GetCommandLineA problem

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int0x50

Joined: 19 Jul 2019
Posts: 54

int0x50 19 Jul 2019, 11:58

I have made the following code to print arguments passed.

Code:

format PE64 console
entry start

include 'D:\fasmw17312\INCLUDE\win64a.inc'

section '.data' data readable writeable

printf.format.string db '%s', 0

arg1 db 'arg1: ', 13, 10, 0
arg2 db 'arg2: ', 13, 10, 0

GetCommandLine.String dq 1 dup 0h
GetCommandLine.String1 dq 1 dup 0h
GetCommandLine.String2 dq 1 dup 0h


section '.text' code readable executable
start:
push rbp
mov rbp, rsp

push rsi                ; before GetCommandLineA
xor rsi, rsi

invoke GetCommandLineA
mov [GetCommandLine.String], rax
mov rsi, rax

walkgetcommandline1:
        cmp byte [rsi], 32d
        je walkgetcommandline2
        inc rsi
        jmp walkgetcommandline1

walkgetcommandline2:
        inc rsi
        mov [GetCommandLine.String1], rsi

walkgetcommandline3:
        cmp byte [rsi], 32d
        je walkgetcommandline4
        inc rsi
        jmp walkgetcommandline3

walkgetcommandline4:
        inc rsi
        mov [GetCommandLine.String2], rsi

xor rsi, rsi
pop rsi                 ; after GetCommandLineA
        
        invoke printf, printf.format.string, arg2

invoke printf, printf.format.string, [GetCommandLine.String2]

mov rsp, rbp
pop rbp
ret

running it as : example.exe 11111 22222

while running this program under cmd.exe, the output is 11111 22222
while running this program under the debugger, the output is 22222

Why there is a difference?

Edit by revolution: Added code tags

19 Jul 2019, 11:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 19 Jul 2019, 12:10

There shouldn't be any difference.

Which debugger are you using? How are you starting the program for debugging?

19 Jul 2019, 12:10

int0x50

Joined: 19 Jul 2019
Posts: 54

int0x50 19 Jul 2019, 12:19

x64dbg.

19 Jul 2019, 12:19

edfed

Joined: 20 Feb 2006
Posts: 4353
Location: Now

edfed 19 Jul 2019, 12:24

hum... maybe under the debugger you don't pass all the command line....

in cmd, you will pass the full command, including the executable name.

in the debugger, it's maybe possible this token is not present.

if it's that, the solution would be to ignore the first token only if it's equal the executable name.

19 Jul 2019, 12:24

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 19 Jul 2019, 13:14

You can do a comparison by simply printing the full string returned by GetCommandLineA and look for any differences.

19 Jul 2019, 13:14

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 19 Jul 2019, 14:15

Seems to be the case of which parameters CreateProcess ends up being passed by different means of running the program. Since you skip everything until the first space whenever command line parameters get passed through a separate parameter to CreateProcess the first space becomes the one after your 11111 parameter.

19 Jul 2019, 14:15

bitRAKE

Joined: 21 Jul 2003
Posts: 4073
Location: vpcmpistri

bitRAKE 19 Jul 2019, 19:23

When I use x64dbg and change the commandline, it will provide the executable name as the first parameter by default - there is no need to leave it unchanged unless one wants the same effect of using the shell. The shell execution interface is a subset of what CreateProcess/x64dbg allows.

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19 Jul 2019, 19:23

int0x50

Joined: 19 Jul 2019
Posts: 54

int0x50 20 Jul 2019, 03:45

cmd.exe adds extra space between the program name and the first argument. that's the reason my validation failed. The debugger does not do this, hence my validation worked.

Now whenever I run it in cmd.exe, I have to add ' inc esi '. Remark this line whenever I debug it.

thank you all.

20 Jul 2019, 03:45

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 20 Jul 2019, 05:09

If the user types two or more spaces between arguments then it will fail also.

20 Jul 2019, 05:09

int0x50

Joined: 19 Jul 2019
Posts: 54

int0x50 21 Jul 2019, 08:58

revolution, exactly. I am trying to figure out how to solve this problem. I think I have to traverse till i get a character.

21 Jul 2019, 08:58

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20451
Location: In your JS exploiting you and your system

revolution 21 Jul 2019, 11:51

int0x50 wrote:

I think I have to traverse till i get a character.

That is the usual solution. Skip all whitespace between arguments.

You can make it a function call to make the code easier to understand.

21 Jul 2019, 11:51

Furs

Joined: 04 Mar 2016
Posts: 2565

Furs 22 Jul 2019, 11:49

You also need to handle quotes and stuff if you don't want to confuse the user, the "rules" are not that straightforward. Better use CommandLineToArgvW.

22 Jul 2019, 11:49

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