CVE-2020-8293 in Nextcloud Server
Summary
by MITRE • 01/26/2021
A missing input validation in Nextcloud Server before 20.0.2, 19.0.5, 18.0.11 allows users to store unlimited data in workflow rules causing load and potential DDoS on later interactions and usage with those rules.
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Analysis
by VulDB Data Team • 02/20/2021
The vulnerability described in CVE-2020-8293 represents a critical input validation flaw within Nextcloud Server that affects versions prior to 20.0.2, 19.0.5, and 18.0.11. This issue specifically targets the workflow rules functionality within the Nextcloud platform, which allows administrators and users to define automated processes that trigger based on specific conditions. The missing input validation creates a scenario where malicious actors can exploit the system by crafting workflow rules that store excessive amounts of data, effectively bypassing normal storage limitations and constraints that should govern such operations.
The technical flaw manifests in the insufficient validation of user-supplied data within the workflow rule creation and management interfaces. When users create or modify workflow rules, the system fails to properly validate the size and complexity of the data being stored within these rules. This allows attackers to inject large payloads or malformed data structures that can grow exponentially over time, particularly when workflow rules reference or contain nested data elements. The vulnerability falls under CWE-20, which specifically addresses improper input validation, and represents a classic example of how insufficient sanitization can lead to resource exhaustion attacks. The lack of proper data size limits and structural validation enables attackers to create workflow rules that can consume unlimited storage space within the system's metadata structures.
The operational impact of this vulnerability extends beyond simple storage consumption, creating significant risks for system availability and performance. When workflow rules accumulate unlimited data, they can cause substantial delays in system operations, particularly during rule evaluation and execution phases. The cumulative effect of numerous oversized workflow rules can lead to system degradation, increased latency in user interactions, and potentially complete service disruption. This vulnerability directly maps to ATT&CK technique T1499.004, which describes resource exhaustion attacks, and can be leveraged to perform denial of service attacks against Nextcloud instances. The potential for DDoS-like effects becomes particularly pronounced when multiple users or automated systems create these oversized workflow rules simultaneously, overwhelming system resources and making legitimate user interactions increasingly difficult or impossible.
Mitigation strategies for CVE-2020-8293 should prioritize immediate patching of affected Nextcloud installations to versions 20.0.2, 19.0.5, or 18.0.11 where the input validation has been properly implemented. System administrators should implement monitoring solutions that track workflow rule size and complexity, establishing alerts for when rules exceed predetermined thresholds. Access controls and user permissions should be reviewed to limit who can create or modify workflow rules, particularly in multi-tenant environments where users may have elevated privileges. Additionally, implementing automated cleanup mechanisms that periodically review and prune oversized workflow rules can provide an additional layer of defense. Organizations should also consider implementing rate limiting and resource quotas for workflow rule operations to prevent single users from creating excessive data structures. The vulnerability highlights the importance of proper input validation in web applications and demonstrates how seemingly minor validation gaps can lead to significant security and operational risks, emphasizing the need for comprehensive security testing and validation of all user-supplied data within enterprise platforms.