system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are buffer overflows and how do people hack software with them?
gpt
A buffer is a place where data is stored.  Buffers have a fixed size.  A program must be very careful to not put more data into the buffer than its size allows, or else the buffer will "overflow" and damage the data in surrounding memory.  It would be like if your toilet overflowed and water spilled onto your floor, damaging your flooring and drywall.

If a malicious hacker has control over the data that is put into a buffer, sometimes they can control what data overflows out of the buffer as well.  This allows them to replace *other* data in memory, with the data they chose.  This can change the behavior of the program, allowing the hacker to manipulate it to his/her will.

For a more concrete example, take a login and password box on a website.  Usually logins are less than 12 characters or so, so let's pretend the programmer made a buffer for them that only held 30 characters (to give a safe margin).  Suddenly a hacker comes along and enters 5,000 characters into the login box.  If the programmer hasn't checked for this condition, the program might end up writing 5,000 characters into a buffer that was meant for maximum 30 characters.  Those extra 4,970 characters can overwrite other data in memory, allowing the hacker to change the operation of the program.