xpdf PDF viewer client earlier than 0.91 does not properly launch a web browser for embedded URL's, which allows an attacker to execute arbitrary commands via a URL that contains shell metacharacters.
http://www.securityfocus.com/bid/1624
http://www.redhat.com/support/errata/RHSA-2000-060.html
http://www.debian.org/security/2000/20000910a
http://www.calderasystems.com/support/security/advisories/CSSA-2000-031.0.txt