This time I want to speak on the 'printf' function. Everybody has heard of software vulnerabilities and that functions like 'printf' are outlaw. But it's one thing to know that you'd better not use these functions, and quite the other to understand why. In this article, I will describe two classic software vulnerabilities related to 'printf'. You won't become a hacker after that but perhaps you will have a fresh look at your code. You might create similar vulnerable functions in your project without knowing that. STOP. Reader, please stop, don't pass by. You have seen the word "printf", I know. And you're sure that you will now be told a banal story that the function cannot check types of passed arguments. No! It's vulnerabilities themselves that the article deals with, not the things you have thought. Please come and read it.