For all you computer security types
I’m sure all the CS people reading this (and maybe even some of the non-CS types!) are familiar with buffer overflow attacks, and know how to both protect against them and exploit them in other people’s code, or at least have a vague idea about how to do it. However, fewer people have heard of format string attacks. Here’s a fairly detailed explanation, but I’ll summarize:
If, in your C or C++ code, you write printf(foo) (where foo is typically a const char*), it will just print foo to the screen. The one exception here is when foo contains the percent sign, in which case it prints corresponding things from the stack. If there are more %’s in the string than there are other things in the stack frame, it will begin printing out previous parts of foo itself. If foo was defined as input from a clever yet malicious user, they can craft strings that do nasty things to your program. Most importantly, they can read from (using %08x) and even write to (using %n) arbitrary locations in memory. Given that, they can pretty much do anything they want on your machine. Nifty!
The simple and obvious way to avoid this attack is to change all instances of printf(foo) in your code to printf("%s", foo) instead. The less obvious but much better solution is to not code in C or C++ ever again, and instead use a modern, high-level language like Python or Java (or if you’re Michael and worry about the speed of your program, use an actual low-level language like Assembly).