CVE-2017-0936 in Nextcloud Server
Summary
by MITRE
Nextcloud Server before 11.0.7 and 12.0.5 suffers from an Authorization Bypass Through User-Controlled Key vulnerability. A missing ownership check allowed logged-in users to change the scope of app passwords of other users. Note that the app passwords themselves where neither disclosed nor could the error be misused to identify as another user.
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Analysis
by VulDB Data Team • 01/17/2020
The vulnerability identified as CVE-2017-0936 represents a critical authorization bypass flaw in Nextcloud Server versions prior to 11.0.7 and 12.0.5. This issue stems from a fundamental weakness in the application's access control mechanisms, specifically within the app password management system. The vulnerability manifests when logged-in users can manipulate the scope of app passwords belonging to other users without proper authorization. This authorization bypass occurs due to a missing ownership verification check that should have prevented users from modifying app password configurations they do not own. The flaw exists in the server-side validation logic where the system fails to properly authenticate the user requesting changes to another user's app password settings.
The technical implementation of this vulnerability allows for unauthorized privilege escalation through a user-controlled key mechanism. When users attempt to modify app password scopes, the system does not properly validate whether the requesting user has legitimate ownership rights over the target app password. This missing validation creates a pathway for malicious actors or compromised accounts to potentially expand their access privileges beyond their intended scope. The vulnerability is classified under CWE-285, which specifically addresses Authorization Bypass Through User-Controlled Key, a category that encompasses flaws where attackers can manipulate access control parameters through user-controllable inputs. The issue demonstrates a classic case of insufficient access control validation, where the system assumes proper authorization without adequate verification steps.
From an operational impact perspective, this vulnerability poses significant security risks to Nextcloud deployments, particularly in environments where multiple users share a single server instance. The ability to modify other users' app password scopes could potentially allow attackers to gain extended access to files, folders, and system resources that they would not normally be authorized to access. While the vulnerability does not directly expose app passwords themselves or enable user impersonation, it creates a vector for privilege escalation that could be combined with other attacks to achieve more severe outcomes. The flaw particularly affects organizations relying on app passwords for third-party application integration or automated processes, as unauthorized modifications could compromise the security posture of connected services.
Security mitigations for CVE-2017-0936 involve immediate deployment of patched Nextcloud Server versions 11.0.7 and 12.0.5, which include proper ownership validation checks for app password modifications. Organizations should also implement comprehensive monitoring of app password usage and modifications to detect anomalous activities that might indicate exploitation attempts. The fix addresses the underlying authorization bypass by enforcing proper ownership verification before allowing any changes to app password scopes, aligning with the principle of least privilege and proper access control implementation. Additionally, system administrators should review existing app password configurations and consider revoking and regenerating passwords for all users as a precautionary measure. This vulnerability highlights the importance of implementing robust access control validation mechanisms and adheres to ATT&CK technique T1078 which covers Valid Accounts and Defense Evasion through unauthorized access to legitimate user accounts. The remediation process should include thorough testing of the patched functionality to ensure that legitimate administrative operations continue to function properly while preventing the specific authorization bypass scenario that was exploited.