NSURLSession needs access to "kern.ostype", so the Networking and WebContent sandboxes should allow it. I corrected this for the iOS Network sandbox in Bug 208346. I should have handled macOS and the WebContent process (on both platforms) at the same time.
<rdar://problem/60010603>
Created attachment 392590 [details] Patch
Committed r257935: <https://trac.webkit.org/changeset/257935> All reviewed patches have been landed. Closing bug and clearing flags on attachment 392590 [details].