CVE-2026-114647 in C# Driver
Summary
by MITRE • 10/22/2026
The MongoDB C# Driver does not reject replacement documents whose top-level field names begin with a dollar sign in FindOneAndReplace, unlike other replace operations. An application that builds such a document from user-supplied field names can have the server apply update operators instead of a full replacement. The effect is limited to the document matched by the application's own filter, whose contents can be modified in ways the application did not intend.
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Analysis
by VulDB Data Team • 10/22/2026
The MongoDB C# Driver contains a validation inconsistency within its FindOneAndReplace operation that allows for unintended server-side behavior when processing documents with specific field naming conventions. Specifically, the driver fails to reject replacement documents where top-level field names begin with a dollar sign character. In standard MongoDB operations, fields prefixed with a dollar sign are reserved for update operators such as $set or $inc, which modify existing document fields rather than replacing them entirely. While other replace operations in the driver correctly enforce this restriction and treat such input as invalid, FindOneAndReplace proceeds to send these documents to the database server without raising an exception locally. This discrepancy creates a divergence between client-side validation expectations and actual server execution logic, leading to unpredictable outcomes when user-supplied data is used to construct replacement documents.
When an application constructs a replacement document using field names derived from untrusted or user-controlled input, it may inadvertently include fields starting with the dollar sign character. Because the driver does not filter these out before transmission, the MongoDB server interprets these top-level keys as update operators rather than literal field names for replacement. Consequently, instead of performing a full document replacement where all existing content is discarded and replaced by the new structure, the database applies the specified operator logic to the matched document. This results in partial modifications that deviate significantly from the application's intended state transition. The vulnerability effectively allows an attacker who controls the input used for field names to influence how updates are applied to specific documents within their own scope of access.
The operational impact is confined to documents targeted by the application’s existing filter criteria, meaning the attack requires prior knowledge or control over which document will be matched. However, because the contents can be modified in ways not intended by the developer, this flaw poses a significant risk for integrity violations and logic bypasses. For instance, if an application relies on FindOneAndReplace to atomically update user profiles or transaction states based on dynamic inputs, an attacker could manipulate field names to trigger unintended operator behaviors, potentially escalating privileges, corrupting data structures, or causing denial of service through malformed state changes. The severity is heightened by the fact that this behavior bypasses standard security assumptions regarding input validation in replacement operations, making it a subtle but dangerous logic flaw rather than a simple injection vector.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation and CWE-94 Code Injection, as the application fails to properly sanitize or validate user-supplied field names before processing them through an update mechanism. In terms of attack patterns, it relates to MITRE ATT&CK techniques involving data manipulation and potential privilege escalation if the modified fields control access controls or administrative flags within the document structure. The core issue lies in the lack of consistency between different replacement methods provided by the driver, which undermines the principle of least surprise for developers relying on strict input validation.
Mitigation strategies should focus on enforcing consistent validation across all database interaction methods and implementing robust server-side checks where possible. Developers must ensure that any field names derived from user input are strictly validated against a whitelist of allowed characters or patterns before being used in replacement operations. It is critical to avoid using dynamic keys for top-level fields in documents intended for full replacement, as this inherently risks collision with reserved operator syntax. Upgrading the MongoDB C# Driver to versions where this inconsistency has been addressed by enforcing uniform validation rules across FindOneAndReplace and other replace methods is essential. Additionally, applications should implement defense-in-depth measures such as input sanitization libraries that explicitly strip or escape special characters like the dollar sign in dynamic field names, ensuring that only safe, literal identifiers are transmitted to the database server for structural replacement tasks.