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Even if the site does further checks, such as verifying the user s web browser, you can still get in. Note that when we record the user s cookies, they send a HTTP request to cookie-stealing script, meaning that you know their HTTP user-agent string and their IP address. However, just because somebody has your cookies for a given site doesn t mean they can automatically log in under your account. Ultimately, it depends on how the targeted site is coded. Let s now look at how you can both prevent the XSS attack and how you can protect against session theft.
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Strategy 1: Remove Unwanted Tags from Input Data
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Not allowing users to enter any tags at all is easy. This is typically how you want to treat data on a signup form, such as a user s name or e-mail address. On the other hand, in a forum system, you may want to allow users to format their code or post links or images. To remove all HTML tags (including script tags), you can use PHP s strip_tags() function. This function also allows you to pass a list of allowed tags, which it will ignore when stripping the rest of the tags. This is effectively a white list of safe tags. The problem with using PHP s strip_tags() to do this is that it doesn t alter attributes. For example, if you wanted to remove every tag except the strong tag, you would use something along the lines of $str = strip_tags ($str, '<strong>');. A malicious user, however, could still enter the following:
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Don't mouse over the <b onmouseover="alert('I told you not to!')">bold text!</b>
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Or they could enter something more damaging, such as in the previous examples. To combat this, you must also filter out attributes from allowed tags. You can achieve this using preg_replace() on the resulting data from strip_tags().
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< php $str = strip_tags($str, '<strong>'); $str = preg_replace('/<(.*)\s+(\w+=.* )>/', '', $str); >
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If you were to now run the preceding user input through this code, you would end up with Don't mouse over the <strong>bold text!</strong>, just as you had hoped. Another solution some web applications (such as forum software) use is fake HTML tags, such as [b] instead of <b>. When they output posted messages using this markup, the application searches through the code and replaces each tag with a safe HTML tag (which will never have dangerous attributes in it, as the tags will be hardcoded to be clean).
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Strategy 2: Escape Tags When Outputting Client-Submitted Data
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This is in a way the opposite treatment to strategy 1, in which you remove any unwanted tags, and then proceed to output the remaining data as is. Instead of filtering the data, you output it exactly as it was submitted. The difference now is that you re not treating your data as HTML, so therefore you must escape it. You do this with the PHP htmlentities() function. This will convert the < character to <, the > character to >, the " character to ", the ' character to ', and the & character to &. Not only does this protect against HTML tags being directly output, but it also keeps your HTML valid and stops your page from breaking. If you are outputting usersubmitted data in form elements, you should also be using this.
<p>< php echo htmlentities($userSubmittedPost) ></p> <input type="text" name="someInput" value="< php echo htmlentities($someData) >" />
Strategy 3: Protect Your Sessions
Unfortunately, it can be quite difficult to completely protect your sessions. As stated, if a user s cookie data is captured using the XSS attack outlined previously, then their useragent can also be captured. Additionally, since a user s IP address may change from one request to the next (which frequently occurs for users behind a web proxy), then you can t rely on their IP address to identify them. Because of this, you should take at least the following precautions: Regenerate a user s session ID using PHP s session_regenerate_id() after a change in their permission level (and destroy their old session using session_destroy()). Give users the option to log out (thereby destroying their session data when they do). Remove session data after a period of inactivity (e.g., if the user does nothing for 30 minutes, then their session is invalid). Remove session data after an absolute period of time (e.g., after a day, their session ID is no longer valid regardless of how recently the session ID was used). Add password protection to critical operations. Even if it appears that the user is valid, ask them to reauthenticate when they try to do something important (and remember to then regenerate their session ID and destroy their old session).
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