Theory

Buffer overflow is a common security vulnerability that occurs when more data is written to a buffer than it can hold. A buffer is a contiguous block of memory that is allocated to store data temporarily, such as a string of text or an array of integers. When a program writes data beyond the bounds of a buffer, it can overwrite adjacent memory locations, leading to unpredictable behavior. This vulnerability is particularly prevalent in languages like C and C++, where memory management is manual, and there are no built-in safeguards to prevent overwriting memory.

Key Concepts of Buffer Overflow

Memory Management: In programming languages like C and C++, memory management is handled manually. This means developers must allocate and deallocate memory as needed. If they allocate a buffer but don't ensure that the data written to it fits within its boundaries, a buffer overflow can occur.

Stack: Buffers often reside in the stack, a memory region used for local variables and function call management. Stack-based buffer overflows are the most common because local variables are stored contiguously. When a buffer overflows, it can overwrite adjacent memory in the stack frame, including critical elements like return addresses and function pointers, leading to unpredictable behavior or exploitation.

How Buffer Overflow Happens?

Lack of Bounds Checking: Many standard C library functions, such as strcpy, gets, and scanf, do not check the size of the destination buffer. For instance, if a program uses strcpy to copy a 20-character string into a 10-character buffer, the extra 10 characters overwrite whatever comes after the buffer in memory.

User Input: Buffer overflows often occur when a program accepts user input without validating its size. For example, a program might prompt for a username and store it in a fixed-size buffer. If the user enters a string longer than the buffer’s capacity, the excess data overflows into adjacent memory.