A growing literature is emerging on the believability and spread of disinformation, such as fake news, over social networks. However, little is known about the degree to which malicious actors can use social media to covertly affect behavior with disinformation. A lab-based randomized controlled experiment was conducted with 233 undergraduate students to investigate the behavioral effects of fake news. It was found that even short (under 5-min) exposure to fake news was able to significantly modify the unconscious behavior of individuals. This paper provides initial evidence that fake news can be used to covertly modify behavior, it argues that current approaches to mitigating fake news, and disinformation in general, are insufficient to protect social media users from this threat, and it highlights the implications of this for democracy. It raises the need for an urgent cross-sectoral effort to investigate, protect against, and mitigate the risks of covert, widespread and decentralized behavior modification over online social networks. • Fake news and disinformation can covertly modify the behavior of individuals • It can do this by manipulating implicit attitudes and emotions • Current mitigation methods do not prevent behavior modification. • It is urgent to address this threat to democracy and individual autonomy