HIGH
Niels Provos Systrace before 1.6f on the x86_64 Linux platform allows local users to bypass intended access restrictions by making a 64-bit syscall with a syscall number that corresponds to a policy-compliant 32-bit syscall.
http://scary.beasts.org/security/CESA-2009-001.html
http://scarybeastsecurity.blogspot.com/2009/01/bypassing-syscall-filtering.html
http://www.citi.umich.edu/u/provos/systrace/
http://www.securityfocus.com/archive/1/500377/100/0/threaded