Possession is X.A.N.A.'s own ability to infect other systems like a virus. This has been developed so its spectres can meld with real objects, devices, and living things. In X.A.N.A.'s case, they allow it to have total control over the vessels down to their molecular structure, becoming an instrument of X.A.N.A.'s will.
X.A.N.A. is a multi-agent system that takes over other systems like a virus. By activating a tower on Lyoko, it uses its own digital impulses that it sends through networks to connect to machines and devices and possess them, becoming tools to control at will. When they emerge as spectres, they can do the same thing to real objects and living things when planted inside, turning them into unnatural threats to attack the Earth.
In A Great Day, it was revealed that going back in time doubles X.A.N.A.'s power. When it hijacked the program, each trip increased its power, to the point where it could possess humans, transforming them into pixelized beings that do its bidding on Earth.
In Mister Pück, it attempted to possess Odd and Ulrich, but failed. It then tried to possess Jeremie and succeeded. It was later learned that people who have been scanned into the Supercomputer and virtualized onto Lyoko at least more than once are immune to possession.
Anything that has been possessed by X.A.N.A.'s spectres can be modified in almost any way that it sees fit, gaining modifications that people consider to be "supernatural". Additionally, X.A.N.A.'s possessed victims are usually granted special knowledge, including how to get to the factory and how to interact with the supercomputer.
In Lyoko Minus One, it is revealed that the Scyphozoa, one of X.A.N.A.'s monsters, has the ability to implant X.A.N.A.'s virus inside of virtualized people with its power to manipulate digital memory, effectively possessing them. The Lyoko Warriors' immunity to possession in the real world doesn't apply in the virtual world, ensuring that this is the only guaranteed method to take hold of them and the only way to end the possession is to devirtualize them or use Supercomputer Reboot Program due to Scyphozoa implanting an inhibitor in victim's brain to prevent manual devirtualization.
- In certain instances, the behavior of humans possessed by X.A.N.A. seems to imitate their real nature. Examples of this include the X.A.N.A.-possessed Jeremie from Mister Pück, who displayed his usual calculating and patient manner. Similarly, Saint Valentine's Day saw a possessed Aelita mockingly apologize to Yumi as she knocked her to the ground, while a possessed Jim in Final Mix acted cocky and boastful as he enacted X.A.N.A.'s will.
- Whenever X.A.N.A. possesses a large crowd of people at once (examples include Lyoko Minus One and Bad Connection), the effect seems to be less extreme. Most of the targets do not get enhanced abilities or powers, and their focus seems far less refined. The Lyoko Warriors were occasionally able to make use of these weaknesses (most notably in Bad Connection, where Odd was actually able to convince his attackers to make him a sandwich before killing him).