xvfb-run 1.6.1 in Debian GNU/Linux, Ubuntu, Fedora 10, and possibly other operating systems place the magic cookie (MCOOKIE) on the command line, which allows local users to gain privileges by listing the process and its arguments.
https://exchange.xforce.ibmcloud.com/vulnerabilities/50348
http://www.vupen.com/english/advisories/2010/1185
http://www.ubuntu.com/usn/USN-939-1
http://www.securityfocus.com/bid/34828
http://www.openwall.com/lists/oss-security/2009/05/05/4