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PicoWebServer Remote Unicode Stack Overflow
From: Dennis Elser <dennis () backtrace de>
Date: Sat, 28 May 2005 18:13:03 +0200

Title: PicoWebServer Remote Unicode Stack Overflow
    Release Date: 28. May 2005
          Author: Dennis Elser (dennis backtrace de)

          Vendor: Newmad Technologies (http://www.newmad.se)
   Vendor Status: Contacted
         Product: PicoWebServer
Affected Version: 1.0

        Platform: Microsoft Windows CE / PocketPC
  Processor Type: ARM

   Vulnerability: Missing user-input length check
            Risk: High (Remote code execution)
       Discovery: 23. May 2005
          Impact: A remote attacker can gain full access to the device
                  or crash the application



PicoWebServer is a free "light weight" HTTP web server for PocketPCs
running the Windows CE operating system.
By default, the PicoWebServer application is configured to listen for
incoming connections on TCP port 80.

Bug Description:

If the supplied URL is large enough, an attack can trigger a stack overflow
and the application crashes. If successfully exploited, a remote attacker can
gain control over the device.

Technical Description / Proof of Concept:

An attacker gains control over the Program Counter (PC) by supplying
an outsized HTTP GET request.

1.) The maximum number of bytes that an attacker can supply is limited by
a loop checking for the occurence of a "0D 0A 0D 0A" byte sequence.


.text:00015664 SUB R3, R8, R6 ; R3 = 0x400 = 1024
   .text:00015668 SUBS    R7, R3, #1                      ; R7 = 1023
   .text:0001566C MOVMI   R7, R11
.text:00015670 MOV R2, R7 ; len = R7 = R2 = 1023
   .text:00015674 ADD     R1, R6, R4                      ; buf
   .text:00015678 MOV     R0, R10                         ; s
   .text:0001567C BL      _recv

                                                          ; check for \n\n
.text:00015704 MOV R0, R4 ; char * 0D,0A,0D,0A
   .text:00015708 MOV     R1, R5                          ; char *
   .text:0001570C BL      strstr
   .text:00015710 MOVS    R3, R0
   .text:00015714 BEQ     get_more_data_from_attacker

If the above-mentioned byte sequence was found, the buffer might look similar to this one:

   GET /xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx[...snip...]\x0D\x0A\x0D\x0A

2.) This buffer is then converted to unicode and passed to an unchecked
swprintf() call, where the stack overflow occurs:

.text:00013738 ADD R3, R4, #0xC ; aGetXxxxxxxxxxxxxxxx
                                                          ; snip
.text:00013774 ADD R0, SP, #0x820+var_7EC ; wchar_t *destination .text:00013778 LDR R1, =aD_D_D_DDDDDDDS ; wchar_t *format_string ; [%d.%d.%d.%d - %d/%d/%d () %d:%d:%d] "%s"
                                                          ; snip
   .text:00013798 BL      swprintf

The call to swprintf() uses a stack variable (var_7EC) to store
the resulting formatted string. It can hold a maximum number of 0x7EC (2028) bytes.
Once the string has been formatted and saved to the destination buffer,
a possible result can be:

   [ - 2005/5/12 () 12:30:11] "GET /"

The string is filled with an ip address, the current date and time and the
GET request made by an attacker.
In this example (depending on the ip, date, time and so on), an attacker would have to supply 1974 additional bytes in order to take control over the Program Counter.

   sizes       |    description
       2028    |    size of var_7EC
- 82 | size of unicode string: [ - 2005/5/12 () 12:30:11] "GET /"
   +      4    |    size of R4 (saved at function prologue)
   +      4    |    size of R5 (saved at function prologue)
   +      4    |    size of R6 (saved at function prologue)
   +      4    |    size of R7 (saved at function prologue)
   +      4    |    size of R8 (saved at function prologue)
   +      4    |    size of R9 (saved at function prologue)
   +      4    |    size of PC (saved at function prologue)
   =   1974         size of user-supplied buffer needed to overflow stack

At the function epilogue of function sub_000136A4 the instruction at address 0x000137D0 modifies the stack pointer (SP) to point to offset 0x7D0 of the formatted string
containing user-supplied input (potential shellcode).
The instruction at 0x000137D4 restores R4, R5, R6, R7, R8, R9 and PC from the stack.

   .text:000137D0 ADD     SP, SP, R12
   .text:000137D4 LDMFD   SP!, {R4-R9,PC}

Registers R4 to R9 and PC are under control of the attacker.

PicoWebServer.exe: The instruction at 0x780078 referenced memory at 0x780078.
   The memory could not be read (0x00780078 -> 00780078)

An attacker has full control over the device if he is able to let the overwritten return address point to a "0D F0 A0 E1" ("MOV PC, SP") equivalent byte sequence. Since SP is the only register pointing into the potential shellcode supplied by an attacker,
the aim of an attacker is to let PC equal SP.


debug1218:2211E90C 78 00 78 00 DCD 0x780078 ; R4 debug1218:2211E910 78 00 78 00 DCD 0x780078 ; R5 debug1218:2211E914 78 00 78 00 DCD 0x780078 ; R6 debug1218:2211E918 78 00 78 00 DCD 0x780078 ; R7 debug1218:2211E91C 78 00 78 00 DCD 0x780078 ; R8 debug1218:2211E920 78 00 78 00 DCD 0x780078 ; R9 debug1218:2211E924 04 00 07 00 DCD addr_MOV_PC_SP ; PC debug1218:2211E928 ; ------------------------------------------------- debug1218:2211E928 78 00 78 00 RSBEQS R0, R8, R8,ROR R0 ; shellcode
   debug1218:2211E92C 78 00 78 00 RSBEQS  R0, R8, R8,ROR R0
   debug1218:2211E930 78 00 78 00 RSBEQS  R0, R8, R8,ROR R0
   debug1218:2211E934 78 00 78 00 RSBEQS  R0, R8, R8,ROR R0
   debug1218:2211E938 78 00 78 00 RSBEQS  R0, R8, R8,ROR R0


Dennis Elser, 28.05.2005

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