• CROSS-SITE TRACING (XST)


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    • Abstract: CROSS-SITE TRACING (XST)THE NEW TECHNIQUES AND EMERGING THREATS TO BYPASS CURRENT WEB SECURITY MEASURES USINGTRACE AND XSS.Jeremiah Grossman// Warranties and Disclaimers        “ ”     , 

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CROSS-SITE TRACING (XST)
THE NEW TECHNIQUES AND EMERGING THREATS TO BYPASS CURRENT WEB SECURITY MEASURES USING
TRACE AND XSS.
Jeremiah Grossman
//
Warranties and Disclaimers
        “ ”     , 
  , ,    ,     ,
   ,  -.  , ..  
          ,  
.
     , .     ,   
     , ,     
  ,  ,      , , , 
    ,             
.
        .  
     ;       
   .  , .    /  
 (), (), / ()    .
Overview
October 23 2002, Microso issued a press release describing a new browser/server based
protective security measure within of internet explorer 6 sp1. is new feature, dubbed
“httponly”, helps guard http cookies against xss (cross-site scripting) attack. WhiteHat
Security, heavily focused on web application security research and technology, began to
investigate the feature in order to determine what it meant to web security. First of all,
anything that attempts to help prevent the xss plague on the web is a good thing. Most of
us in the web application security field already know the great pains required to prevent the
ever-present existence of xss issues.
Aer much security review, I posted to bugtraq stating that the new httpOnly security
feature, which is nicely effective for the intended purpose, is limited in xss protection scope.
Limited in that the security feature only prohibits the exposure of cookie data through the
“document.cookie” object. However, Microso has taken an excellent first step in the right
direction to prevent xss as a whole.
A week later into testing of httpOnly, WhiteHat staff discovered a new web security
attack technique that is able not only to bypass the httpOnly mechanism present in i.e. 6
service pack 1, but in addition the ability to xss “just about” anything from “just about”
anywhere. is technique allows client-side scripting languages, such as javascript, and
possibly other client-side technologies like vbscript, flash, java, etc., the ability access http
web authentication credentials, with the added bonus of achieving this result over ssl. is
ability has never before been previously possible. ese new exposures will be explained
with detail in the proceeding sections to illustrate the concepts.
Background Information
TRACE Request Method
“Trace” is used simply as an input data echo mechanism for the http protocol. is request
method is commonly used for debug and other connection analysis activities.
e http trace request (containing request line, headers, post data), sent to a trace
supporting web server, will respond to the client with the information contained in the
request. Trace provides any easy to way to tell what an http client is sending and what the
server is receiving. Apache, IIS, and iPlanet all support trace as defined by the HTTP/1.1
RFC and is currently enabled by default. Very few system administrators have disabled
this request method either because the method posed no known risk, default settings were
considered good enough or simply had no option to do so.
e following is an example of a TRACE request:
$ telnet foo.com 80
Trying 127.0.0.1...
Connected to foo.bar.
Escape character is ‘^]’.
TRACE / HTTP/1.1
Host: foo.bar
X-Header: test
HTTP/1.1 200 OK
Date: Mon, 02 Dec 2002 19:24:51 GMT
Server: Apache/2.0.40 (Unix)
Content-Type: message/http
TRACE / HTTP/1.1
Host: foo.bar
X-Header: test
As shown in the example, the server responded with the information sent by the client to
the server. What sites currently have TRACE enabled?
• www.passport.com
• www.yahoo.com
• www.disney.com
• www.securityfocus.com
• www.redhat.com
• www.go.com
• www.theregister.co.uk
• www.sun.com
• www.oracle.com
• www.ibm.com
(Many other web sites)
httpOnly Cookie Option
httpOnly is a HTTP Cookie option used to inform the browser (IE 6 only until other
browsers support httpOnly) not to allow scripting languages (JavaScript, VBScript, etc.)
access to the “document.cookie” object (normal XSS attack target). e syntax of an
httpOnly cookie is as follows:
Set-Cookie: name=value; httpOnly
Using JavaScript we can test the effectiveness of the feature. (Code tested in IE 6 SP1)
Code Example 1.
Screen Shot 1: aer pressing the “Display Normal Cookie” button.
Screen Shot 2 : Aer pressing the “Display HTTPONLY Cookie’” button.
By testing the above code, you can quickly see that when httpOnly setting is in use, the
“document.cookie” function allows access to the object, but the string returns empty. is
becomes a useful security enhancement for many web applications.
Analysis
e first challenge is to gain access to the cookie data string normally contained in
“document.cookie” while httpOnly is in use. e idea became to identify where the
data within “document.cookie” is located besides within, of course, “document.cookie”.
is is where TRACE’s usefulness for our purposes becomes clear. TRACE will echo the
information you send in the HTTP Request. is includes cookie and Web Authentication
strings, since they are just simple HTTP headers themselves.
However, it is not a simple process forcing Internet Explorer to send a TRACE request, even
while first considering the use HTML Form (METHOD=POST). In fact, Internet Explorer
does not support request methods other than GET or POST while using an HTML form. To
resolve this limitation, we had to utilize extended client-side scripting technologies to create
and send a specially formatted HTTP request to a target web server. Many technologies are
capable of performing specially craed HTTP request.
Code Example 2. (Will need to change the URL in the code)
Screen Shot 3: Results of the TRACE request response from the server. Note the cookie
string contained and accessible by means other than “document.cookie”.
e above code, using the ActiveX control XMLHTTP, will send a TRACE request to the
target web server. e server will then echo, if it supports TRACE, the information sent
within the HTTP request. Internet Explorer will send general browser headers by default
that will be displayed via a resulting JavaScript alert window. If your browser happens to
have a cookie from the target domain or is logged into the target web server using web
authentication, you will be able to see your cookies and credentials present within the alert.
is technique successfully grants the code ability bypass “httpOnly”, while accessing
cookie data without the use of “document.cookie”. We now have the desired capability to
pass sensitive credentials off-domain to a third-party. Also as stated in the overview, the
ability to access web authentication credentials not before possible using client-side script.
To restate, all the sensitive information is still accessible even over an SSL link.
It is important to note two things at this point. e first is ability to do these types of
request using a web browser is NOT limited to Internet Explorer. Other web browsers such
as Mozilla/Netscape possess the ability as well. Specifically, TRACE requests have been
achieved in Mozilla using XMLDOM object scripting. e second, XMLHTTP, is only one
of several ActiveX controls and other technologies, which appear have this control over
HTTP within a browser environment.
ere is however at this point a limiting factor preventing wider a danger escalation. e
TRACE connection made by the browser, will NOT be allowed by the browser, to connect
to anything other than the domain hosting the actual script content. A foo.bar script
domain will only be able to TRACE and connect to a foo.bar domain host. is is a browser
implemented domain restriction security policy. e domain restriction policy helps
prevent XSS and other similar attacks from occurring. is technical exploit limitation
does prevent further abuse, however, this hurdle can be bypassed as well as shown below.
To increase the exposure of the exploit, we are in need of a domain-restriction-bypass
vulnerability within Internet Explorer (or web browser of choice). As it turns out, these
issues are quite numerous and can be commonly found posted to public resource forums
such as bugtraq. Recently and currently, there have been known unresolved issue with
the IE Domain Restriction policies. ese un-patched Internet Explorer 6 flaws, allow the
ability to bypass the domain restriction security policy, and increase the overall severity of
the problem. is IE issue uses the “external” browser flaw in the caching mechanism. And
was first identified by GreyMagic Security.

Code Example 3. (Code will not work post the MS02-068 roll-up which resolves the issue).
However a working code example (4) below, as of this writing, does function. URLs in the
code will need to be changed to identify a target.
function xssDomainTraceRequest(){
var exampleCode = “var xmlHttp = new ActiveXObject(\
”Microsoft.XMLHTTP\”)\;xmlHttp.open(\”TRACE\”,\”http://foo.bar\”,false)\
;xmlHttp.send()\;xmlDoc=xmlHttp.responseText\;alert(xmlDoc)\;”;
var target = “http://foo.bar”;
cExampleCode = encodeURIComponent(exampleCode + ‘;top.close()’);
var readyCode = ‘font-size:expression(execScript(decodeURIComponen
t(“’ + cExampleCode + ‘”)))’;
showModalDialog(target, null, readyCode);
}
Code Example 4. (Functional as of this writing) is IE issue uses a flaw within
“showModalDialog”. Gathered from or Larholm on http://www.pivx.com/ e URLs in
the code will need to be changed to identify a target.
Screen Shot 4: Results of the TRACE request response from the server. Note the base64
authentication string contained and now accessible.
ese scripts now have the ability to connect to any domain, access cookies, and web
authentication information, while NOT utilizing document.cookie and/or being restricted
by domain security policy. What does this mean for exposure scenarios? Read On.
Exposure Scenarios
We will outline a few exposure scenarios while using varying degrees of security
assumptions. Attempting to organize the scenarios by level of risk severity.
Defining some necessary technologies and acronyms to better understand exposure at
several levels.
Domain Restriction Bypass (DRB) e ability for a client-side script to bypass domain
restriction security policy enabled within a web browser.
HTTP Request Enabling Technology (HRET) Client-side technologies resident within
a web browser, which allow for the creation and sending of specially formatted HTTP
Requests. ese technologies may include, but not all confirmed, JavaScript, VBScript,
Flash, Java, ActiveX, Jscript, Action Script, Shockwave, etc..
TRACE Method Support (TMS) A target web server that currently supports the TRACE
request method.
“Credentials” will include cookie data and web authentication credentials.
Scenarios assume the following:
A user visits a malicious web site or views malicious content hosted by a trusted source
(message board, web mail, etc..) and loads code similar to code example 3 & 4.
Scenario 1. (DRB, HRET, TMS)
All the required insecurities are present in today’s environment. Code may access any
and all of the user credentials from any domain that supports TRACE including bypass
httpOnly. XSS “just about” anyone from “just about anywhere”.
Scenario 2. (HRET, TMS)
Code may access any and all of the user credentials from the “hosting code” domain
including bypass httpOnly.
Scenario 3. (DRB, HRET)
User credentials from target domain are safe due to the server not supporting or disallowing
TRACE. However, other security concerns beyond the scope of this paper are present.
Scenario 4. (HRET)
User credentials from target domain are safe due to the server not supporting or disallowing
TRACE. No other security concerns beyond the scope of this paper persist.
General Recommendations
1. Sufficiently patch all web browsers against known domain restriction bypass flaws. is
is a more important part of security policy now more than ever.
2. Disable or disallow the TRACE Request method on production and development (unless
needed) web servers.
3. Web server vendors should update their web server packages to disable TRACE by
default.
4. Web server vendors should inform their users on how to disable or disallow TRACE on
existing web servers.
5. ActiveX controls supporting arbitrary HTTP request should be marked unsafe for
scripting by default. Other such technology vendors (Flash, Java, Shockwave, VBScript,
etc..) should attempt to implement greater security mechanisms regarding disallowing
unauthorized HTTP requests.
6. Users have the ability to disable all active scripting and increase the safety of their
credentials. However, this may negatively impact the functionality of many web sites.
Server Specific Recommendations
(Resolutions should be confirmed by appropriate vendor)
IIS
• URL Scan
Apache
• Source Code Modification
• Mod_Rewrite Module
RewriteEngine on
RewriteCond %{REQUEST_METHOD} ^TRACE
RewriteRule .* – [F]
(ank you to Rain Forest Puppy)
** e Limit or LimitExcept directive in the httpd.conf file does not appear to be able to
restrict TRACE. **
Netscape iPlanet
(Procedures for removing unwanted Request Methods)
cd ${IPLANET_ROOT}
mkdir secure_lib
cp bin/https/lib/libns-httpd40.so secure_lib
cd secure_lib
emacs libnc-httpd40.so
e supported methods appear in lists like: HEAD^@GET^@PUT^@POST^@DELETE^
@TRACE^@OPTIONS^@MOVE^@INDEX^@MKDIR^@RMDIR
• Find all occurrences of these lists and change the methods as required to be GET
padded with spaces to match the length of the word. I.e. DELETE becomes ‘GET ‘ (three
spaces)
• edit the start script for the web server to protect and prepend the secure_lib at the front
of the LD_LIBRARY_PATH. i.e. LD_LIBRARY_PATH=${IPLANET_ROOT}/secure_lib:
• re-start the web server and test it still works!
(Many thanks to Alastair Davie and Robert Rodger.)
References
Some Answered Questions.
Q: Does this affect only Internet Explorer?
A: No, this new technique may affect all browser supporting HTTP Request Enabling
Technologies (HRET).
Q: Does this exploit technique require ActiveX?
A: ActiveX is used in our examples, however research has shown other similar technologies
posses the same abilities.
Q: If I turn off Active Scripting, as a user, am I safe?
A: You could be “safer” but not safe. As previously said, other technologies such as Flash
and Java may still pose a threat even if Active Scripting is disabled.
Q: As a web server administrator, if I disable TRACE are my users credentials safe?
A: Yes, this appears to be the case. Users of your “domain” would be safe against this
new technique since your web server no longer echoes sensitive information in TRACE
requests.
Q: Are my users credentials at risk even though my applications are not vulnerable to XSS
at the application layer?
A: Yes. e particular attack vector of this XSS issue targets the web server itself rather than
the web application layer.
Q: Why should I have to reconfigure my web server, this sounds like a browser client-side
issue?
A: e security of the web browsers in use should be indeed secured as well as the web
server. However, if the web server itself is not configured to deny TRACE, then the security
of the domain credentials will reside in the security of the web browser. Not a good idea.
Issue Discovery & Disclosure Time line
November 1, 2002.
httpOnly bypass issue identified.
November 28, 2002.
Increased Exposure identified.
December 4, 2002.
Issue disclosed and confirmed by Tim Mullen
December 4, 2002.
Issue disclosed and confirmed by Ryan Russell
December 5, 2002.
Issue disclosed and confirmed by Steve Christey (Mitre)
December 6, 2002.
Issue disclosed and confirmed by Rain Forest Puppy (Wiretrip.net)
December 10, 2002.
Issue disclosed and confirmed by CERT.
January 20, 2003.
Issue publicly disclosed by WhiteHat.
Credits & anks
Robert Auger: For help with security research, vulnerability identification, and mirror
help.
Rain Forest Puppy: For help with vulnerability confirmation, security resolutions research
and the XST title.
Tim Mullen: For help with vulnerability confirmation and security resolutions
Steve Christey: For help with vulnerability confirmation and feedback
Ryan Russell: For help with vulnerability confirmation
Robert Rodger: For help with iPlanet vulnerability confirmation and remediation
Alastair Davie: For help with iPlanet vulnerability confirmation and remediation


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