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Windows 2000 and NT4 IIS .ASP Remote Buffer Overflow
11 April 2002
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Release Date:

High (Remote code execution)
IWAM_MACHINE Privilege Level

Systems Affected:
Microsoft Windows NT 4.0 Internet Information Services 4.0
Microsoft Windows 2000 Internet Information Services 5.0


A vulnerability in the ASP (Active Server Pages) ISAPI filter, loaded by default on all NT4 and Windows 2000 server systems (running IIS), can be exploited to remotely execute code of an attackers choice. The fault lies within the decoding and interpretation of form data received by malicious clients. By chunk encoding form data we can force IIS to overwrite 4 bytes of arbitrary memory with data we supply.

This is a very serious vulnerability and eEye suggests that administrators install the Microsoft supplied patch as soon as possible.

The following example will show the vulnerable condition. We will use a default .asp page left after install on a Windows 2000 server with the latest service packs.


**************Begin Session****************
POST /iisstart.asp HTTP/1.1
Accept: */*
Host: eeye.com
Content-Type: application/x-www-form-urlencoded
Transfer-Encoding: chunked

**************End Session******************

Technical Description:

The example session above causes the default exception handler to execute from within the dllhost child process. When the default exception handler executes a window will open with this message:

DLLHOST.EXE - Application error

The instruction at 0x77fcb397 referenced memory at 0x54534544

Notice that 0x54534544 is the hex representation of "TSED", or the value "DEST" in little endian format. The DLLHOST.EXE process is trying to copy "DATA" to "DEST". Because there isn't writeable memory at 0x54534544, an access violation occurs and the structured exception handling (SEH) within the NT kernel catches it and kills the child dllhost.exe process.

The crux of this problem lies in the fact that the memory we overwrite with our data contains Heap Management header structures, in our case being used by AllocateHeap(). Specifically, as we overwrote the header, we control two four byte addresses within it. These addresses are associated with the population and use of lookaside lists. The first four-byte address, which in our example is overwritten by "DATA", is an address that gets copied to the second four-byte address specified in header. We have also overwritten the second address, this time with "DEST". By overwriting these two addresses, we can put four bytes anywhere in memory that the child dllhost.exe has privileges to write to. This allows us to overwrite function pointers, saved instruction pointers, exception handlers, or anything else that will allow us to control the flow of execution into our payload. We have been most successful in exploitation by overwriting a structured exception handler address on the stack. Due to the fact that we supplied addresses that aren't associated with valid lookaside lists, an exception handler will be called, and when it does, it will call our modified routine, which points directly into payload code.

It should be noted that while this vulnerability exists in the .ASP ISAPI, a mechanism is still required to get the malicious request to hit the vulnerable functions within the .ASP ISAPI. Although pages with form submissions make it easier to demonstrate this vulnerability, there are other methods for causing code to execute beyond the form variable referencing. In the above example we used a default .asp file that has script code within it that deals with .ASP Server Variables. When the .ASP ISAPI performs processing on the Server Variables, we are able to cause an overflow and execute code. There are .asp files by default in IIS that allow processing of Server Variables, which make it possible to demonstrate the existence of this vulnerability on default installations.

Like most of the IIS vulnerabilities eEye has discovered in the past, firewalls and intrusion detection systems do not protect from this vulnerability.

SecureIIS - Application Firewall for Microsoft IIS

It should be noted that clients using SecureIIS 1.2.5 and above are secure from this vulnerability. This vulnerability was discovered by the eEye team while testing a new version of SecureIIS to help further its protection abilities. To learn more visit http://www.eeye.com/SecureIIS

Vendor Status:
Microsoft has released a security bulletin and patch:

Discovery: Riley Hassell
Exploitation Research: Riley Hassell and Ryan Permeh

To all the people who continue to make the security industry more exciting with innovative research. Also to the rest of eEye, who help make all this magic possible.

Copyright (c) 1998-2002 eEye Digital Security Permission is hereby granted for the redistribution of this alert electronically. It is not to be edited in any way without express consent of eEye. If you wish to reprint the whole or any part of this alert in any other medium excluding electronic medium, please e-mail alert@eEye.com for permission.

The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties with regard to this information. In no event shall the author be liable for any damages whatsoever arising out of or in connection with the use or spread of this information. Any use of this information is at the user's own risk.

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