Facebook says it has largely brought the attack under control. The site was hit by a spam attack that unleashed massive quantities of violent and pornographic images. The images spread via the attack included images of Justin Bieber, who’d been “Photoshopped” into a sexual situation, as well as pictures of a dead dog. Facebook declined to name which browser had the vulnerability, but “self-XSS” refers to a cross-site scripting (XSS) exploit that’s launched by a user.”]
Source: https://www.darkreading.com/attacks-breaches/facebook-blames-porn-attack-on-browser-bugs