NoSQL injection via array replacement bypassing update shape validation in driver write path

Impact

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.

Patches

From driver version 3.11.1, a replacement that is not a document is rejected with an ArgumentException instead of being sent to the server.

Workarounds

Make sure the replacement you pass to ReplaceOne, FindOneAndReplace or a bulk-write replace is a document, not a raw value such as an array.

References

[CSHARP-6158](https://jira.mongodb.org/browse/CSHARP-6158), [Fix](https://github.com/mongodb/mongo-csharp-driver/commit/27bc81ddeb407348fff57b3370aba360a13c5404), [CVE](https://www.cve.org/CVERecord?id=CVE-2026-81528)