Security bug in .NET Forms Authentication

From: Toby Beaumont (toby_at_CREATOR.CO.UK)
Date: 09/14/04

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    Date:         Tue, 14 Sep 2004 12:42:28 +0100


    We believe we have discovered a serious flaw in .NET forms authentication
    when used to secure sub folders.

    A standard forms authentication setup requires the presence of "web.config"
    to set the authentication method and login procedure. The presence of this
    file prevents access to certain files (.aspx files for example) unless


    The webroot for your website is:


    You want to secure files in a sub directory "secure"


    A request to http://localhost/secure/somefile.aspx would then redirect the
    user to a predefined authentication page, as defined in web.config, before
    allowing the user access to "somefile.aspx".


    1. Using Mozilla not IE, you make a request to
    http://localhost/secure\somefile.aspx The use of a backslash rather than a
    forward slash appears to bypass the expected authentication model invoked in
    .NET forms authentication
    2. Using IE, you make a request to http://localhost/secure\somefile.aspx -
    IE automatically replaces the backslash "\" with a forward slash "/" and
    everything appears fine. However, replace the backslash "\" with %5C (%5C
    being hex value for \) and all is not so fine:
    Interestingly (and I guess now somewhat amusingly) Microsoft point out in
    the article "Design Guidelines for Secure Web Applications"
    "Be Careful with Canonicalization Issues:
    Data in canonical form is in its most standard or simplest form.
    Canonicalization is the process of converting data to its canonical form.
    File paths and URLs are particularly prone to canonicalization issues and
    many well-known exploits are a direct result of canonicalization bugs. For
    example, consider the following string that contains a file and path in its
    canonical form."
    And then goes on to define the exploit ;-)
    (Russ - I have not posted this message anywhere as yet, nor have I contacted
    Microsoft. If you indeed confirm this exploit, you are the first to know).
    Toby Beaumont
    Director of Technology
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