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Has your site got the 3 basic security measures?

By: Gim Yeap, Fri Dec 9th, 2005 05:10:43 PM

In recent weeks, attacks on prominent sites such as Yahoo and Ebay have brought home a very pressing point - site security. Anywhere you have a dynamically-generated page, you could be open to attacks where malicious HTML is embedded into your pages. Your pages could be rewritten to substitute your customers' names with "Dummy." Or, credit card information could be intercepted and sent to a secret depository for later use. What can we do about this?

There are many methods by which a hacker may attack or take control of a site. I am focusing this discussion on attacks that come via form input. That is, anywhere you have input coming in from your web user, e.g. a registration form, user login or even a search on your site. Scripts could be sent to your server by entering < script> some malicious code < /script> in your input fields. The following are steps you can take to minimise the risk of this happening. These measures will not make your site hacker-proof (no site can be if a hacker really has it in for you), but it can make it less of an easy target. Step 1: Place character limits on your inputs You do this by adding the "maxlength" attribute into your text input tags

e.g. < input type="text" name="firstname" maxlength="15">

The example above restricts the user to a 15 character input for that field. The "< script>" and "< /script>" tags alone will take 17 characters so the smaller you limit your "maxlength" attribute to, the harder it will be to include rogue codes in your inputs. Of course, you must ensure that you impose a suitable limit so that actual input from your valid users will not be excluded. Step 2: Filtering your data All data received from your site should be filtered, you can either filter your data when it comes into your server as user input, or when it goes out as results for your user's browser. Whether you should filter input or output, depends on your site and its requirements, there is a good discussion on this at http://www.cert.org ech_tips/malicious_code_mitigation.html/ . Filters can be written in any language, here is an example in Perl :

# This function checks the input, $firstname, for the following symbols ;<>?*/'&$!#()[]{}:"' # and tells the user to re-enter his/her firstname if any of the symbols is found if($firstname =~ /([;<>?*/'&$!#()[]{}:'"])/) { print p('Invalid input found, please use only alphanumerical input. Please re-enter your FIRSTNAME'); } You can see this script at work on our site : http://www.payingads.com/freesignup.html . Step 3: Setting the character encoding

Some HTML editors already set this while it creates a page, but those of you who have older HTML editors or like me, like to code the page from scratch will need to include the following line in our HTML pages: < META http-equiv="Content-Type" content="text/html; charset=IS0-8859-1"> It should go as high as possible on your webpage, I normally place it just after the < /head> tag, before the < title> tag. This META tag tells the browser to use the "ISO-8859-1" character set, which is suitable for most Western European languages, rather than let the browser choose it's own character encoding, which may or may not be ISO-8859-1.

Why is it important to explicitly set it? The character encoding basically tells browsers how to display a particular character. For example, in the ISO-8859-1character set, "A" represents the letter "A" while "©" represents the copyright symbol "©" (You can try this out by typing < p>A< /p> or < p>©< /p> in a html file then call it up on a browser). Some character sets, have more than one representation for special characters such as "<" or ">", so your filter program may not toss out all the representations of the character you have asked it to exclude. So when it serves a new page back to the browser, the browser, because it has not been told what encoding to use, can still read the malicious script intact.

So there you have it, 3 steps that should be incorporated into every website. Use them as a base to further build on. Because every site is different, you (or the security consultant you hire) will need to assess your site's own vulnerabilities and implement appropriate security measures. To do this you need to take into account your site's risk factor, your budget and your available resources.

On a final note, I'd like to stress the importance of keeping up with the latest threats and developments in site security. A good site for checking out security alerts is the CERT Coordination Center http://www.cert.org/nav/index.html or better yet sign up for their Security Advisory that is sent via email.

About the author: Gim Yeap Email : gim@payingads.com Site : www.payingads.com

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