For El Capitan, the fix is Security Update 2016-001.
For Yosemite, the fix is Security Update 2016-005.
To check for Mac software updates, open the App Store app on your Mac, then click Updates in the toolbar. If updates are available, click the Update button to download and install them. If you don't have the App Store on your Mac, get OS X updates by choosing Software Update from the Apple menu.
Updated 26 August: Brief update - here is a link to the original (and in-depth) report by Citizen Lab, the firm that identified the vulnerabilities and ferreted out the origin of the attack.
When a mobile phone provider sends you an update for your phone, it's usually a good idea to install it. Sometimes it's a better idea than others.
This is one of those times: Apple just released an update for iPhones, fixing three very serious bugs that together have been exploited in secret to spy on apparent Middle Eastern targets. Through the flaws, merely clicking on a link can "jailbreak" an iPhone - defeating the security measures Apple has built in and giving the attacker complete control of the device (and any private information on the device).
Your iPhone will prompt you to update to iOS 9.3.5 very shortly. Do it.
Motherboard has an article describing how the flaw was discovered and how it was being used to spy on individuals.
The SANS Internet Storm Center has a concise description of the three flaws and how they work together to compromise a device.
Here is Apple's release bulletin for iOS, and Apple's release bulletin for OS X.
What do you need to do?
Open your iPhone or iPad's Settings tool and go to General -> Software Update in your device's Settings app, or connect to iTunes on your Mac or PC. If you are running iOS 9.3.5 (the latest update as of this writing), your device will show that it is up-to-date. If you are running an older version, your device will show an update is available. Install it!