CVE-2017-0886 in Server
Summary
by MITRE
Nextcloud Server before 9.0.55 and 10.0.2 suffers from a Denial of Service attack. Due to an error in the application logic an authenticated adversary may trigger an endless recursion in the application leading to a potential Denial of Service.
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Analysis
by VulDB Data Team • 08/25/2020
The vulnerability identified as CVE-2017-0886 affects Nextcloud Server versions prior to 9.0.55 and 10.0.2, representing a critical denial of service weakness that can be exploited by authenticated attackers. This vulnerability stems from a logical error within the application's processing mechanisms that allows an attacker with valid credentials to initiate a recursive loop that consumes system resources and ultimately renders the service unavailable to legitimate users. The flaw specifically manifests as an infinite recursion condition that occurs during normal application operations when certain user inputs are processed through the server's internal logic pathways.
The technical implementation of this vulnerability involves a flaw in the application's input handling and processing flow where authenticated users can craft specific requests that trigger recursive function calls within the Nextcloud server codebase. This recursive behavior occurs when the server processes user data through pathways that lack proper termination conditions or depth limits, causing the application to enter an endless loop that consumes cpu cycles and memory resources. The vulnerability is classified under CWE-674 which describes "Uncontrolled Recursion" as a weakness that leads to resource exhaustion and system instability. The recursive nature of the flaw means that each invocation of the vulnerable code path can trigger additional recursive calls, exponentially increasing resource consumption until the system becomes unresponsive or crashes entirely.
From an operational perspective, this vulnerability presents a significant risk to Nextcloud deployments as it requires only authenticated access to exploit, meaning that any user with valid credentials can potentially disrupt service availability. The impact extends beyond simple service interruption as the recursive processing can cause the server to become unresponsive to all users, including administrators, effectively creating a complete denial of service condition. Attackers can leverage this vulnerability to target specific Nextcloud instances without requiring specialized tools or deep technical knowledge beyond having legitimate user credentials. The vulnerability affects the core functionality of the Nextcloud platform and can be particularly damaging in enterprise environments where continuous access to file sharing and collaboration services is critical for business operations.
Mitigation strategies for CVE-2017-0886 primarily focus on upgrading to patched versions of Nextcloud Server, specifically versions 9.0.55 or 10.0.2 and later, which contain fixes that prevent the recursive processing conditions that lead to the denial of service. Organizations should also implement monitoring solutions to detect unusual resource consumption patterns that might indicate exploitation attempts, and establish access controls to limit the number of authenticated users with elevated privileges. Network-level protections such as rate limiting and connection tracking can help reduce the impact of exploitation attempts, while regular security audits and penetration testing should be conducted to identify similar recursive patterns in other applications. The vulnerability aligns with ATT&CK technique T1499 which covers "Network Denial of Service" and demonstrates how authenticated access can be leveraged to compromise system availability. Additionally, implementing proper input validation and recursion depth checking mechanisms in application code can prevent similar vulnerabilities from manifesting in future versions of the software.