The session object keeps a strong reference to the delegate until your app exits or explicitly invalidates the session. If you do not invalidate the session, your app leaks memory until it exits.

NSURLSession-invalidate.swift

import UIKit

class ModalViewController: UIViewController, NSURLSessionDelegate {

    var session: NSURLSession!

    override func viewDidLoad() {
        super.viewDidLoad()
        session = NSURLSession(configuration: NSURLSessionConfiguration.ephemeralSessionConfiguration(), delegate: self, delegateQueue: nil)

        dispatch_async(dispatch_get_main_queue()) { [weak self] in
            for i in 0...100 {
                self?.session.dataTaskWithRequest(NSURLRequest(URL: NSURL(string: "https://google.pt")!), completionHandler: { data, response, error in
                    print("session.dataTaskWithRequest \(i): done")
                }).resume()
            }
        }
    }

    override func viewDidDisappear(animated: Bool) {
        super.viewDidDisappear(animated)
        // Comment the line after to check that the view controller isn't released.
        session.finishTasksAndInvalidate()
    }

    @IBAction func didTouchClose(sender: AnyObject) {
        self.dismissViewControllerAnimated(true, completion: nil)
    }

    deinit {
        print("ViewController.deinit")
    }

}