Windows 2000 and NT4 IIS .HTR Remote Buffer Overflow

Release Date:
June 12, 2002

Severity:
High (Remote code execution)

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

A vulnerability in transfer chunking, in combination with the processing of
HTR request sessions can be exploited to remotely execute code of an
attackers choice on the vulnerable machine. By sending a carefully crafted
session, an attacker can overwrite a section of the heap. Data structures in
the overwritten heap can be manipulated to move attacker-supplied data to
attacker supplied memory addresses, thereby altering the flow of execution
into an attacker supplied payload.

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. The dllhost.exe
child process will silently die because the developers have replaced the
default exception filter. So if you want to examine this closer, load a
debugger up on the dllhost child process before you send this example
session over the wire.

**************Begin Session****************
POST /EEYE.htr HTTP/1.1
Host: 0day.big5.com
Transfer-Encoding: chunked

20
XXXXXXXXXXXXXXXXXXXXXXXXEEYE2002
0
[enter]
[enter]
**************End Session******************

Technical Description:

The example session above overwrites a section of the heap that contains
data structures related to the memory management system. By manipulating the
content of these structures we can overwrite an arbitrary 4 bytes of memory
with an attacker supplied address.

While many may believe that the risk for these types of vulnerabilities is
fairly low due to the fact that addressing is dynamic and brute force
techniques would need to be use in an attack, eEye strongly disagrees. This
premise is false as successful exploitation can be made with one attempt,
across dll versions. An attacker can overwrite static global variables,
stored function pointers, process management structures, memory management
structures, or any number of data types that will allow him to gain control
of the target application in one session.

SecureIIS(tm) Application Firewall for Microsoft IIS

It should be noted that clients using any version of SecureIIS from eEye
Digital Security 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 from similar classes of attack. To learn
more visit http://www.eeye.com/SecureIIS

Vendor Status:
Microsoft has released a security bulletin and patch:
http://www.microsoft.com/technet/security/

Beyond installing the Microsoft security patch it is also recommend to
disable the .htr ISAPI filter if you have not already done so. Microsoft’s
security advisory references more information on the steps of how to disable
the .htr ISAPI filter.

Credit: Riley Hassell

Greetings: Caesar, K2, Dark Spyrit, Solar Designer, Joey, Halvar, Gera,
Scut, Ilfak Guilfanov. And last but not least, Kasia and Jenn ;) and as
always, www.securityfocus.com.

Copyright (c) 1998-2002 eEye Digital Security
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