A security researcher has discovered a critical vulnerability in Google-owned YouTube that could allow anyone to make the comment posted by any celebrity or public figure on some YouTube video appear on his or her own YouTube video. Ahmed Aboul-Ela and Ibrahim M. El-Sayed, two Egyptian security researchers, found a simple trick that allowed him to copy any comments from any video on the popular video sharing website to his video, even without any user-interaction. The vulnerability have been fixed after the researcher reported it to Google.
Source: https://thehackernews.com/2015/04/hacking-youtube-comments.html