Exploring Process Environment Block

Exploring Process Environment Block

A very brief look into the PEB memory structure found, aiming to get a bit more comfortable with WinDBG and walking memory structures.

Basics

First of, checking what members the _PEB structure actually entails:

dt _peb

There are many fields in the structure among which there are ImageBaseAddresss and ProcessParameters which are interesting to us for this lab:

Getting the PEB address of the process:

0:001> r $peb
$peb=000007fffffd5000

The _PEB structure can now be overlaid on the memory pointed to by the $peb to see what values the structure members are holding/pointing to:

0:001> dt _peb @$peb

_PEB structure is now populated with the actual data pulled from the process memory:

Let's check what's in memory at address 0000000049d40000 - pointed to by the ImageBaseAddress member of the _peb structure:

0:001> db 0000000049d40000 L100

Exactly! This is the actual binary image of the running process:

Another way of finding the ImageBaseAddress is:

0:001> dt _peb
ntdll!_PEB
//snip
      +0x010 ImageBaseAddress : Ptr64 Void
//snip

0:001> dd @$peb+0x010 L2
000007ff`fffd5010  49d40000 00000000

// 49d40000 00000000 is little-endian byte format - need to invert
0:001> db 0000000049d40000 L100

Convenience

We can forget about all of the above and just use:

!peb

This gets us a nicely formatted PEB information of some of the key members of the structure:

Finding Commandline Arguments

One of the interesting fields the PEB holds is the process commandline arguments. Let's find them:

dt _peb @$peb processp*
ntdll!_PEB
   +0x020 ProcessParameters : 0x00000000`002a1f40 _RTL_USER_PROCESS_PARAMETERS

dt _RTL_USER_PROCESS_PARAMETERS 0x00000000`002a1f40

We can be more direct and ask the same question like so:

0:001> dt _UNICODE_STRING 0x00000000`002a1f40+70
ntdll!_UNICODE_STRING
 ""C:\Windows\system32\cmd.exe" "
   +0x000 Length           : 0x3c
   +0x002 MaximumLength    : 0x3e
   +0x008 Buffer           : 0x00000000`002a283c  ""C:\Windows\system32\cmd.exe" "

or even this:

0:001> dd 0x00000000`002a1f40+70+8 L2
00000000`002a1fb8  002a283c 00000000
0:001> du 00000000002a283c
00000000`002a283c  ""C:\Windows\system32\cmd.exe" "

Since we now know where the commandline arguments are stored - can we modify them? Of course.

Forging Commandline Arguments

0:001> eu 00000000002a283c "cmdline-logging? Are You Sure?"

_PEB_LDR_DATA

Getting a list of loaded modules (exe/dll) by the process:

// get the first _LIST_ENTRY structure address
0:001> dt _peb @$peb ldr->InMemoryOrderModuleList*
ntdll!_PEB
   +0x018 Ldr                          : 
      +0x020 InMemoryOrderModuleList      : _LIST_ENTRY [ 0x00000000`002a2980 - 0x00000000`002a1e40 ]


// walking the list manually and getting loaded module info
dt _LIST_ENTRY 0x00000000`002a2980
// cmd module
dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2980

dt _LIST_ENTRY 0x00000000`002a2980 
// ntdll module
dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2a70

dt _LIST_ENTRY 0x00000000`002a2a70
// kernel32 module
dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2df0

...loop...

If we check the loaded modules with !peb, it shows we were walking the list correctly:

Here is another way to find the first _LDR_DATA_TABLE_ENTRY:

dt _peb @$peb
dt _PEB_LDR_DATA 0x00000000`774ed640

dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2980

A nice way of getting a list of linked-list structure addresses is by providing address of the first list_entry structure to the command dl and specifying how many list items it should print out:

0:001> dl 0x00000000`002a2980 6
00000000`002a2980  00000000`002a2a70 00000000`774ed660
00000000`002a2990  00000000`00000000 00000000`00000000
00000000`002a2a70  00000000`002a2df0 00000000`002a2980
00000000`002a2a80  00000000`002a2f70 00000000`774ed670
00000000`002a2df0  00000000`002a2f60 00000000`002a2a70
00000000`002a2e00  00000000`002a3cb0 00000000`002a2f70
00000000`002a2f60  00000000`002a3ca0 00000000`002a2df0
00000000`002a2f70  00000000`002a2e00 00000000`002a2a80
00000000`002a3ca0  00000000`002a41f0 00000000`002a2f60
00000000`002a3cb0  00000000`002defc0 00000000`002a2e00
00000000`002a41f0  00000000`002a3ff0 00000000`002a3ca0
00000000`002a4200  00000000`002e1320 00000000`002a4000

Another way of achieving the same would be to use the !list command to list through the list items and dump the info:

!list -x "dt _LDR_DATA_TABLE_ENTRY" 0x00000000`002a2980

Continuing further:

Abusing PEB

It is possible to abuse the PEB structure and masquerade one windows processes with another process. See this lab for more:

Masquerading Processes in Userland via _PEB

References

Common WinDbg Commands (Thematically Grouped)windbg.info

PEB LDR DATA - aldeidwww.aldeid.com

!list (WinDbg) - Windows driversMicrosoftLearn