Not spy messages, but steganography has been used for malvertizing campaigns, hiding malicious code inside ad images to prevent automated detection by ad networ

Not spy messages, but steganography has been used for malvertizing campaigns, hiding malicious code inside ad images to prevent automated detection by ad networks. See for example https://www.proofpoint.com/us/threat-insight/post/massive-adgholas-malvertising-campaigns-use-steganography-and-file-whitelisting-to-hide-in-plain-sight https://www.proofpoint.com/us/threat-insight/post/massive-ad....