(Proof of Concept Exploit Described) Re: Microsoft Internet Information Server Buffer Overflow in Chunked Encoding Mechanism Lets Remote Users Run Arbitrary Code on the Server
SecurityTracker Alert ID: 1004012|
SecurityTracker URL: http://securitytracker.com/id/1004012
(Links to External Site)
Date: Apr 10 2002
Denial of service via network, Execution of arbitrary code via network, Root access via network, User access via network|
Fix Available: Yes Vendor Confirmed: Yes Exploit Included: Yes |
Version(s): 4.0, 5.0, 5.1|
Two buffer overflow vulnerabilities have been reported in Microsoft's Internet Information Server (IIS) in the chunked encoding transfer processing mechanism for Active Server Pages (ASP). A remote user could cause arbitrary code to be executed.|
It is reported that a remote user could overrun heap memory on the system, causing the IIS service to crash or causing arbitrary code to be executed. The code would run with System level privileges on IIS 4.0 and with Web Application Manager on later versions of IIS. The vulnerability is apparently due to an arithmetic error in the ISAPI ASP extension that enables data to be uploaded to a web server via 'chunked encoding'. The bug causes IIS to allocate a buffer that is not large enough to store the uploaded data. This reportedly makes it possible for a remote user send a chunk of data to overwrite most or all of the memory on the system.
According to the vendor, customers that have applied the IIS Lockdown Tool to configure their servers as static web servers are already protected against this vulnerability. However, the vendor still recommends applying the patch.
The vendor has assigned this flaw a maximum severity rating of 'Critical' for Internet and Intranet servers.
Microsoft credits eEye Digital Security with discovering one of the two buffer overflows. The other was reportedly discovered by Microsoft.
eEye Digital Security has provided the following demonstration exploit transcript:
POST /iisstart.asp HTTP/1.1
In this example, the default exception handler will execute from within the dllhost child process and try to copy "DATA" to "DEST". However, writeable memory is not available at the destination (0x54534544 in this example), so an access violation occurs and the structured exception handling (SEH) within the NT kernel catches it and kills the child dllhost.exe process.
A remote user could cause the IIS service to crash or could cause arbitrary code to be executed on the server. The code would execute with System level privileges (IIS 4.0) or Web Application Manager privileges (IIS 5).|
The vendor has released a fix:|
For Microsoft IIS 4.0:
For Microsoft IIS 5.0:
For Microsoft IIS 5.1:
The IIS 4.0 patch can be installed on systems running Windows NT 4.0 SP6a. The IIS 5.0 patch can be installed on systems running Windows 2000 SP1 or SP2. The IIS 5.1 patch can be installed on systems running Windows XP Professional Gold.
Microsoft notes that the IIS 5.0 fixes will be included in Windows 2000 SP3 and the IIS 5.1 fixes will be included in Windows XP SP1.
For IIS 4.0 and 5.0, this patch reportedly supersedes the one previously provided in Microsoft Security Bulletin MS01-044.
There is a very large list of 'caveats' associated with this patch. See the Vendor URL for the list.
The vendor will issue Microsoft Knowledge Base article Q319733 shortly, to be available on the Microsoft Online Support web site.
Vendor URL: www.microsoft.com/technet/security/bulletin/MS02-018.asp (Links to External Site)
|Underlying OS: Windows (NT), Windows (2000), Windows (XP)|
This archive entry is a follow-up to the message listed below.|
Source Message Contents
Subject: [VulnWatch] Windows 2000 and NT4 IIS .ASP Remote Buffer Overflow|
Windows 2000 and NT4 IIS .ASP Remote Buffer Overflow
High (Remote code execution)
IWAM_MACHINE Privilege Level
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.
POST /iisstart.asp HTTP/1.1
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
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
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
Copyright (c) 1998-2002 eEye Digital Security
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