CVE-2018-3775 in Nextcloud Server
Summary
by MITRE
Improper Authentication in Nextcloud Server prior to version 12.0.3 would allow an attacker that obtained user credentials to bypass the 2 Factor Authentication.
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Analysis
by VulDB Data Team • 03/15/2020
The vulnerability identified as CVE-2018-3775 represents a critical authentication flaw in Nextcloud Server versions prior to 12.0.3 that fundamentally undermines the security posture of user accounts. This issue stems from a design weakness in how the system handles authentication flows, specifically creating a pathway where authenticated users can bypass the mandatory two-factor authentication mechanism. The vulnerability operates at the intersection of authentication and authorization controls, creating a scenario where an attacker who has already compromised user credentials can exploit a flaw in the authentication sequence to gain unauthorized access without completing the second factor verification process.
The technical implementation of this vulnerability occurs within the Nextcloud authentication subsystem where the system fails to properly validate the authentication state after initial credential verification. When a user successfully provides their username and password, the system should enforce the mandatory second factor authentication before granting full access to the account. However, in affected versions, this validation step becomes bypassable, allowing attackers to leverage stolen credentials to access accounts that have 2FA enabled. This flaw essentially creates a backdoor in the authentication flow where the system's security controls are circumvented through improper state management and insufficient validation checks. The vulnerability demonstrates a classic failure in authentication protocol implementation where the system does not properly maintain the authenticated state throughout the multi-factor verification process, enabling attackers to manipulate the authentication sequence.
The operational impact of this vulnerability extends far beyond simple credential theft, as it effectively neutralizes the security benefits of two-factor authentication for affected Nextcloud installations. Organizations relying on Nextcloud for file storage and collaboration services face significant risk exposure when this vulnerability is present, as attackers can exploit it to gain access to sensitive user data without the need to overcome the second authentication factor. This creates a scenario where even users who have properly configured 2FA protection become vulnerable to unauthorized access, fundamentally weakening the security model that organizations have implemented to protect their data. The vulnerability is particularly concerning in enterprise environments where Nextcloud serves as a primary collaboration platform, as it allows attackers to bypass security controls that were specifically implemented to prevent unauthorized access to corporate data and user information.
Security mitigations for this vulnerability require immediate patching of Nextcloud Server installations to version 12.0.3 or later, where the authentication flow has been properly corrected to maintain the integrity of the multi-factor authentication process. Organizations should also implement additional monitoring of authentication events to detect anomalous access patterns that might indicate exploitation attempts. The fix addresses the root cause by ensuring proper state management during authentication flows and implementing robust validation checks that prevent the bypass of mandatory second factor authentication. This vulnerability aligns with CWE-287, which addresses improper authentication issues, and represents a clear violation of the principle of least privilege in authentication systems. From an ATT&CK framework perspective, this vulnerability maps to T1078, which covers valid accounts and T1531, which addresses account access removal, as it allows adversaries to maintain access through compromised credentials without proper authentication controls. Organizations should also consider implementing additional security measures such as account lockout policies and enhanced monitoring of authentication events to further reduce the risk of exploitation.