The vfprintf function in stdio-common/vfprintf.c in libc in GNU C Library (aka glibc) 2.12 and other versions does not properly calculate a buffer length, which allows context-dependent attackers to bypass the FORTIFY_SOURCE format-string protection mechanism and cause a denial of service (stack corruption and crash) via a format string that uses positional parameters and many format specifiers.
http://rhn.redhat.com/errata/RHSA-2012-1098.html
http://rhn.redhat.com/errata/RHSA-2012-1200.html
http://www.openwall.com/lists/oss-security/2012/07/11/17
http://www.ubuntu.com/usn/USN-1589-1
https://bugzilla.redhat.com/show_bug.cgi?id=833703