CVE-2018-20802 in MongoDB
Summary
by MITRE • 11/23/2020
A user authorized to perform database queries may trigger denial of service by issuing specially crafted queries with compound indexes affecting QueryPlanner. This issue affects: MongoDB Inc. MongoDB Server v3.6 versions prior to 3.6.9, v4.0 versions prior to 4.0.3.
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Analysis
by VulDB Data Team • 03/04/2026
This vulnerability represents a critical denial of service flaw within the MongoDB database server that specifically targets the QueryPlanner component responsible for optimizing database query execution. The issue arises when authorized users submit carefully constructed queries that leverage compound indexes, creating a scenario where the database server becomes unresponsive or crashes entirely. The vulnerability affects MongoDB versions 3.6.x prior to 3.6.9 and 4.0.x prior to 4.0.3, indicating that the flaw existed across multiple major releases and was not properly addressed in the initial patch cycles. The technical nature of this vulnerability aligns with CWE-400, which categorizes it as a weakness related to resource management failures, specifically in how the database server handles query planning under certain index conditions.
The operational impact of this vulnerability extends beyond simple service interruption as it can be exploited by authenticated users who already possess database access privileges, making it particularly dangerous in environments where database users have elevated permissions. Attackers can craft queries that force the QueryPlanner to enter an infinite loop or consume excessive system resources, effectively rendering the database service unavailable to legitimate users. This creates a scenario where authorized personnel can inadvertently or maliciously disrupt database operations without requiring elevated system privileges or specialized attack vectors. The vulnerability demonstrates how seemingly benign database operations can be weaponized to cause significant operational disruption, particularly in mission-critical applications where database availability is paramount.
The root cause of this issue lies in the improper handling of compound index queries within the QueryPlanner's optimization logic, where the system fails to properly validate or limit the complexity of query execution paths. This flaw allows for a resource exhaustion condition that can be triggered through specific combinations of query parameters and index structures, leading to system instability and potential complete service unavailability. Organizations running affected MongoDB versions face significant risk as this vulnerability can be exploited without requiring advanced technical knowledge or specialized tools, making it accessible to a broad range of threat actors. The issue highlights the importance of proper input validation and resource management within database query optimization engines, as outlined in various cybersecurity frameworks and best practices for database security.
Mitigation strategies for this vulnerability primarily involve upgrading to the patched versions of MongoDB where the QueryPlanner has been modified to properly handle compound index scenarios and prevent resource exhaustion conditions. System administrators should prioritize patching affected installations and implement monitoring solutions to detect unusual query patterns that might indicate exploitation attempts. Additionally, organizations should consider implementing database access controls and query monitoring to limit the potential impact of authenticated users who might attempt to exploit this vulnerability. The remediation process should include comprehensive testing of patched systems to ensure that legitimate database operations continue to function correctly while preventing the specific query patterns that trigger the denial of service condition. This vulnerability underscores the importance of regular security updates and proper vulnerability management processes within database environments.