PicoWebServer Remote Unicode Stack Overflow

From: Dennis Elser (dennis_at_backtrace.de)
Date: 05/28/05

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    Date: Sat, 28 May 2005 18:13:03 +0200
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                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

    --------------------------------------------------------------------------

    Background:
    -----------

    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.

    get_more_data_from_attacker:
        ;snip

        .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

        ;snip
                                                               ; 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:

        [127.0.0.1 - 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: [127.0.0.1 -
    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.

        stack:

        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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