EF Core provider: pipeline-form ExecuteUpdate evaluates a user string setter as a $-field reference

The MongoDB Entity Framework Core provider does not neutralize special elements in values passed to SetProperty when an ExecuteUpdate call also contains a setter that reads the entity being updated. In that case the update is sent as an aggregation pipeline, and a caller-supplied string beginning with $ is evaluated as an expression rather than stored as data. A party who can influence such a value may modify or remove fields in the records being updated, or copy other fields of the same record into them.