Vulnerable Windows Images Tdll At 0X7C90Beef

A better reason for preferred base addresses is to ensure that only one copy of a DLL needs to be in memory.

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Breaking ASLR is an area of active research and can get incredibly complicated.

In addition to covering what ASLR accomplishes to improve security posture, we aim to give defenders advice on how to improve the security of their software, and to give researchers more insight into how ASLR works and ideas for investigating its limitations.

This may crash the program, or worse, provide the attacker full control over the system.

Memory corruption vulnerabilities have plagued software for decades, despite efforts by large companies like Apple, Google, and Microsoft to eradicate them.

A silver bullet would be a mechanism to make exploits so tricky and unreliable that buggy code can be left in place, giving developers the years they need to fix or rewrite code in memory-safe languages.

Unfortunately, nothing is perfect, but address space layout randomization (ASLR) is one of the best mitigations available.

A common example is the locations of gadgets used in return-oriented programming (ROP), which is often used to defeat the defense of data execution prevention (DEP).

ASLR mixes up the address space of the vulnerable processthe main program, its dynamic libraries, the stack and heap, memory-mapped files, and so onso that exploit payloads must be uniquely tailored to however the address space of the victim process is laid out at the time.

Writing a worm that propagates by blindly sending a memory corruption exploit with hard-coded memory addresses to every machine it can find is bound to fail.

So long as the target process has ASLR enabled, the exploits memory offsets will be different than what ASLR has selected.

This crashes the vulnerable program rather than exploiting it.

Pre-Vista versions of Windows lacked ASLR; worse, they went to great lengths to maintain a consistent address space across all processes and machines.

One might assume that prior versions simply didnt randomize the address space, and instead simply loaded DLLs at whatever location was convenient at the timeperhaps a predictable one, but not necessarily the same between two processes or machines.

Unfortunately, these old Windows versions instead went out of their way to achieve what well call Address Space Layout Consistency.

Table 1 shows the preferred base address of some core DLLs of Windows XP Service Pack 3.

Vulnerable Windows Images Tdll At 0X7C90Beef

If an attacker finds a useful ROP gadget in ntdll at 0x7c90beef, for example, the attacker can assume that it will always be available at that address until a future service pack or security patch requires the DLLs to be reorganized.

This means that attacks on pre-Vista Windows can chain together ROP gadgets from common DLLs to disable DEP, the lone memory corruption defense on those releases.

Especially on 32-bit machines, if Windows DLL code needs to reference a global variable, the runtime address of that variable gets hardcoded into the machine code.

If the DLL gets loaded at a different address than was expected, relocation is performed to fix up such hardcoded references.

If the DLL instead gets loaded as its preferred base address, no relocation is necessary, and the DLLs code can be directly mapped into memory from the file system.