A vulnerability exists in the apcd package, as shipped in Debian GNU/Linux 2.1. By sending the apcd process a SIGUSR1, a file will be created in /tmp called upsstat. This file contains information about the status of the APC device. This file is not opened securely, however, and it is possible for an attacker to create a symlink with this name to another place on the file system. This could, in turn, lead to a compromise of the root account.
apcd is used to monitor information from APC uninterruptable power supplies. The ups will inform the apcd that power has been removed, and the apcd will shut down the machine.
ln -sf /tmp/upsstat /.rhosts
(wait for SIGUSR1 to be sent)
echo + + >> /.rhosts
rsh localhost -l root