The Russian invasion of Crimea in 2014 was a violation of the Budapest Memorandum, so that agreement was dissolved at that point and no longer in effect. This means that, from that point forward, any country could supply Ukraine with any weapon system whatsoever, up to and including nuclear weapons.
This also means that any country, at any point in time, can simply paint a Ukrainian flag on any of their weapon systems and transfer ownership to Ukraine, and as long as a Ukrainian citizen pushes the button, it's completely legit.
Western countries have already stated that they will support Ukraine with lethal weapons in their fight against Russia. These countries possess weapon systems that can easily penetrate Russian defenses.
@PatI I don't believe your reasoning makes sense... supplying guns is clearly legal, but I don't think this means anyone can do anything: if Germany starts shooting missiles, this is a war declaration