Token Abuse for Privilege Escalation in Kernel

The purpose of this lab is to understand at a high level (will not be writing any kernel code, rather playing around with WinDBG) how kernel exploits abuse tokens for privilege escalation.

I will look briefly into two techniques:

• Token stealing/replacement - low privileged token is replaced with a high privileged token

• Token privilege adjustment - adding and enabling more privileges to an existing token

Key Structures

Before proceeding, there's a couple of kernel memory structures we need to know about.

_EPROCESS

_EPROCESS is a kernel memory structure that describes system processes (or in other words - each process running on a system has its corresponding _EPROCESS object somewhere in the kernel) as we know them as it contains details such as process image name, which desktop session it is running in, how many open handles to other kernel objects it has, what access token it has and much more.

Below is a snippet of the structure:

dt _eprocess

_TOKEN

_TOKEN is a kernel memory structure that describes process's security context and contains information such as process token privileges, logon id, session id, token type (i.e primary vs. impersonation) and much more.

Below is a snippet of the _TOKEN structure:

dt _token

Let's now see how we can use the above information about processes and tokens to elevate a medium integrity process to a system integrity process the way kernel exploits do it.

1. Replacing Tokens for Privilege Escalation

One way kernel exploits escalate privileges is by replacing a low privileged token with a high privileged token. Below are some key points in explaining the exploitation process:

• Each process running on the system has its corresponding _EPROCESS kernel structure

• _EPROCESS structure contains a pointer to a _TOKEN memory structure that describes process's security context

• Kernel exploit finds address of the _TOKEN structure of a low privileged process - the one it wants to escalate from

• Kernel exploit finds address of the _TOKEN structure of a privileged process, running as NT\SYSTEM

• Kernel exploit replaces the low privileged process's token with the high privileged token

In this lab, I'm going to replace the authentication token of a low privileged powershell process with a high privileged token of the system process (always a PID 4) following the above described process, except I will do it manually using WinDBG.

Below is an attempt to visually represent the above described process with a high level diagram:

• Boxes with blue headings represent a MEDIUM integrity process, running as WS02\spotless

  • WS02 is my lab machine name

  • spotless is a low privileged local user.

• Boxed with red headings indicate a SYSTEM integrity process, effectively running as NT\SYSTEM

  • WS02$ is my lab computer account

  • OFFENSE is the domain the machine is a member of

• Red dotted line signifies that the low privileged process powershell will assume the high privileged token from the process system once the _TOKEN kernel memory structure is manipulated.

Let's now try to see how we can replace process tokens using WinDBG.

Listing Processes

First off, listing all running processes on the system in WinDBG can be done like so:

!process 0 0

Below is a snippet of some of the processes running on the system and highlighted are addresses pointing to _EPROCESS structures for given processes:

Medium Integrity Process

Next, let's launch powershell (this is the process for which we will replace the low privileged token with a high privileged token) as a medium integrity/non-elevated process (in my case running as a local non-admin user ws02\spotless) and get its process ID:

Let's get a process summary in WinDBG for our powershell process with PID 2648 (0xa58):

!process a58 0

Below confirms we're looking at our powershell.exe process. Note the _EPROCESS location ffffdc8fbe1f1080:

Finding Powershell Token

Once we have powershell's _EPROCESS location in the kernel, we can inspect its contents like so:

kd> dt _eprocess ffffdc8fbe1f1080

Since we're interested in swapping the token, the key member of the _EPROCESS memory structure we are after is Token located at offset 0x358:

Let's read memory contents of the pointer the _EPROCESS.Token is pointing to, which is ffffc507`dab7799f in my case:

kd> dq ffffdc8fbe1f1080+0x358 l1
ffffdc8f`be1f13d8  ffffc507`dab7799f

If we try inspecting the memory location ffffc507`dab7799f with !token ffffc507dab7799f command, we are told that this address does not point to a token object, which we may find a bit odd:

However, this is where the _EX_FAST_REF comes into play. It was pointed out earlier that _EPROCESS.Token actually points to a _EX_FAST_REF structure rather than a _TOKEN structure.

Let's overlay the address stored in _EPROCESS.Token which is ffffdc8f`be1f13d8 (_EPROCESS location plus the Token member offset (ffffdc8fbe1f1080+0x358)) with the _EX_FAST_REF structure and see what's inside:

kd> dt _EX_FAST_REF ffffdc8fbe1f1080+0x358
ntdll!_EX_FAST_REF
   +0x000 Object           : 0xffffc507`dab7799f Void
   +0x000 RefCnt           : 0y1111
   +0x000 Value            : 0xffffc507`dab7799f

Notice how all three members have the same offset and Object and Value are pointing to the same address, but the interesting piece is the RefCnt with 4 bits on (equals to 0xF, which looks like it is the last digit of both Object and Value members are pointing to - 0xffffc507`dab7799f).

If we inspect the _EX_FAST_REF without data, based on the symbols, it's defined like so:

ntdll!_EX_FAST_REF
   +0x000 Object           : Ptr64 Void
   +0x000 RefCnt           : Pos 0, 4 Bits
   +0x000 Value            : Uint8B

Which indicates and confirms that the last 4 bits (the last hex digit of the Object or Value) of the value pointed to by members Object and Value (in my case 0xffffc507`dab7799f) is used to denote the reference count to this token, which means it's not part of the token address, which means we should be able to zero it out and get an actual _TOKEN structure address for our powershell process.

Essentially, if Object and Value are 0xffffc507`dab7799f, we should be able to just swap the last f with 0 which would give us 0xffffc507`dab77990 and it should be our _TOKEN address.

In fact, if we inspect our powershell process with a more verbose output like so:

!process ffffdc8fbe1f1080 1 
// or !process 0xa58 1

..we see that indeed the Token is pointing to 0xffffc507`dab77990 - note the last digit is 0 rather than f, which confirms that we can always zero out the last digit pointed to by _EX_FAST_REF to get the effective _TOKEN structure address:

We can mask out the last digit with a bitwise AND operation as shown below:

kd> ? (ffffc507dab7799f & 0xFFFFFFF0); !token (ffffc507dab7799f & 0xFFFFFFF0)

Now, if we try the !token command again with the last digit of _EPROCESS.Token->Value set to 0, we no longer see the error message suggesting there's no token at that address and we start seeing some actual token details like user group it belongs to, etc.:

Confirming SIDs

We can double check we're actually looking at the right token - the SID's seen in the output of whoami /all and the !token (ffffc507dab7799f & 0xFFFFFFF0) match:

Finding SYSTEM Token

Now let's find the address of the high privileged _TOKEN - the token that our low privileged powershell process will assume.

Below shows some information about the SYSTEM process - we're interested in it's _TOKEN location which is at ffffc507d8818040 as shown below:

kd> !process 4 1
Searching for Process with Cid == 4
PROCESS ffffdc8fbdad3040
    SessionId: none  Cid: 0004    Peb: 00000000  ParentCid: 0000
    DirBase: 001aa002  ObjectTable: ffffc507d88032c0  HandleCount: 3042.
    Image: System
    VadRoot ffffdc8fbdad1170 Vads 8 Clone 0 Private 21. Modified 76433. Locked 0.
    DeviceMap ffffc507d8818eb0
    Token                             ffffc507d8818040

Swapping Tokens

We now have all the required information to successfully swap the powershell process token (located at ffffdc8fbe1f1080+0x358) with that held by the SYSTEM process (ffffc507d8818040) by simply writing the SYSTEM process's token address to the the _EPROCESS.Token of our powershell process:

eq ffffdc8fbe1f1080+0x358 ffffc507d8818040

Below shows the above in action and how prior to the token manipulation, the powershell was running as ws02\spotless and nt authority\system after:

2. Modifying Token Privileges

Another interesting (and abused for privilege escalation) member of the _TOKENstructure is Privileges at offset 0x040, defined as _SEP_TOKEN_PRIVILEGES structure:

dt _token 0xffffc507dab77990

Enabling Existing Privileges

We can overlay our low privileged powershell token address + offset 0x40 to inspect the _sep_token_privileges structure:

dt _sep_token_privileges 0xffffc507dab77990+0x40

In essence, _sep_token_privileges shows which privileges the token has and which of them are enabled/disabled - the info that we can also check from the userland with whoami /priv.

Note how _sep_token_privileges Present and Enabled values do not match and this is what results in Enabled/Disabled privileges that we see in the whoami /priv State column:

We can manipulate the kernel memory and make Present and Enabled values match like so:

eq 0xffffc507dab77990+0x40+8 0x00000006`02880000

After manipulating the memory and matching the Present and Enabled values, we can now see how all the privileges in the State column of the whoami /priv output are Enabled:

Adding More Privileges

Let's see if we can try to add more privileges to that exsiting token rather than just enabling those that already exist.

How do we know what is a valid value in the Present field that would give us more/elevated privileges? We can get a good hint by inspecting the Present value of the SYSTEM process (PID 4) token:

!process  4 1
sep_token_privileges 0x40+ffffde8fe9a06040

From the above, Present value is 0x0000001f`f2ffffbc - this represents all the privileges the SYSTEM process token has.

Let's see if we can assign this value to our powershell process's token to both Present and Enabled fields. If successful, we should have all the SYSTEM privileges enabled for our low privileged powershell process running in the context of the user ws02\spotless:

kd> eq 0x40+ffffde8ff8cde5f0+8 0x0000001f`f2ffffbc
kd> eq 0x40+ffffde8ff8cde5f0 0x0000001f`f2ffffbc

Let's check if the new values got assigned to our _sep_token_privileges structure:

kd> dt _sep_token_privileges 0x40+ffffde8ff8cde5f0
nt!_SEP_TOKEN_PRIVILEGES
   +0x000 Present          : 0x0000001f`f2ffffbc
   +0x008 Enabled          : 0x0000001f`f2ffffbc
   +0x010 EnabledByDefault : 0x40800000

Running whoami /priv now shows that we have all the SYSTEM privileges and all of them are enabled:

References

token-priv/abusing_token_eop_1.0.txt at master · hatRiot/token-privGitHub

Starting with Windows Kernel Exploitation – part 3 – stealing the Access Tokenhasherezade's 1001 nights