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.
https://security.gentoo.org/glsa/201503-04
https://bugzilla.redhat.com/show_bug.cgi?id=833703
http://www.ubuntu.com/usn/USN-1589-1
http://www.openwall.com/lists/oss-security/2012/07/11/17