CVE-2026-81527 in C# Driver
Summary
by MITRE • 08/27/2026
A NoSQL/expression injection weakness exists in the LINQ-to-aggregation query translation layer of the MongoDB C# Driver, in both aggregation expression and query filter translation. When application-supplied values are embedded in certain query constructs, special elements contained within those values are not properly escaped before the resulting query is transmitted to the database, so portions of the value may be interpreted by the database as query logic rather than as data. A user able to supply values that an application incorporates into an affected query may thereby cause unintended data to be returned or query results to be altered.
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability identified in the MongoDB C# Driver represents a critical security flaw within its LINQ-to-aggregation query translation layer, specifically affecting both aggregation expression and query filter translations. This issue stems from an improper input validation mechanism where application-supplied values are embedded directly into specific query constructs without adequate sanitization or escaping of special characters. In secure software design, user inputs must be treated as data rather than executable code to prevent injection attacks. However, in this instance, the driver fails to distinguish between literal string content and structural query syntax when processing certain complex expressions. Consequently, if an attacker can control a portion of the input that is passed into these vulnerable translation functions, they can inject malicious NoSQL or expression elements that bypass intended logical boundaries.
From a technical perspective, the core weakness lies in how the driver translates LINQ queries into MongoDB-specific query documents and aggregation pipelines. When special characters such as dollar signs, parentheses, or specific operator symbols are present within user-supplied values, the translation logic incorrectly interprets these sequences as part of the database command structure rather than as literal data points. This misinterpretation allows an attacker to alter the semantics of the original query. For example, by injecting operators that modify logical conditions like $or, $and, or $where, a malicious actor can manipulate which documents are selected for retrieval or modification. The vulnerability is particularly dangerous because it occurs at the driver level, meaning applications using this library may appear secure while actually being susceptible to exploitation through seemingly innocuous input fields.
The operational impact of this vulnerability is severe, primarily manifesting as unauthorized data access and potential integrity violations. An attacker who successfully exploits this injection flaw can cause unintended data to be returned by bypassing authentication or authorization checks embedded in the query filters. This leads directly to a breach of confidentiality, where sensitive information stored within MongoDB collections becomes accessible to unauthorized users. Furthermore, if the affected queries are used for update operations rather than just reads, the attacker could potentially alter or delete critical records, compromising data integrity. The ability to alter query results also undermines the reliability of application logic that depends on accurate database responses, potentially leading to further downstream vulnerabilities such as privilege escalation or business logic manipulation within the host application.
This vulnerability aligns with Common Weakness Enumeration (CWE) ID 943, which describes Improper Neutralization of Special Elements used in a NoSQL Query, and is closely related to CWE-78 for OS Command Injection principles applied to database queries. In terms of the MITRE ATT&CK framework, this exploitation technique falls under T1190, Exploit Public-Facing Application, as it leverages input vectors exposed through application interfaces to interact with backend data stores. It also reflects aspects of T1564, Hidden Fields and Elements, in that the malicious payload is hidden within legitimate-looking user inputs. The attack vector typically involves web applications or APIs that accept untrusted input and pass it directly into MongoDB queries via this specific driver version without implementing parameterized query patterns or strict allow-listing for special characters.
Mitigation strategies must focus on both immediate patching and long-term architectural improvements. Organizations should immediately upgrade the MongoDB C# Driver to a patched version where the translation layer has been hardened to properly escape or neutralize special elements within user-supplied values. Developers must also review existing codebases that utilize LINQ-to-aggregation translations, particularly those involving dynamic query construction based on user input. Implementing strict input validation using allow-lists for expected character sets can provide an additional layer of defense against injection attempts. Furthermore, adopting parameterized queries or object mapping techniques that separate data from logic more rigorously than the vulnerable translation methods will reduce the attack surface. Regular security audits and static code analysis focused on NoSQL injection patterns are recommended to ensure that similar weaknesses do not exist in other parts of the application stack.