A man-in-the-browser is a type of online threat, where a hacker uses a trojan horse virus to gain access to your computer. From there, the hacker manipulates the content you see within your web browser, which can allow them to record your personal information and passwords, as well as manipulate your transactions so that the money you think you are spending on an online product actually goes to the hacker, without anything looking any different from normal on that webpage.