around authentication events will help detect and respond to potential exploitation attempts that could leverage this vulnerability.