The most common vulnerability in C programming is buffer overflows. The buffer, as the name suggests, represents a temporary area of memory storage in the RAM that a program uses to run itself. Usually, all the variables used in a program are assigned temporary buffer storage for keeping the values assigned to them. Some functions don't limit the data within the buffer boundaries while assigning larger values (larger than the assigned buffer) to the variables, leading to overflowing of the buffer. The overflowing data corrupts or overwrites data of other buffers.
These buffer overflows might be used by hackers or malicious users to damage files or data or to extract sensitive information. That is, an attacker might enter such an input that leads to buffer overflows.