CVE-2026-81528 in C# Driver
Summary
by MITRE • 08/27/2026
A MongoDB C# driver document-replacement code path omits the element-name/shape validation that the equivalent write paths apply, so a value supplied as a replacement is forwarded to the server without neutralization of query-language special elements. An application that passes untrusted, loosely-typed input as a replacement value therefore allows that input to be interpreted by the database as update logic rather than as data, executing under the application's own database credentials. Applications using strongly-typed document mappings are not affected.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 08/28/2026
The vulnerability described involves a critical security flaw within the MongoDB C# driver, specifically in its document-replacement code path. This issue arises from an inconsistency in how input validation is handled compared to other write operations supported by the same library. In standard database interactions, particularly those involving updates or replacements of documents, it is imperative that user-supplied data be strictly treated as literal values rather than executable commands. However, this specific implementation fails to apply the necessary element-name and shape validations that are present in equivalent write paths. Consequently, when an application utilizes untrusted or loosely-typed input for document replacement, the driver does not neutralize query-language special elements embedded within that input. These special elements, which typically serve as operators or structural markers in MongoDB queries, are forwarded directly to the database server without sanitization.
This lack of validation creates a severe risk of NoSQL injection attacks. When an attacker supplies maliciously crafted input containing these special characters or structures, the database engine interprets them not merely as data fields but as part of the update logic itself. This misinterpretation allows the attacker to manipulate the structure and content of documents in ways that were not intended by the application developer. The impact is particularly acute because this execution occurs under the credentials of the application connecting to the database. If the application uses a high-privilege account, such as an administrator or a user with broad read-write access across multiple collections, the potential for damage escalates significantly. An attacker could potentially overwrite critical data, extract sensitive information by manipulating query conditions, or even cause denial-of-service conditions through resource exhaustion attacks facilitated by malformed update operations.
From a technical perspective, this flaw aligns closely with CWE-94, which covers Improper Control of Generation of Code (Code Injection), and more specifically CWE-610, Externally Controlled Reference to a Resource in Another Domain if the injection leads to data exfiltration beyond local scope. In terms of offensive security frameworks like MITRE ATT&CK, this vulnerability facilitates techniques associated with Data Manipulation or Collection, where an adversary alters stored information to maintain persistence, evade detection, or achieve their objectives without direct system access controls being bypassed in the traditional sense. The core issue is a failure in input validation and output encoding principles, which are foundational to secure software development practices.
It is important to note that this vulnerability does not affect applications using strongly-typed document mappings. Strongly-typed systems typically enforce schema constraints at compile time or through rigorous runtime type checking, ensuring that only expected data structures and types are passed to the driver. This inherent validation acts as a safeguard against the injection of unexpected special characters or structural elements. Therefore, organizations relying on loosely-coupled dynamic documents with unvalidated user input are primarily at risk. To mitigate this vulnerability, developers should ensure they upgrade to patched versions of the MongoDB C# driver that implement consistent validation across all write paths. Additionally, application-level mitigations include implementing strict schema validation for incoming data before it reaches the database layer and avoiding the direct use of unsanitized user inputs in document replacement operations. Where possible, switching to strongly-typed models can provide an additional layer of defense by limiting the types of data that can be submitted through these interfaces.