The Xserver uses the mechanism of "privates" to store additional data to its own objects, each private has an associate "type". Each private is allocated for the relevant size of memory that is declared at creation.

The cursor structure in the Xserver goes as far as having two keys, one for the cursor itself and another one for the bits that make the cursor shape.

XSELINUX also uses privates but it's a bit of a special case because it uses the same privates keys for all different objects.

What happens here is that the cursor code in both Xephyr and Xwayland uses the wrong type of private at creation, using the cursor bits type with the cursor private and when initiating the cursor, the overwrites the XSELINUX context.