The gdImageScaleTwoPass function in gd_interpolation.c in the GD Graphics Library (aka libgd) before 2.2.0, as used in PHP before 5.6.12, uses inconsistent allocate and free approaches, which allows remote attackers to cause a denial of service (memory consumption) via a crafted call, as demonstrated by a call to the PHP imagescale function.
https://github.com/libgd/libgd/issues/173
https://github.com/libgd/libgd/commit/4751b606fa38edc456d627140898a7ec679fcc24
http://www.ubuntu.com/usn/USN-2987-1
http://www.php.net/ChangeLog-5.php