There is the possibility. That there is at least four hostile alien civilization lurking in Milky Way.

Artists' impression of hypothetical alien craft that observes Earth. 

The fact is that the SETI program is the most interesting, exciting, and frightening program in the history of mankind. In the shadows of the SETI program are created the models that are making the things like quantum computers possible. Maybe quantum computers are the first things that can open alien messages. 

There is the possibility that some of the FRBs:s (Fast Radio Bursts) are packed with alien burst messages. But the problem is how to separate those synthetic signals from natural signals. There is the possibility that aliens use natural radio signals like pulsar radiation as carrier waves. Or maybe they are using the neutrinos as the long-range qubits or quantum-based communication. 

The problem with detecting alien communication is that we don't know anything about that thing. The numeric system of the aliens might be different, what the binary numeric system that people use is. And in the case that aliens use neutrinos in quantum communication capturing those particles is very difficult. And then the system must know what kind of states that qubit has. But before we can make that kind of thing we must find the neutrino qubit. 

There are lots of neutrinos in the universe and the only way that can make it possible to find the neutrino that is carrying information is simply by measuring the weight of the neutrino. If some neutron that has the same origin as other neutrinos is heavier than those other neutrinos. There is the possibility that this particular neutrino carries synthetic data. 

And then the system can start breaking the information. That is stored in that particle. But that thing is extremely hard to make in real life. The aliens would send their message by sharing data in multiple qubits. But if we someday can make that thing that confirms the existence of intelligent life. 

But then we must ask: could that hypothetical intelligent life be dangerous? And the answer is that intelligent species are all somehow dangerous. There is the possibility that when the interstellar probe enters some solar system its brake fails. And the craft accidentally impacts to planet. And if 20 000 tonnes of the material impact Earth with the speed of 25% of the speed of light. This impact destroys the entire planet. In some other visions, the leak in the antimatter engine detonates the alien craft in the air. And that detonation destroys the entire planet. 

The image above: 

David A. Hardy painted this famous "Space ark" painting. The conflict between civilizations can begin from this kind of case. When the alien craft will slow to the orbit of the planet. The civilization of that planet sends their fighters to space. And that thing causes conflict. This kind of conflict can happen if the hypothetical aliens have seen a planet like Earth in the time of dinosaurs. 

They might not predict that humans are suddenly turned the masters of the planet because of an asteroid impact. So they might not predict that there are intelligent species on that planet. And that thing can cause hostile actions between species. 

There is the possibility that some aliens collect the DNA for making their genomes stronger.  And that makes them dangerous for individuals. But the dangerous civilization would be the civilization that just invented interstellar travel. There is the possibility that some civilizations are traveling in space by using the asteroid arks. Those asteroid arcs are good systems for intelligence and recon work. 

If an asteroid ark is entering the orbit of some planet that thing can cause the planetary civilization simply thinks that the incoming thing is the dangerous asteroid and shoots that thing by using missiles. In those cases, hostile actions are caused by mistakes. There is the possibility that the crew of an asteroid craft will open the fire by using orbital antimatter bombardment. 

But there is of course possible, that the civilization that just created technology that is useful for interstellar travel just wants to demonstrate its power. The thing is that the slavery of another civilization or species is not as safe as we might want to think. The slave species would always sooner or later rebel. 

https://www.space.com/malicious-alien-civilizations-odds