parasite.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/stat.h>

#define PARASITE_SIZE 8824

int main(int argc, char **argv) {

    /* Execute parasite code */
    printf("Parasite code is running!\n");

    /* Move to the beginning of the host executable */
    FILE *fp = fopen(argv[0], "r");
    fseek(fp, 0L, SEEK_END);
    int file_size = ftell(fp);

    fseek(fp, PARASITE_SIZE, SEEK_SET);

    /* Read host executable into buffer */
    int host_size = file_size - PARASITE_SIZE;
    char *host_buffer = calloc(host_size, sizeof(char));
    fread(host_buffer, sizeof(char), host_size, fp); 

    /* Write host executable into a temp file */
    FILE *temp_fp = fopen("temp.out", "w");
    fwrite(host_buffer, sizeof(char), host_size, temp_fp);

    /* Clean up */
    fclose(temp_fp);
    fclose(fp);
    free(host_buffer);

    chmod("temp.out", S_IXOTH | S_IXGRP | S_IXUSR);

    /* Execute the host executable */
    int ret = execl("temp.out", "temp.out", (char *) 0);
    if (ret == -1) {
        perror("execl");
        exit(1);
    }

    return 0;
}