TUCoPS :: Web :: IIS :: ciacl139.txt

CIAC L-139 Microsoft IIS %25u encoding IDS bypass vulnerability


                       The U.S. Department of Energy
                     Computer Incident Advisory Center
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                             INFORMATION BULLETIN

              Microsoft IIS "%u encoding IDS bypass vulnerability"

September 6, 2001 23:00 GMT                                       Number L-139
PROBLEM:       The Microsoft Internet Information Server (IIS) has a non-HTTP 
               standard encoding method.  This Microsoft encoding method
               is named "%u encoding". The encoding style is not recognized
               by a variety of Intrusion Detection Systems (IDS). 
PLATFORM:      Microsoft IIS Web Servers.
DAMAGE:        The use of "%u encoding" allows a malicious party to circumvent 
               an affected IDS security detection. Attackers using this 
               methodology would be hidden from the IDS view. A degradation of 
               the security of a system can take place by attacks not being 
               detected. Automated warning systems based on IDS defenses would
               be defeated. 
SOLUTION:      The following IDS products were listed as affected:
                  Cisco Secure Intrusion Detection System
                     (formerly known as NetRanger, Sensor component)
                  Cisco Catalyst 6000 Intrusion Detection System Module.
                  Dragon Sensor 4.x 
                  RealSecure Network Sensor 5.x and 6.x before XPU 3.2
                  RealSecure Server Sensor 6.0 for Windows 
                  RealSecure Server Sensor 5.5 for Windows 
                  Snort prior to version 1.8.1 

               Patches/fixes are available as listed in the advisory. Review
               supplemental advisory links, as provided in this bulletin. 
VULNERABILITY  The risk is HIGH. This vulnerability has been widely reported 
ASSESSMENT:    in the media. IDS systems should be immediately patched. 
 CISCO Advisory:
 Securiteam Advisory: 
 X-Force Advisory 

           All Products - http://www.iss.net/eval/eval.php 
           RealSecure Network Sensor - 
           Cisco Secure Intrusion Detection System Sensor 
                     (read Cisco advisory concerning this fix) 
           Cisco Catalyst 6000 Intrusion Detection Module
              (Will be repaired with the release of service pack 3.0,
               scheduled for October 2001 release).
           Contracted Customers 

[******  Begin eEye Advisory ******]

%u encoding IDS bypass vulnerability

Release Date:
September 5, 2001


Systems Affected:
Cisco Secure Intrusion Detection System, formerly known as NetRanger, Sensor
Cisco Catalyst 6000 Intrusion Detection System Module
ISS RealSecure Network Sensor 5.x and 6.x before XPU 3.2
ISS RealSecure Server Sensor 6.x prior to 6.0.1
ISS RealSecure Server Sensor 5.5
Dragon Sensor 4.x
Snort prior to 1.8.1
NFR (Network Flight Record) is believed to be vulnerable however they have
not responded to our eMails.
Symantec and NAI were contacted but we were told that none of their products
are vulnerable.
Other Intrusion Detection style products (Network based pattern matching)
are probably affected... contact your vendor to be positive if your software
is affected or not.

For an Intrusion Detection system to function properly it must have the
ability to be able to decode (break down) various forms of HTTP encoded
requests such as UTF and hex encoding. Most commercial and freeware IDS
(Intrusion Detection Systems) do have the ability to break down UTF and hex
encoded request in an effort to analyze them for attack strings.

The two mainstream ways of encoding a url would be UTF (%xx%xx) or just
plain hex encode (%xx) where xx are the relevant hex values. Microsoft's IIS
Web server does include both of these types of encoding however it also
includes a third style of encoding that is not a HTTP standard. Therefore
most IDS systems were not aware of this "different" encoding and therefore
do not try to decode it.

This "different" style of encoding is known as %u encoding. The purpose of
this %u encoding seems to be for the ability to represent true Unicode/wide
character strings.

Since %u encoding is not a standard and IDS systems do not decode %u
strings, it is possible for an attacker to %u encode his/her attack against
an IIS web server without an IDS system detecting the attack. Therefore
allowing an attacker to successfully perform scans and attacks against IIS
web servers without IDS systems detecting the attacks.

A good example of how this could have been used in the real world would have
been a "stealth CodeRed". The CodeRed worm used the .ida buffer overflow
vulnerability to be able to exploit systems to propagate itself. CodeRed was
detected because IDS systems had signatures for the .ida attacks. However if
CodeRed would have had a polymorphic %u encoding mechanism then it would
have easily slipped past most IDS systems because they detected the .ida
attack by looking for ".ida" (or any .ida signature string) in a web

So if an attacker sent a %u encoded request then they could bypass IDS's
checking for ".ida". An example request would look like:
GET /himom.id%u0061 HTTP/1.0

The above request will translate himom.id%u0061 to himom.ida and therefore
the request will work properly. The problem is that since %u encoding is not
a standard IDS systems did not know about this IIS specific encoding and
therefore are not properly decoding %u requests and will not detect these

Vendor Status:

"Products that are not affected because they do NOT implement
de-obfuscation, and do not implement attack signatures targeted at Microsoft
operating systems and applications.
Cisco Secure PIX Firewall
Cisco IOS Firewall Feature Set with Intrusion Detection
To get information on how to patch and protect your Cisco products, visit:

ISS (Internet Security Systems)
"ISS X-Force has included a patch for this vulnerability in RealSecure
Network Sensor X-Press Update 3.2.  ISS X-Force recommends that all
RealSecure customers download and install the update immediately. RealSecure
X-Press Update 3.2 is now available.  RealSecure Network Sensor customers
can download XPU 3.2 from the following address:
RealSecure Server Sensor version 6.0.1 includes a fix for this
vulnerability. RealSecure Server Sensor 6.0.1 will be available for download
on September 4, 2001.  ISS X-Force recommends that all RealSecure customers
upgrade their Windows Server Sensors to version 6.0.1.  A patch is being
developed for RealSecure Server Sensor 5.5 and will be available on or
before August 31, 2001 at the ISS Download Center:
BlackICE products are not affected by this vulnerability.  Attempts to
exploit this vulnerability will trigger the "HTTP URL bad hex code"
signature.  The next BlackICE product update will specifically address "%u"

"Dragon Sensor 4.x was affected. Signatures to detect the new IIS UNICODE
encoding flaw have been available, and a modification to the Web processing
engine is already included in Dragon Sensor 5.0. To obtain dragon products,
visit http://dragon.enterasys.com"

"Snort 1.8.1 fixes this encoding bug. You can receive it from

This technique first came to our attention by an exploit written by HSJ. The
%u encoding technique was used in HSJ's .ida buffer overflow exploit however
it was not used to mask the attack to bypass Intrusion Detection Systems
when performing attacks against IIS systems.

Finding security holes is easy, writing advisories that are not dry boring
piles of cow dung, is hard.

Radiohead. Stringbeans. CodeRed.

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

[******  End eEye Advisory  *******]

CIAC wishes to acknowledge the contributions of eEye Digital Security for the 
information contained in this bulletin.

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