CVE-2018-3761 in Nextcloud Server
Summary
by MITRE
Nextcloud Server before 12.0.8 and 13.0.3 suffer from improper authentication on the OAuth2 token endpoint. Missing checks potentially allowed handing out new tokens in case the OAuth2 client was partly compromised.
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Analysis
by VulDB Data Team • 02/25/2020
The vulnerability described in CVE-2018-3761 represents a critical authentication flaw within Nextcloud Server versions prior to 12.0.8 and 13.0.3, specifically affecting the OAuth2 token endpoint implementation. This issue stems from insufficient validation mechanisms that fail to properly verify the integrity of OAuth2 client requests, creating a pathway for unauthorized token issuance. The flaw occurs when OAuth2 clients experience partial compromise, allowing attackers to exploit the missing authentication checks to obtain new tokens without proper authorization. Such vulnerabilities fall under the CWE-287 category of Improper Authentication, which directly impacts the fundamental security posture of any system relying on OAuth2 for authentication and authorization processes. The vulnerability is particularly concerning because it undermines the core principles of OAuth2 security model where proper client authentication is essential for token issuance.
The technical implementation of this flaw manifests in the OAuth2 token endpoint's failure to validate client credentials adequately during token grant requests. When an OAuth2 client is partially compromised, attackers can leverage this vulnerability to perform unauthorized token generation by bypassing the standard authentication mechanisms that should verify client identity and legitimacy. The missing checks typically involve verification of client secrets, redirect URIs, or other client-specific identifiers that should be validated before token issuance occurs. This weakness creates a scenario where an attacker with access to compromised client credentials can potentially escalate privileges or gain unauthorized access to protected resources within the Nextcloud environment. The vulnerability aligns with ATT&CK technique T1566 which covers credential harvesting through social engineering and compromised credentials, and more specifically T1078 which addresses valid accounts and legitimate credentials for persistence and privilege escalation.
The operational impact of CVE-2018-3761 extends beyond simple unauthorized token issuance, potentially enabling broader security breaches within Nextcloud deployments. An attacker exploiting this vulnerability could gain access to user data, perform unauthorized file operations, or establish persistent access to the Nextcloud instance. The implications are particularly severe for organizations that rely heavily on Nextcloud for collaborative file sharing and document management, as compromised OAuth2 tokens could provide attackers with extended access to sensitive corporate information. This vulnerability also affects the integrity of the authentication system, potentially allowing attackers to impersonate legitimate users and access resources they should not have authorization to view or modify. Organizations using Nextcloud in environments where security is paramount, such as healthcare, finance, or government sectors, face heightened risk due to the potential for data breaches and unauthorized access to classified information.
Mitigation strategies for CVE-2018-3761 require immediate patching of affected Nextcloud installations to versions 12.0.8 or 13.0.3 where the authentication checks have been properly implemented. System administrators should also conduct thorough audits of all OAuth2 clients to identify and remediate any partially compromised systems that could be exploited. Additional security measures include implementing strict client credential management practices, monitoring token issuance patterns for suspicious activity, and ensuring that all OAuth2 clients are properly configured with appropriate redirect URI validation. Organizations should also consider implementing additional authentication layers such as multi-factor authentication for critical Nextcloud services and regular security assessments of their OAuth2 implementations. The vulnerability highlights the importance of proper authentication implementation in web applications and serves as a reminder of the critical need for comprehensive security testing and validation of authentication mechanisms, particularly those involving third-party integration and external service authentication flows.