Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.4",
  "pubDate": "2026-08-27T20:18:51.340Z",
  "pubdate": "2026-08-27T20:18:51.340Z",
  "executiveSummary": "The MongoDB C# driver is susceptible to a database injection vulnerability arising from inconsistent input validation within specific document-replacement code paths.\nThis security flaw stems from the omission of essential element-name and shape validation, a protection mechanism that is correctly implemented in other write operations but absent during replacement procedures.\nWhen an application processes untrusted, loosely-typed input as a replacement value, the driver forwards this payload to the database without appropriate neutralization. Consequently, the database interprets the user-supplied input as executable query-language logic rather than literal data.\nThe vulnerability allows an attacker to manipulate database operations, potentially leading to unauthorized data modification, unauthorized data access, or denial of service, depending on the injected query operators.\nThe risk is primarily concentrated in applications that dynamically construct document replacements from arbitrary user input. Systems utilizing strongly-typed BSON mappings or POCOs (Plain Old CLR Objects) are inherently protected, as their serialization process enforces schema strictness.\nSuccessful exploitation requires the application to accept untrusted data and pass it directly to the driver's replacement methods without sanitation. No specific authentication is required if the entry point is exposed, as the malicious commands execute under the privilege level of the application's established database connection.",
  "technicalDetails": "The root cause of this vulnerability lies in the architectural discrepancy within the MongoDB C# driver's write path logic. While standardized write operations perform rigorous shape and element-name validation to ensure that incoming data conforms to expected structures, the document-replacement code path fails to apply these same security controls.\nIn a secure implementation, the driver acts as an intermediary, sanitizing input to prevent control-plane injection. Because the replacement code path bypasses this validation layer, it fails to neutralize MongoDB query operators or 'dollar-sign' directives that may be embedded within the input data. When an application passes loosely-typed data—typically objects represented as dictionaries, generic BSON documents, or raw JSON strings—directly to the driver, the lack of sanitization allows an attacker to inject MongoDB query language (MQL) operators.\nThe attack flow follows a predictable sequence: First, the attacker identifies an application endpoint that accepts user input to perform a full document replacement (e.g., using 'ReplaceOne' or similar methods). Second, the attacker crafts a malicious payload containing MQL injection operators such as '$set', '$inc', or '$rename' within the document structure. Third, the application receives this input and, due to its loosely-typed handling, wraps the payload in the document replacement call without filtering the injected MQL operators.\nThe driver subsequently transmits this unneutralized, attacker-controlled structure to the MongoDB server. The server, interpreting the payload as valid administrative logic rather than static document data, executes the injected operators. For instance, an attacker could potentially overwrite critical fields, change document schemas, or exfiltrate data by forcing the server to evaluate specific fields in the context of the replacement.\nThis vulnerability is restricted to applications utilizing loosely-typed mappings, such as 'BsonDocument' or 'ExpandoObject', where the driver cannot enforce structural integrity. Applications leveraging strongly-typed classes (POCOs) are unaffected because the driver's serialization layer enforces strict data types, effectively neutralizing any attempt to inject MQL operators into the schema-defined fields. The exploit executes with the same permissions as the application itself; therefore, the impact is bound by the specific database privileges assigned to the application's connection string. Post-exploitation impact may include data corruption, unauthorized escalation of data access, or the bypass of application-level business logic that assumes static document structure."
}