qpopper 4.0.5 and earlier does not properly drop privileges before processing certain user-supplied files, which allows local users to overwrite or create arbitrary files as root.
http://www.gentoo.org/security/en/glsa/glsa-200505-17.xml
http://www.debian.org/security/2005/dsa-728
http://secunia.com/advisories/15505
http://secunia.com/advisories/15478