NoteMy answer is incorrect. Please refer to other answers.
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As a client program assembles a query in MongoDB, it builds a BSON object, not a string. Thus traditional SQL injection attacks are not a problem.
For details follow the documentation
UPDATE
Avoid expression like eval which can execute arbitrary JS. If you are taking input from user and running eval like expressions without cleaning the input you can screw up. As pointed by JoBu1324, operations like where, mapReduce and group permit to execute JS expressions directly.