The ADDW macro in stdio-common/vfscanf.c in the GNU C Library (aka glibc or libc6) before 2.21 does not properly consider data-type size during a risk-management decision for use of the alloca function, which might allow context-dependent attackers to cause a denial of service (segmentation violation) or overwrite memory locations beyond the stack boundary via a long line containing wide characters that are improperly handled in a wscanf call.
http://www.ubuntu.com/usn/USN-2519-1
http://www.securityfocus.com/bid/72499
http://www.oracle.com/technetwork/topics/security/linuxbulletinoct2015-2719645.html