legally speaking, your current company can only state the fact

that you are or have been employed at the company from when to when, unless there are criminal issues they need to disclose to future potential employers. That said, they cannot comment anything on your performance that may cause you to lose the potential opportunity. If they do so and you have the evidence, you can sue your current employer for doing so.

Let's take one step back, many potential employers may require you to disclose contact/references from your current employer, and you DON'T have to tell them. It all comes down to negotiation and you are NOT obligated to disclose such information.