The history of the EncodePointer function for obfuscating pointers

Raymond Chen

Raymond

Windows XP introduced the function Encode­Pointer whose mission was to take a pointer value and obfuscate it in a way that is different for each process. The purpose is to add a layer of defense in depth: If an attacker is able to extract the encoded pointer value, they nevertheless don’t get very much information about the address space layout of the target process. This makes it harder to attack the target with, say, write-what-where attack to overwrite a function pointer: Before they can overwrite the pointer, they need to be able to encode it in the specific way that is custom to the target, so that when the target decodes it, the result is the value the attacker desires.

Windows XP took the extra step of taking the information required to encode and decode pointers, and moving some of it out of the process address space. So even if somebody got free access to the entire process address space, they still couldn’t figure out how to encode and decode pointers.

Some of the critical information was kept in kernel mode, and each time the target process wanted to encode or decode a pointer, it called into the kernel to say, “Please do your extra secret magic sauce.”

In Windows 10 version 1809, the secret magic sauce was brought into the process to avoid a kernel transition every time a pointer needed to be encoded or decoded. This was good news, bad news, and mitigated news.

The good news is that this makes encoding and decoding pointers much, much faster.

The bad news is that this makes all of the information needed to encode and decode pointers visible to user-mode.

The mitigated news is that in the time since pointer encoding was introduced, there have been a lot of changes which mitigate the security impact. For example, address space layout randomization (ASLR) makes it harder to predict where that last piece of information went. And control flow guard (CFG) makes it harder to get control to jump through a function pointer to an address of your choosing.

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  • Henke37
    Henke37

    With the introduction of DecodeRemotePointer the point is kinda lost, since most attackers that can get the pointer can also open a handle to the process.

    As for where the secret sauce was stored, I actually looked into it a while back. For those curious it’s accessed using an undocumented NtQueryInformationProcess class. Which is very much unsupported. Just use DecodeRemotePointer and you get the mangling for free.

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      Jan Kraetzschmar

      To be fair, according to the documentation EncodeRemotePointer and DecodeRemotePointer have never been part of the public API of a stable Windows 10 release, but still live on their undocumented lives in NTDLL, probably unfazed by this change. Before them, you already could use various means of remote code execution to deobfuscate pointers using DecodePointer in the target process, so this is only a more convenient way to attack yourself.
      EncodeRemotePointer already seems to work with a process handle with mere PROCESS_VM_WRITE access rights. But since you can’t access another process’s memory without going through the kernel, it might as well keep providing the secret sauce in this case.