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Multiple .NET Null Byte Injection Vulnerabilities
From: "Paul Craig" <paul.craig () security-assessment com>
Date: Wed, 11 Jul 2007 09:46:12 +1200

========================================================================
= Multiple .NET Null Byte Injection Vulnerabilities
=
= Vendor Website: 
= http://www.microsoft.com
=
= Affected Version:
=    .NET FrameWork v1.1 SP1
=    .NET FrameWork v2.0.50727
=
= Vendor Notified   - October, 2006
= Public Disclosure - July 11th, 2007
=
http://security-assessment.com/files/advisories/2007-07-11_Multiple_.NET_Nul
l_Byte_Injection_Vulnerabilities.pdf
========================================================================

== Overview ==
Security-Assessment.com recently completed research into the .NET 
Framework in relation to the affect a Null byte (%00) has on 
various aspects of the .NET Common Language Runtime.
This advisory details the findings of that research conducted
by Paul Craig Paul.Craig<at>security-assessment.com.

It was found that certain .NET methods in various sections of the
.NET namespace are vulnerable to Null byte injection attacks.
Null byte injection occurs when the .NET CLR incorrectly handles
user supplied Null bytes.

The .NET CLR considers Null bytes as 'data', .NET strings are
not Null byte terminated.
However, native POSIX compliant function calls terminate all
strings at the first found Null byte.
Interoperability issues are encountered when data containing a Null
byte is used by .NET to directly call a native C function call.

Native function calls terminate strings at the injected Null
byte allowing a remote user to arbitrarily terminate a string
parameter used by the vulnerable method.

Security-Assessment.com has discovered five vulnerable methods
in the .NET framework which are exploited through Null byte injection.
Three of the discovered vulnerabilities allow strings to be
arbitrary terminated through String Termination vulnerabilities.
The remaining two resulted in an Arbitrary File Disclosure
condition where a remote user is capable of accessing arbitrary
files from within the web root. 

.NET has a history surrounding Null byte input flaws and associated
logic.
On September 8th, 2003 WebCohort Research <research () webcohort com>
released an advisory titled "Microsoft ASP.NET Request Validation 
Null Byte Filter Bypass Vulnerability". Where by the .NET request
validation routine could be bypassed when using a Null byte
injection.

Null byte injection is not a new class of attack, and is a well
known exploitive method but this is the first time a Null byte
injection vulnerability has been found in methods within the
.NET framework.
Security researchers should be aware of Null byte injection 
attacks within the framework itself and .NET developed 
applications.


== Exploitation ==

The following examples can be found at
http://ha.cked.net/examples.zip

Exploit 1: Server.MapPath
------------------------------------------------------------------------
A Null byte injected within the filename parameter of the
Server.MapPath method will terminate any returned string,
removing any string data concatenated to the user supplied
value.
This can be seen in example 1 below.

name = test.aspx%00

Sub Page_Load()
        dim name as string
        dim realname as string
        name = request("name") & ".uploaded"
        realname = Mappath(".") & "\" & name
        response.write("Mappath value of name variable: " & MapPath(name) &
"<br>")
  response.write("The real value is: " & realname & "<br>")
End Sub


Output:
Mappath of name variable = C:\Inetpub\exploit1\test.aspx
The Real value is : C:\Inetpub\exploit1\test.aspx.uploaded

1.      Two variables are assigned, name and realname. 

2.      Name is a user supplied filename, and for security reasons
                .uploaded is appended to this value.

3.      Real name is the virtual path for the current directory,
                with the name variable appended (This is the correct 
                location of the file)

4.  Inserting a Null byte suffix into the 'name' variable
                (name=test.aspx%00) is able to terminate the string returned
                 from MapPath and any data concatenated to the user supplied
                 value is removed.


Exploit 2: Server.Execute and Server.Transfer
------------------------------------------------------------------------
Server.Execute and Server.Transfer were found to both be vulnerable
to Null byte injection. Here the Null byte produces an arbitrary file 
disclosure vulnerability.

If a remote user is able to control the input used in a Server.Execute
or Server.Transfer method, the method can be used to disclose the contents
of any file within the document root.
As seen in example 2, below.

Sub Page_Load()
        Server.Transfer(request("page"))
End Sub

Security-Assessment.com has witnessed Server.Transfer and Server.Execute
being used in page redirection functionality where .NET sessions are
transferred to a user supplied page variable.

According to MSDN:
"The page transferred to should be another .aspx page. For instance,
a transfer to an .asp or .asmx page is not valid. The Transfer 
method preserves the QueryString and Form collections."

If a user attempts to transfer to web.config, or any other none .ASPX
page an exception is created. However, when the page variable is
suffixed with a Null byte the complete file contents of the page 
is returned to the remote user.

A Server.Transfer or Server.Execute to page=web.config%00 will
display the contents of the web.config file.

This vulnerability can also be used to view any file within the
document root.


Exploit 3: String.Compare
------------------------------------------------------------------------
String.Compare was also found to be affected by Null bytes. Although does
not
produce an exploitable condition. 
String.Compare demonstrates .NET's inability to correctly handle Null bytes.

This is demonstrated in the example below.

Sub Page_Load()
        dim allowed, sFirstItem, sSecondItem as string
        sFirstItem = Request("first")
        sSecondItem = Request("second")
        response.Write ("String.Compare - First item = " & sFirstItem &
"<br>")
        response.Write ("String.Compare - Second item = " & sSecondItem &
"<br>")
        if String.Compare(sFirstItem, sSecondItem) =0 then
                response.Write ("<b>String.Compare - Matched! Strings are
the same</b>" & "<br>") 
        else
                response.Write ("<b>String.Compare - FAILED!! Strings are
not the same</b>" & "<br>")
        End If
        if sFirstItem=sSecondItem then
                response.Write ("Direct eval - Matched! Strings are the
same" & "<br>")
                else
                response.Write ("<b>Direct eval - FAILED! Strings are not
the same</b>" & "<br>")
        End If
End Sub

1.      Two variables are supplied, "first" and "second"
2.      String.Compare is called to compare the two strings, and then a
direct
                evaluation is performed.
3.  String.Compare should act identically to a direct evaluation.
4.      The result of first=test&second=test can be seen below. 

        String.Compare - First item = test
        String.Compare - Second item = test
        String.Compare - Matched! Strings are the same
        Direct Eval - Matched! Strings are the same
 
 This is the correct response, as both the first and second were defined as
'test'

5. Now, add a Null byte suffix to the 'first' variable and try again.
The result of Example3.aspx?first=test%00&second=test can be seen below.

        String.Compare - First item = test
        String.Compare - Second item = test
        String.Compare - Matched! Strings are the same
        Direct Eval - Failed! Strings are not the same
        
String.Compare terminates the string at the first Null byte, and the
two values are seen as identical. However direct evaluation correctly
determines that they are different, due to the extra Null byte suffix.  

Although this is not exploitable it is an interesting finding.

Exploit 4: System.Net.Mail.SmtpMail.Send
------------------------------------------------------------------------

The object System.Net.Mail.SmtpMail.Send was found to be vulnerable to
string parameter termination similar to the vulnerability discovered in
Server.MapPath.

This is demonstrated in the example below.

Private Sub Page_Load(sender As Object, e As System.EventArgs)
        Dim m As New MailMessage()
        m.From = "securityguy () security-assessment com"
        m.To = request("to") & "@security-assessment.com"
        m.Subject = request("subject") & ": FromWebsite"
        m.Body = request("body") & "This message was submitted by a user."
        Response.Write("Sending mail to: " & m.to)
        
m.Fields.Add("http://schemas.microsoft.com/cdo/configuration/smtpauthenticat
e", "1")
        
m.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendusername";,
"username") 
        
m.Fields.Add("http://schemas.microsoft.com/cdo/configuration/sendpassword";,
"password")
        SmtpMail.SmtpServer = "mail.server.com"
        SmtpMail.Send(m)
End Sub

In this example the recipient property (To) is defined as a user supplied
variable with “ () security-assessment com” concatenated.
Security-Assessment.com
has seen this development technique being used to ensure mail is only be
sent to users of a particular domain.

However, if a user supplies a recipient (To) variable as
paul.craig () microsoft com%00
the mail will be sent to the Microsoft.com domain and string is again
terminated at
the first Null byte removing the concatenated "@security-assessment.com"
value.

Similarly the From, Subject and Body values were found to be vulnerable to
the
same method of Null byte injection

== Solutions ==

Security-Assessment.com has been in contact with Microsoft and a new .NET
patch
has been released to address the discovered vulnerabilities.
Install patch KB928365 (Security Update for Microsoft .NET Framework 2.0)
and/or
KB928366 (Security Update For Microsoft .NET Framework 1.1)

== Credit ==

Discovered and advised to Microsoft October, 2006 by Paul Craig of
Security-Assessment.com - Paul.Craig<at>Security-Assessment.com


== About Security-Assessment.com ==

About Security-Assessment.com
Security-Assessment.com is Australasia’s leading team of Information 
Security consultants specialising in providing high quality Information
Security services to clients throughout the Asia Pacific region. Our 
clients include some of the largest globally recognised companies in
areas such as finance, telecommunications, broadcasting, legal and
government. Our aim is to provide the very best independent advice
and a high level of technical expertise while creating long and lasting
professional relationships with our clients.

Security-Assessment.com is committed to security research and development,
and its team continues to identify and responsibly publish vulnerabilities
in public and private software vendor's products. Members of the
Security-Assessment.com R&D team are globally recognised through their
release of whitepapers and presentations related to new security research.

Security-Assessment.com is an Endorsed Commonwealth Government of Australia
supplier and sits on the Australian Government Attorney-General's Department
Critical Infrastructure Project panel. We are certified by both Visa and
MasterCard under their Payment Card Industry Data Security Standard
Programs.


Paul Craig
Security Consultant
Security-Assessment.com



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