This is a genuine photograph of Tom Cruise sitting on top of the Burj Khalifa, but he is almost certainly wearing a harness.
A practically unbelievable photograph of actor Tom Cruise sitting on top of the Burj Khalifa, the world's tallest building at a height of more than a half mile, is frequently shared online along with the claim that the actor wasn't wearing a harness during the stunt:
While this is a genuine picture of Cruise on the top of the Burj Khalifa, and while there doesn't appear to be a harness visible in this shot, the actor is almost certainly wearing some form of safety tether.
This photograph was taken in 2011, during the filming of the fourth installment in the "Mission Impossible" series. Cruise did a number of stunts in that movie that took place inside (and outside) the building. Here's a behind-the-scenes clip showing some of these stunts:
At one point during filming, Cruise and the stunt team decided to take a helicopter to the top of the building to get a few photographs. Cruise talked about this experience during an appearance on "The Graham Norton Show."
While the picture is real, there's plenty of reasons to doubt the claim that Cruise was without a harness during this stunt. For one, when Cruise filmed these stunts (as seen in the behind-the-scenes clip above) he is wearing a harness. In fact, Tom Peitzman, the visual effects producer for the movie, said in 2011 that everyone involved in filming these stunts (even those who were inside while Cruise was hanging on the outside of the building) were also wearing a harnesses.
"Special mounts had to be made for the 65-millmeter Imax cameras, special safety had to be put in place, because in a building that's 800 meteres tall [it's 2,723 feet] you couldn't run the risk of anything falling. Even all of us who are working inside the building, we all had to harness ourselves because the window was open."
With all of the safety precautions that went into filming these stunts, it seems highly improbable that Cruise would be allowed to sit on the top of this building without a harness.
It should also be noted that while there are pictures of Cruise sitting on the Burj Khalifa and of the helicopter near the Burj Khalifa, there are no photographs of Cruise getting in or out of the helicopter. While it seems likely Cruise simply hopped out of the helicopter for a quick pic, there was likely a little more involved (such as hooking up some sort of security cable.)
It's also worth noting that helicopter is not the only way to access the top of this spire. This column is actually hollow and can be accessed via an internal ladder. This is how maintenance crews would access this area. In 2013, photographer Joe McNally was allowed to access this area to take some images.
It seems likely that someone was waiting for Cruise as he transitioned from the helicopter to the spire, and then helped him attach a safety harness.