Heap-based attacks depend on a combination of memory management errors and an exploitable memory allocator. We analyze a range of widely-deployed memory allocators, including those used in Windows, Linux, FreeBSD, and OpenBSD. We show that despite numerous efforts to improve their security, they remain vulnerable to attack. We present the design and security analysis of DieHarder, a memory allocator that provides the highest degree of security from heap-based attacks of any practical allocator.