Here is a very simple example of a global buffer overflow. See more on https://blog.gypsyengineer.com/fun/security/global-buffer-overflows.html

main.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
 
char buffer[16];
 
int main(int argc, char **argv) {
    if(argc < 2) {
        printf("no parameters specified\n");
        exit(-1);
    }
 
    // global buffer overflow may occur here 
    // if argv[1] has more than 16 symbols
    strcpy(buffer, argv[1]);
}