Example - 1

Consider the following source code from a (hypothetical) FTP server:

fwrite(buf, 1, sizeof(buf), network);
chroot("/var/ftproot");...fgets(filename, sizeof(filename), network);localfile = fopen(filename, "r");while ((len = fread(buf, 1, sizeof(buf), localfile)) != EOF) {}fclose(localfile);

This code is responsible for reading a filename from the network, opening the corresponding file on the local machine, and sending the contents over the network. This code could be used to implement the FTP GET command. The FTP server calls chroot() in its initialization routines in an attempt to prevent access to files outside of /var/ftproot. But because the server does not change the current working directory by calling chdir("/"), an attacker could request the file "../../../../../etc/passwd" and obtain a copy of the system password file.