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:
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dt _peb
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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:
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0:001> r $peb
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$peb=000007fffffd5000
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The _PEB structure can now be overlaid on the memory pointed to by the $peb to see what values the structure members are holding/pointing to:
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0:001> dt _peb @$peb
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_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:
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0:001> db 0000000049d40000 L100
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Exactly! This is the actual binary image of the running process:
Another way of finding the ImageBaseAddress is:
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0:001> dt _peb
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ntdll!_PEB
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//snip
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      +0x010 ImageBaseAddress : Ptr64 Void
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//snip
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​
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0:001> dd @$peb+0x010 L2
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000007ff`fffd5010  49d40000 00000000
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​
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// 49d40000 00000000 is little-endian byte format - need to invert
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0:001> db 0000000049d40000 L100
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Convenience
We can forget about all of the above and just use:
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!peb
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This gets us a nicely formatted PEB information of some of the key members of the structure:
Finding Commandline Arguments
One of the interesting fields the PEB holds is the process commandline arguments. Let's find them:
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dt _peb @$peb processp*
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ntdll!_PEB
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   +0x020 ProcessParameters : 0x00000000`002a1f40 _RTL_USER_PROCESS_PARAMETERS
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​
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dt _RTL_USER_PROCESS_PARAMETERS 0x00000000`002a1f40
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We can be more direct and ask the same question like so:
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0:001> dt _UNICODE_STRING 0x00000000`002a1f40+70
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ntdll!_UNICODE_STRING
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 ""C:\Windows\system32\cmd.exe" "
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   +0x000 Length           : 0x3c
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   +0x002 MaximumLength    : 0x3e
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   +0x008 Buffer           : 0x00000000`002a283c  ""C:\Windows\system32\cmd.exe" "
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or even this:
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0:001> dd 0x00000000`002a1f40+70+8 L2
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00000000`002a1fb8  002a283c 00000000
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0:001> du 00000000002a283c
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00000000`002a283c  ""C:\Windows\system32\cmd.exe" "
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Since we now know where the commandline arguments are stored - can we modify them? Of course.
Forging Commandline Arguments
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0:001> eu 00000000002a283c "cmdline-logging? Are You Sure?"
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_PEB_LDR_DATA
Getting a list of loaded modules (exe/dll) by the process:
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// get the first _LIST_ENTRY structure address
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0:001> dt _peb @$peb ldr->InMemoryOrderModuleList*
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ntdll!_PEB
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   +0x018 Ldr                          : 
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      +0x020 InMemoryOrderModuleList      : _LIST_ENTRY [ 0x00000000`002a2980 - 0x00000000`002a1e40 ]
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​
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​
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// walking the list manually and getting loaded module info
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dt _LIST_ENTRY 0x00000000`002a2980
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// cmd module
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dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2980
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​
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dt _LIST_ENTRY 0x00000000`002a2980 
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// ntdll module
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dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2a70
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​
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dt _LIST_ENTRY 0x00000000`002a2a70
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// kernel32 module
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dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2df0
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​
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...loop...
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If we check the loaded modules with !peb, it shows we were walking the list correctly:
Here is another way to find the first _LDR_DATA_TABLE_ENTRY:
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dt _peb @$peb
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dt _PEB_LDR_DATA 0x00000000`774ed640
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dt _LDR_DATA_TABLE_ENTRY 0x00000000`002a2980
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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:
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0:001> dl 0x00000000`002a2980 6
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00000000`002a2980  00000000`002a2a70 00000000`774ed660
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00000000`002a2990  00000000`00000000 00000000`00000000
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00000000`002a2a70  00000000`002a2df0 00000000`002a2980
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00000000`002a2a80  00000000`002a2f70 00000000`774ed670
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00000000`002a2df0  00000000`002a2f60 00000000`002a2a70
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00000000`002a2e00  00000000`002a3cb0 00000000`002a2f70
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00000000`002a2f60  00000000`002a3ca0 00000000`002a2df0
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00000000`002a2f70  00000000`002a2e00 00000000`002a2a80
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00000000`002a3ca0  00000000`002a41f0 00000000`002a2f60
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00000000`002a3cb0  00000000`002defc0 00000000`002a2e00
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00000000`002a41f0  00000000`002a3ff0 00000000`002a3ca0
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00000000`002a4200  00000000`002e1320 00000000`002a4000
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Another way of achieving the same would be to use the !list command to list through the list items and dump the info:
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!list -x "dt _LDR_DATA_TABLE_ENTRY" 0x00000000`002a2980
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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
https://docs.microsoft.com/en-us/windows/desktop/api/winternl/ns-winternl-_peb_ldr_data
docs.microsoft.com
Common WinDbg Commands (Thematically Grouped)
PEB LDR DATA - aldeid
list (WinDbg) - Windows drivers
docsmsft
https://docs.microsoft.com/en-us/windows/desktop/api/winternl/ns-winternl-_peb_ldr_data
docs.microsoft.com
WinDbg : the !peb Command
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Instrumenting Windows APIs with Frida

Writing a Custom Bootloader

Last modified 2yr ago

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Contents

Basics

Convenience

Finding Commandline Arguments

Forging Commandline Arguments

_PEB_LDR_DATA

Abusing PEB

References