The PTR_MANGLE implementation in the GNU C Library (aka glibc or libc6) 2.4, 2.17, and earlier, and Embedded GLIBC (EGLIBC) does not initialize the random value for the pointer guard, which makes it easier for context-dependent attackers to control execution flow by leveraging a buffer-overflow vulnerability in an application and using the known zero value pointer guard to calculate a pointer address.
https://security.gentoo.org/glsa/201503-04
http://www.securityfocus.com/bid/61183
http://www.mandriva.com/security/advisories?name=MDVSA-2013:284
http://www.mandriva.com/security/advisories?name=MDVSA-2013:283