In https://jira.mongodb.org/browse/BF-33414 , we detected a case where the fuzzer tries to generate an invalid BSON object, providing null terminators in the field of a BSON object. Although null terminators are allowed in the value of a bson element, they are not permitted in the fieldname of the BSON element. In the failing case, we created an invalid BSON object as such:

 var invalidBson = _buildBsonObj('_id', 2, '\0\0', 3); 

The fix for this, is to prevent creating such a bson object. The implementation is here https://github.com/10gen/mongo/blob/master/src/mongo/shell/shell_utils.cpp#L735 . 

We could assert that the fieldname does not contain null terminators:

        uassert(7587900, str::stream() << "BSON field name must not contain null terminators : " << name,                std::string::npos == name.str().find('\0'));