The video, posted to YouTube, was filmed by camcorder off a computer screen. The dark object is hard to see against the blue colors of the Earth as it slowly rotates below. But an enlargement of the image shows an unusual shape which doesn't resemble any kind of launched spacecraft.
The ISS can be clearly seen on the left hand side of the screen, so there's no doubt about the authenticity of the sighting. The UFO itself defies easy description, other than to say it's shaped very strangely, with several rounded protrusions extending from a main axis.
As it passes through space, it lightens a bit in response to a varying angle of light struck by the Sun. That makes it hard to say it's a CGI image, as that is one of the ways to spot a fake.
So, what is it?