x11.c in xonix 1.4 and earlier uses the current working directory to find and execute the rmail program, which allows local users to execute arbitrary code by modifying the path to point to a malicious rmail program.
https://exchange.xforce.ibmcloud.com/vulnerabilities/15873
http://www.securityfocus.com/bid/10149
http://www.debian.org/security/2004/dsa-484
http://shellcode.org/Advisories/XONIX.txt