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Generator Data Matrix ECC200 in Objective-C 14: Web Site Security

CHAPTER 14: Web Site Security
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Some PHP directives are important in securing PHP code validating input. For example, you can use the open_basedir directive in the php.ini file to limit paths that files can be opened from. You can put all user files into a directory set in the include_path and restrict use outside of your include_path. While input validation is strongly recommended as a security precaution, it must be performed within the code for the forms within the site. Therefore, systems administrators can often only request or require the practice of validating each field, rather than building in the input validation themselves. How scripts react to submitted data becomes another source of concern for many sites.
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Writing scripts can be risky if you don t consider security issues when creating them. Insecure scripts can leave gateways for others to take over your web server. This could mean defacing your web site, but it could also extend to controlling the operating system. Once the operating system has become compromised by a wayward script, the only dependable way to restore integrity to that web server is to reload it from scratch (something we like to refer to as nuke and pave ). There is no magic bullet that can properly secure scripts, which is why they are so risky to implement in the first place. Practicing good scripting techniques is the best way to secure a script. Additionally, you should consider a mixture of editing httpd.conf, using mod_security, and using dosevasive. These (and other) script-hardening techniques are covered in more detail in the book Hardening Apache by Tony Mobily (Apress, 2004). Many of the scripts contained in this book (and anywhere else in the world) are not, in and of themselves, secure. They should be treated with some degree of caution. The same is true for scripts that you find on the web or elsewhere. These scripts, even those from reliable sources, can and often do contain security holes. Before deploying scripts that you find on the Internet, always perform your own security checks on them.
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Securing Your Perl Scripts
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NOTE: This section is intended for scripting professionals. Perl is an extremely powerful language built into every Mac OS X system. One of Perl s strengths is its flexibility in dealing with its variables. Variables can contain almost anything, including file paths. Storing file paths in variables can be an issue if a visitor to a site can change that variable. The visitor can be tricked into running and showing the output of arbitrary commands, showing the wrong files, or showing the contents of directories. To avoid this problem, always specify a redirection statement such as using the > character before a variable. You should also include a space between the redirection statement and the variable that contains a file name. Newer versions of Perl improved the open function to avoid this problem by introducing the three-argument call to use when opening a file.
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CHAPTER 14: Web Site Security
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NOTE: If your Perl code will run only on newer versions of Perl, always use the three-argument version. In Perl, backticks (``) allow the Perl exec function to run external programs. Although easy to use, backticks can result in security problems from their use in environments that involve user input. The following is a simplistic method for validating user input based on the HTTP_REFERER variable, which contains data about the address of the webpage that is sending direction requests to a server. This variable can be faked by visitors, but it is meant to be used as the first line of defense and can thwart a number of attacks. You should always check the HTTP_REFERER header to ensure that data is originating in the proper place. Generally, a user should not be sending data directly to a script. If a user is trying to do so, this is more than likely evidence of malicious activity.
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#!/usr/bin/perl $referer=$ENV{'HTTP_REFERER'}; print "Content-type:text/html\n\n"; if ($referer =~ m#^http://www.apress.com/#){ print "insert processes and code in place of this line"; } else { print "The server has encountered an error. Please go back and try again."; }
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The following code shows a way of capturing and printing information about a visitor to a page. This data could also be compromised by being passed to another variable and captured with a form. Obviously, the security implications for these variables are substantial. Validating the remote address is a good way to add a layer of protection to scripts against the possibility of man-in-the-middle attacks:
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< php echo "<p>IP Address: " . $_SERVER['REMOTE_ADDR'] . "</p>"; echo "<p>Referrer: " . $_SERVER['HTTP_REFERER'] . "</p>"; echo "<p>Browser: " . $_SERVER['HTTP_USER_AGENT'] . "</p>"; >
Another method for protecting Perl scripts is to restrict the script from using any data coming from outside the script. This can include visitor input and environment variables. Alterations to script data are thought of as tainted data. One way to instruct Perl to restrict the use of tainted data is to use the T switch at the end of the first line of your script. This will cause Perl to issue warnings when you try to do something potentially dangerous. You can still use tainted data, but you must first sanitize it. The safest way to do this is to check your input against a list of valid characters and strip away anything else. If you wrote your script using the CGI.pm library, something like the following will sanitize incoming data:
use HTML::Entities (); use CGI qw/:standard/; $ok_chars = 'a-zA-Z0-9 ,-'; foreach $param_name ( param() ) { $_ = HTML::Entities::decode( param($param_name) );
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