CVE 2013-4788
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.
See the 
CVE page on Mitre.org
for more details.
