A massive adware campaign has so far impacted up to a million Mac users, using a tricky steganography technique to hide malware in image files. Steganography is the practice of concealing secret messages, code or information within otherwise innocuous-looking text or images. Once clicked, the malicious ad infects the user with the Shlayer trojan, which masquerades as a Flash upgrade and in turn redirects the victim to an adware installer. The campaign is still ongoing but the bad actor regularly rotates its payload and domains.
Source: https://threatpost.com/malware-in-ad-based-images-targets-mac-users/141115/