CVE-2018-20804 in MongoDB
Summary
by MITRE • 11/23/2020
A user authorized to perform database queries may trigger denial of service by issuing specially crafted applyOps invocations. This issue affects: MongoDB Inc. MongoDB Server v4.0 versions prior to 4.0.10; v3.6 versions prior to 3.6.13.
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Analysis
by VulDB Data Team • 08/24/2025
This vulnerability represents a denial of service condition within the MongoDB database server that specifically targets the applyOps operation functionality. The flaw allows authenticated users with database query privileges to craft malicious invocations that can cause the database server to become unresponsive or crash entirely. The vulnerability is particularly concerning because it leverages legitimate database operations that users might normally perform, making it difficult to distinguish between normal usage and malicious exploitation. The affected versions include MongoDB 4.0 prior to 4.0.10 and 3.6 prior to 3.6.13, indicating this was a significant issue that impacted multiple major release lines of the database server. This type of vulnerability falls under CWE-400, which specifically addresses improper handling of resources leading to denial of service conditions, and aligns with ATT&CK technique T1499.004 for network denial of service attacks.
The technical implementation of this vulnerability involves the applyOps command which is designed to apply operations from a replication oplog to a secondary node in a MongoDB replica set. When users submit specially crafted applyOps invocations, the database server processes these requests in a manner that leads to resource exhaustion or internal state corruption. The flaw likely stems from inadequate input validation or improper resource management within the applyOps processing pipeline, causing the server to consume excessive memory or CPU resources during the execution of these malformed operations. The vulnerability demonstrates how database operations that are intended for replication and backup purposes can be weaponized to disrupt database availability. This exploitation method represents a sophisticated approach to denial of service because it uses legitimate administrative commands rather than attempting to exploit buffer overflows or other low-level vulnerabilities.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the entire database infrastructure. When a MongoDB server becomes unresponsive due to this vulnerability, it can affect all applications and services that depend on database availability, leading to cascading failures throughout the system. The vulnerability affects database servers running in replica set configurations, making it particularly dangerous for production environments where high availability and data consistency are critical requirements. Organizations may experience extended downtime while administrators work to recover from the service disruption, and the vulnerability could be exploited by malicious actors to create persistent availability issues. The fact that this vulnerability affects both 3.6 and 4.0 release lines means that organizations running these versions across multiple environments would need to implement immediate mitigations, as the exploitation could occur through normal database operations that administrators perform regularly.
Organizations should immediately upgrade to MongoDB versions 4.0.10 or later and 3.6.13 or later to address this vulnerability, as these releases contain patches that properly validate applyOps inputs and prevent the resource exhaustion conditions that lead to denial of service. Additionally, implementing network segmentation and access controls to limit which users can execute applyOps operations can provide an additional layer of protection. Monitoring for unusual patterns in applyOps usage and implementing automated alerting for high resource consumption on database servers can help detect exploitation attempts before they cause complete service disruption. Security teams should also consider implementing database activity monitoring solutions that can detect and alert on potentially malicious applyOps invocations, as this vulnerability can be exploited by both external attackers and compromised internal users. The remediation process should include comprehensive testing to ensure that the patch does not introduce compatibility issues with existing database operations while maintaining the security benefits of the fix.