Part IV Securing Your Web Site in Visual Studio .NET

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Part IV Securing Your Web Site
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digest() subroutine using the value of the $name variable It then compares the old and the new digests to determine whether the name field was altered in screen 2 Remember that a name was entered in screen 1 and it was hidden using the HTML hidden tag in screen 2 So, there was a chance that someone could have altered the name value in transit
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If the digests do not match, an alert message is printed using the alert() subroutine, which displays a JavaScript pop-up message Otherwise, the name and the e-mail address are printed on the screen The most interesting subroutine is the create_message_digest() It takes anything as an argument and uses the Digest::MD5 object called $ctx to add the given data and a secret pass phrase (stored in $secret) using the Digest::MD5 object s add() method Then it creates a Base64 MD5 digest using the b64digest() method, which is returned to the calling subroutine When the CGI script is first run, it produces a screen that asks the user to enter his or her name After the user enters his or her name, the user continues to the next screen, where the user is asked to enter an e-mail address The HTML source of this screen is shown in Listing 18-3
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Listing 18-3: HTML Source for Screen 2 of hidden-md5eg
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<!DOCTYPE HTML PUBLIC -//IETF//DTD HTML//EN > <HTML> <HEAD> <TITLE>Multi-Screen Web Application Demo</TITLE> </HEAD> <BODY> <H2>Screen 2</H2> <HR SIZE= 0 COLOR= black > <FORM METHOD= POST ENCTYPE= application/x-www-form-urlencoded > Enter email: <INPUT TYPE= text NAME= email SIZE=30> <INPUT TYPE= hidden NAME= name VALUE= Cynthia > <INPUT TYPE= hidden NAME= digest VALUE= IzrSJlLrsWlYHNfshrKw/A > <INPUT TYPE= submit
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18 Web Security
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NAME= submit VALUE= Next > </FORM> </BODY> </HTML>
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The hidden data are stored using the following lines:
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<INPUT TYPE= hidden NAME= name VALUE= Cynthia > <INPUT TYPE= hidden NAME= digest VALUE= IzrSJlLrsWlYHNfshrKw/A >
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The first hidden data tag line stores name=Cynthia and the second hidden data tag line stores digest=IzrSJlLrsWlYHNfshrKw/A The second piece of data is the message digest generated for the name entered in screen 1 When the user enters an e-mail address in the second screen and continues, the final screen is displayed However, before the final screen is produced, a message digest is computed for the name field in screen 1 This digest is compared against the digest created earlier to verify that the value entered for the name field in screen 1 is not altered in screen 2 Because the MD5 algorithm creates the same message digest for a given data set, any differences between the new and old digests will raise red flags and the script will display an alert message and refuse to complete processing In other words, if a vandal decides to alter the data stored in screen 2 (shown in Listing 18-3) and submits the data for final processing, the digest mismatch will allow the script to detect the alteration and take appropriate action In your real-world CGI scripts (written in Perl), you can use the create_message_digest() subroutine to create a message digest for anything
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You can download and install the latest version of Digest::MD5 from CPAN using perl MCPAN e shell command followed by a install Digest::MD5 command at the CPAN shell prompt
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Keeping user input secure
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As you can see, most of the security holes created by CGI scripts are caused by inappropriate user input The following section discusses a few common problems and solutions
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Part IV Securing Your Web Site
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There are two approaches to ensuring that user input is safe: Scanning the input for illegal characters and to replace or remove them For example, for the whoispl script, you can add the following line:
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$domain =~ s/[\/ ;\[\]\<\>&\t]//g;
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This removes illegal meta-characters This is a common but inadvisable approach, as it requires that the programmer be aware of all possible combinations of characters that could cause trouble If the user uses input not predicted by the programmer, there is the possibility that the program may be used in a manner not intended by the programmer Defining a list of acceptable characters and replacing or removing any character that is not acceptable The list of valid input values is typically a predictable, well-defined set of manageable size I prefer the second approach above, since it does not require the programmer to trap all characters that are unacceptable, leaving no margin for error The recommended approach requires only that a programmer ensure that acceptable characters are identified; thus the programmer can be less concerned about the characters an attacker may try in an attempt to bypass security checks Building on this concept, the whoispl program presented earlier could be sanitized to contain only those characters allowed; for example:
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#!/usr/bin/perl -w # # Purpose: This is a better version of the previous # whoispl script # # Assign a variable the acceptable character # set for domain names # my $DOMAIN_CHAR_SET= -a-zA-Z0-9_ ; # Get the domain name from query string # environment variable # my $domain = $ENV{ QUERY_STRING }; # Now remove any character that does not # belong to the acceptable character set # $domain =~ s/[^$DOMAIN_CHAR_SET]//g; # Print the appropriate content type # Because whois output is in plain text, we # choose to use text/plain as the content-type here # print Content-type: text/plain\n\n ; # Here is the system call:
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