IIS ASP $19.95 hack - IISHack 1.5
Release Date: November 3rd, 2000
Systems Affected: Windows NT 4.0 IIS 4.0 sp6 (vulnerable) Windows NT 5.0 IIS 5.0 (not vulnerable)
Description: There exists a buffer overflow, that can be exploited to gain SYSTEM level access, within the IIS (Internet Information Server) .ASP ISAPI file parsing mechanism.
This is not a remote exploit, it is a local one (However, we will explain later in the advisory how you could pull this exploit off remotely.). It is local in the sense that you need to actually create an "evil" .asp file that when parsed by IIS will cause inetinfo.exe to buffer overflow and therefore allow you to take control of the local server as SYSTEM.
So who is affected by this? Any web hosting company or internet service provider that is running multiple clients within the same NT4 IIS 4 web server. Basically any client (or attacker) who can update their website, i.e. upload a new default.asp or anything.asp, can then execute code as SYSTEM and therefore take complete control of your server. They could then do whatever they want to any of the other client websites hosted on that NT4 server. On the other hand they could install sniffers or crack passwords to further their control of your network.
Hence the title $19.95 hack... Anyone with $20 dollars to spend can potentially break into any web hosting company or internet service provider running NT4+IIS4. Simply buy an account with a web hosting company, upload your evil.asp and request that evil.asp from your website, I.E. http://www.badguy.com/evil.asp, and your executing code on that server as SYSTEM.
Here is an example .asp file that will cause NT4+IIS4's inetinfo.exe to overflow.
----start-cut-of-example.asp---- <SCRIPT LANGUAGE="[buffer]" RUNAT="Server"> </SCRIPT> ----start-cut-of-example.asp----
Where [buffer] is 2220 characters or more. Yes, this overflow is exploitable. See section below.
So you don't have $20 dollars to spend, or a server does not host websites? What then? Well there are going to be situations when a remote attack is possible.
An example situation would be a server that has a guest book/message board system that does not strip out SCRIPT comments when it writes to its returned .asp file. So an attacker could go to a web board and within the "Enter your message here:" box (or whatever) they could simply paste in the contents of the script language overflow and submit the message. Then once they requested the .asp file that has that message (the embedded overflow msg) it would cause IIS to parse that .asp file, overflow, and execute their code.
On another note you could couple this attack with the recent IIS Unicode bug to use cmd.exe to echo the contents of your evil.asp to the remote system then when you request evil.asp your code is executed as SYSTEM. In fact, we have created a sample exploit to show how this .asp language overflow and the IIS Unicode exploit could be used together to remotely compromise any NT4+IIS4+SP6(or lower) system that is behind in installing hot fixes.
Due to the nature of this attack it is likely and quite possible that firewalls will not protect you against attacks like this.
The Exploit: How to remotely exploit a system? Use the IIS Unicode exploit + .asp language overflow. For those of you not familiar with the IIS Unicode exploit, it basically allows you to remotely execute commands against IIS as IUSR_MACHINE. Now since we can execute commands via cmd.exe we could try to make the remote web server connect out to an FTP server to grab a file (sort of like the first IISHack) but you would still be executing code as IUSR_MACHINE. The way our exploit works is by using the Unicode bug to echo our .asp file (complete with shell code, to bind cmd.exe to a port ;-]) to a remote server and then request that .asp file which then causes inetinfo.exe to overflow resulting in a SYSTEM privileged cmd.exe to be bound to a specified port. Now, in most circumstances you should have correct acl's on cmd.exe (that would not allow IUSR_MACHINE to touch it) as well as some other security precautions in place, so we understand this exploit will not work on a secured system. However, this is just proof of concept and as most of us know, most IIS servers do not have proper local security in place. The exploit basically looks like the following when compiled:
C:\we are still hiring good programmers> iishack1.5.exe IISHack Version 1.5 eEye Digital Security http://www.eEye.com Code By: Ryan Permeh & Marc Maiffret eEye Digital Security takes no responsibility for use of this code. It is for educational purposes only.
Usage: IISHack1.5 [server] [server-port] [trojan-port]
C:\send resume to email@example.com> iishack1.5.exe www.[yourowncompany].com 80 6969 IISHack Version 1.5 eEye Digital Security http://www.eEye.com Code By: Ryan Permeh & Marc Maiffret eEye Digital Security takes no responsibility for use of this code. It is for educational purposes only.
Attempting to find an executable directory... Trying directory [scripts] Executable directory found. [scripts] Path to executable directory is [C:\Inetpub\scripts] Moving cmd.exe from winnt\system32 to C:\Inetpub\scripts. Successfully moved cmd.exe to C:\Inetpub\scripts\eeyehack.exe Sending the exploit... Exploit sent! Now telnet to www.[yourowncompany].com on port 6969 and you should get a cmd prompt. C:\> telnet www.[yourowncompany].com 6969 Trying www.[yourowncompany].com... Microsoft(R) Windows NT(TM) (C) Copyright 1985-1996 Microsoft Corp.
C:\WINNT\system32>whoami NT AUTHORITY\SYSTEM
Download the full exploit from: http://www.eEye.com/html/advisories/IISHack1.5.zip
Vendor Status: Microsoft has already fixed this .asp language overflow in prior hot fixes and applied it to most post service pack 6 hot fixes. This flaw has been fixed in the latest versions of W3SVC.DLL. Installing any recent hotfix will fix the problem. Here is a list of some MS advisories and hotfixes that will fix the .asp language overflow vulnerability: MS00-080: Patch Available for "Session ID Cookie Marking" Vulnerability MS00-060: Patch Available for "IIS Cross-Site Scripting" Vulnerabilities MS00-057: Patch Available for "File Permission Canonicalization" Vulnerability MS00-030: Patch Available for "Malformed Extension Data in URL" Vulnerability MS00-023: Patch Available for "Myriad Escaped Characters" Vulnerability MS00-019: Patch Available for "Virtualized UNC Share" Vulnerability MS00-018: Patch Available for "Chunked Encoding Post" Vulnerability
W3SVC versions greater than 720 addresses this issue. So if you've been keeping up to date with your hot fixes then you should not be vulnerable to this issue.
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