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All versions of Microsoft Internet Information Services, Remote buffer overflow (SYSTEM Level Access)
From: "Marc Maiffret" <marc () eeye com>
Date: Mon, 18 Jun 2001 16:50:10 -0700

All versions of Microsoft Internet Information Services, Remote buffer
overflow (SYSTEM Level Access)

Release Date:
June 18, 2001

High (Remote SYSTEM level code execution)

Systems Affected:
Microsoft Windows NT 4.0 Internet Information Services 4.0
Microsoft Windows 2000 Internet Information Services 5.0
Microsoft Windows XP beta Internet Information Services 6.0 beta

There exists a remote buffer overflow vulnerability in all versions of
Microsoft Internet Information Services (IIS) Web server software.

The vulnerability lies within the code that allows a Web server to interact
with Microsoft Indexing Service functionality. The vulnerable Indexing
Service ISAPI filter is installed by default on all versions of IIS. The
problem lies in the fact that the .ida (Indexing Service) ISAPI filter does
not perform proper "bounds checking" on user inputted buffers and therefore
is susceptible to a buffer overflow attack.

Attackers that leverage the vulnerability can, from a remote location, gain
full SYSTEM level access to any server that is running a default
installation of Windows NT 4.0, Windows 2000, or Windows XP and using
Microsoft’s IIS Web server software. With system-level access, an attacker
can perform any desired action, including installing and running programs,
manipulating Web server databases, adding, changing or deleting files and
Web pages, and more.

The Discovery:
Riley Hassell was at it again one day working to further advance eEye's CHAM
(Common Hacking Attack Methods) technology so that Retina could better
search for unknown vulnerabilities in software and so that SecureIIS could
better protect from unknown IIS vulnerabilities.

After a few hours of running some custom CHAM auditing code one of our Web
servers in our lab eventually came to a halt as the IIS Web server process
had suddenly died.

We investigated the vulnerability further and found that the .ida ISAPI
filter was susceptible to a typical buffer overflow attack.

GET /NULL.ida?[buffer]=X HTTP/1.1
Host: werd

Where [buffer] is aprox. 240 bytes.

The Exploit, as taught by Ryan "Overflow Ninja" Permeh:

This buffer overflows in a wide character transformation operation. It takes
the ASCII (1 byte per char) input buffer and turns it into a wide
char/unicode string (2 bytes per char) byte string. For instance, a string
like AAAA gets transformed into \0A\0A\0A\0A. In this transformation, buffer
lengths are not checked and this can be used to cause EIP to be overwritten.

This sounds like any normal overflow to date, however there are a few
sticking points in doing anything useful with this. First, you transform 2
bytes into 4, 2 of which you have no control over. This would be a bad
situation, but not impossible to exploit. However, the 2 bytes that you do
not have control over happen to be nulls. Basically, we need to take this 2
byte string and somehow get it to point to our code. Traditionally, we use
our overwritten EIP to jump to a call esp, or jmp esp, jumping back to code
we have positioned on the stack to implement whatever it is our shellcode
would like to do. In this case, however, there is a problem.

GET /a.ida?[Ax240]=x HTTP/1.0

The above example overwrites EIP with 0x00410041. Again, traditionally, we
insert our shellcode in the same buffer we overflow, however we run into the
problem that then our code would also face the same expansion that our EIP
bytes face. This makes writing shellcode a horrible pain. There are two
methods of doing this:

1. custom shellcode: It might be possible to write shellcode that works fine
with NULL byes every other byte. It would probably have to be very simple,
but this could be possible.

2. encode: You could probably write a decoder that takes a string of 0x0041
and rewrites it on the stack into actual single byte code. This would have
to be written completely in 0x00bb opcodes, most likely a challenge in
itself (similar to the above custom shellcode, but only a decoder would need
to be written).

This would, of course only be possible if we could find a point in memory
that we could reach using only 0x00aa00bb. This gives us only about 65k
spots in memory to look for jump bytes, a pretty dismal situation.

Exploiting Wide Char Strings In Practice
We got lucky using this method. We were basically limited to a very very
small range of memory in which to find jump bytes. We thought we were losing
the battle until we realized that IIS/ISAPI uses 0x00aabbcc as its memory
range for allocated heap. We developed a spray technique in an attempt to
push enough data into the heap so that the bytes we require will be there
when we need to jump to them.

For instance, in Windows 2000 Service Pack 1, we noticed that we had request
bytes at around 0x0042deaa. Since the closest we could get to this was
0x00430001 (by overflowing with C%01 at the end of our overflow string. This
offered us an intriguing possibility -- perhaps we could push more stuff
into a request, causing more heap memory to be used, pushing our request
closer to where we want to be.

GET /a.ida?[Cx240]=x HTTP/1.1
Host: the.victim.com
eEye: [Cx10,000][shellcode]

Now, we overflow the EIP with 0x00430043. With our new much larger request,
0x00430043 happens to be inside the large C buffer we setup. This acts as a
slide in our code, executing down to our shellcode.

The Warning
With this technique of forceful heap violation, everything is relative to
what is there to begin with. We noticed that in any situation, we found 4-5
different copies of our requests in the 0x00aabbcc memory range. This means
that perhaps 0x430043 is not the best spot in memory, however it is the one
we chose in our forthcoming sample exploit (the exploit we will provide only
executes file writing; we provided Microsoft with shell-binding code but
will not publicly release this code). The other potential problem with this
attack is that different systems may have different heap usages. In our
internal tests, we noticed that heap usage differed depending on which ISAPI
extensions were enabled at any time. Also, requests that cause faults
handled by exception handlers that do not free their heaps may cause certain
parts of the heap to become unusable, causing those spots to not be reused.
This is not a problem for Windows 2000 because it is nice enough to restart
itself (giving us a nice clean heap to work with). Windows XP appears to act
similarly, however we did not focus our research with this beta OS. This is,
however, potentially a problem with NT 4, which will crash if exploited
incorrectly. Again, like all other IIS overflows, this attack is not logged,
causing only a fault in IIS and crashing it.

All of the technical talk aside, we do have working exploits for Windows NT
4.0, Windows 2000 and Windows XP systems.

Proof-Of-Concept Exploit
We will be posting a proof-of-concept (file writing) exploit to our Web site
within the next few days.

The Fallout
According to Netcraft (www.netcraft.com), there are roughly 5.9 Million Web
servers running IIS; however, the true number of IIS Web servers is much
larger when you count internal network servers. Any of these Web servers
that have the default .ida ISAPI filter installed are most likely

Note on Windows XP beta:
As stated earlier, all versions of Microsoft’s IIS Web server software are
vulnerable to this flaw. This includes Windows XP beta, Microsoft’s
next-generation Operating System and the version of IIS that is included
with it. Microsoft is taking the necessary steps to patch Windows XP before
the final version ships to customers.

Some people might wonder why this advisory does not contain the typical eEye
humor like most of our other advisories. Basically, the reason is that this
is our 4th remote SYSTEM level IIS vulnerability and well...we've run out of

eEye Note:
Those eEye customers who are using the latest version of SecureIIS are
already protected from this vulnerability. SecureIIS is able to stop known
and unknown IIS vulnerabilities by looking for classes of attacks instead of
specific attack signatures.

Also, Retina 4.02, The Network Security Scanner, will be posted to our Web
site shortly. It includes many new features and functionalities and also
remotely checks for this latest Microsoft IIS vulnerability.

Vendor Status:
Microsoft has released a patch for this vulnerability that can be downloaded
from: http://www.microsoft.com/technet/security/bulletin/MS01-033.asp
Also eEye Digital Security recommends removing the .ida ISAPI filter from
your Web server if it does not provide your Web server with any _needed_

Discovery: Riley Hassell
Exploit: Ryan Permeh

Related Links:
SecureIIS: stop known and unknown IIS Web server vulnerabilities

Retina, The Network Security Scanner

James, Delsea, Rachael, Steve, and Code Red Mountain Dew.

Copyright (c) 1998-2001 eEye Digital Security
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eEye Digital Security
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