A spy, an informer/snitch. Someone who penetrates/enters a circle of friends or organization for the sole purpose of gathering sensitive or classified information and leaking it to an opposing or competing group.
We have a mole in our organization and we don't know who it is, so watch your back and don't give out sensitive information to anyone.
The addict will sometimes penetrate his arm with a syringe to inject drugs directly into his blood stream. This method of delivery is said to produce a more efficient high.
