CVE-2017-0887 in Server
Summary
by MITRE
Nextcloud Server before 9.0.55 and 10.0.2 suffers from a bypass in the quota limitation. Due to not properly sanitizing values provided by the `OC-Total-Length` HTTP header an authenticated adversary may be able to exceed their configured user quota. Thus using more space than allowed by the administrator.
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Analysis
by VulDB Data Team • 08/25/2020
The vulnerability identified as CVE-2017-0887 affects Nextcloud Server versions prior to 9.0.55 and 10.0.2, representing a critical security flaw in the platform's storage quota enforcement mechanism. This issue stems from insufficient input validation within the server's handling of HTTP headers, specifically the OC-Total-Length header that is used to communicate file size information during upload operations. The flaw allows authenticated attackers to manipulate their storage usage by providing malicious values in this header field, effectively circumventing the administrative controls that limit user storage capacity. The vulnerability is particularly concerning because it directly impacts the integrity of the storage management system and could enable unauthorized consumption of server resources beyond the configured limits.
The technical implementation of this vulnerability resides in the improper sanitization of the OC-Total-Length HTTP header value, which is processed without adequate validation or filtering mechanisms. When users upload files through the Nextcloud interface, the system relies on this header to track and enforce storage quotas. However, due to the lack of proper input validation, an attacker who has authenticated access to the system can craft malicious requests with inflated values in the OC-Total-Length header. This allows them to upload files while the system incorrectly believes they are staying within their allocated quota limits. The flaw operates at the application layer and requires authentication, making it a privilege escalation issue rather than a direct denial-of-service vulnerability. According to CWE classification, this represents a weakness in input validation and data sanitization, specifically CWE-20: Improper Input Validation.
The operational impact of this vulnerability extends beyond simple resource consumption, as it fundamentally undermines the administrative controls that organizations rely upon to manage storage costs and prevent unauthorized usage. An attacker with access to a user account can potentially consume significantly more storage space than permitted, leading to increased infrastructure costs, potential denial of service for other legitimate users, and violation of organizational storage policies. The vulnerability is particularly dangerous in multi-tenant environments where administrators must ensure fair resource allocation among users. From an ATT&CK framework perspective, this vulnerability maps to T1078 Valid Accounts and T1496 Resource Hijacking, as it allows for unauthorized consumption of system resources through legitimate authenticated access. The flaw could also enable a broader attack chain where an attacker first establishes a foothold using legitimate credentials and then uses this quota bypass to store malicious files or exfiltrate data.
Mitigation strategies for CVE-2017-0887 require immediate patching of affected Nextcloud installations to versions 9.0.55 or 10.0.2, which contain the necessary fixes for proper header sanitization. Administrators should also implement additional monitoring of storage usage patterns to detect unusual consumption that might indicate exploitation attempts. Network-level controls can be deployed to monitor for suspicious HTTP header values, though this approach is less reliable than proper application-level fixes. Organizations should review their storage quota configurations and consider implementing additional layers of validation beyond the default system controls. The vulnerability highlights the importance of proper input validation in web applications and the need for comprehensive security testing of all HTTP header processing functions. Regular security audits should include verification of input sanitization mechanisms, particularly for headers that carry critical system information such as file sizes, content lengths, and user identifiers.