Cross-Site Scripting (XSS) attacks are a type of injection, in which malicious scripts are injected into otherwise benign and trusted web sites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to a different end user. Flaws that allow these attacks to succeed are quite widespread and occur anywhere a web application uses input from a user within the output it generates without validating or encoding it.
An attacker can use XSS to send a malicious script to an unsuspecting user. The end user’s browser has no way to know that the script should not be trusted, and will execute the script. Because it thinks the script came from a trusted source, the malicious script can access any cookies, session tokens, or other sensitive information retained by the browser and used with that site. These scripts can even rewrite the content of the HTML page.
Cross-Site Scripting (XSS) attacks occur when:
- Data enters a Web application through an untrusted source, most frequently a web request.
- The data is included in dynamic content that is sent to a web user without being validated for malicious content.
Stored and Reflected XSS Attacks
XSS attacks can generally be categorized into two categories: stored and reflected. There is a third, much less well known type of XSS attack called DOM Based XSS that is discussed seperately here.
Stored XSS Attacks
Stored attacks are those where the injected script is permanently stored on the target servers, such as in a database, in a message forum, visitor log, comment field, etc. The victim then retrieves the malicious script from the server when it requests the stored information. Stored XSS is also sometimes referred to as Persistent or Type-I XSS.
Reflected XSS Attacks
Reflected attacks are those where the injected script is reflected off the web server, such as in an error message, search result, or any other response that includes some or all of the input sent to the server as part of the request. Reflected attacks are delivered to victims via another route, such as in an e-mail message, or on some other web site. When a user is tricked into clicking on a malicious link, submitting a specially crafted form, or even just browsing to a malicious site, the injected code travels to the vulnerable web site, which reflects the attack back to the user’s browser. The browser then executes the code because it came from a "trusted" server. Reflected XSS is also sometimes referred to as Non-Persistent or Type-II XSS.
Other Types of XSS Vulnerabilities
In addition to Stored and Reflected XSS, another type of XSS, DOM Based XSS was identified by Amit Klein in 2005. OWASP recommends the XSS categorization as described in the OWASP Article: Types of Cross-Site Scripting, which covers all these XSS terms, organizing them into a matrix of Stored vs. Reflected XSS and Server vs. Client XSS, where DOM Based XSS is a subset of Client XSS.
XSS Attack Consequences
The consequence of an XSS attack is the same regardless of whether it is stored or reflected (or DOM Based). The difference is in how the payload arrives at the server. Do not be fooled into thinking that a “read only” or “brochureware” site is not vulnerable to serious reflected XSS attacks. XSS can cause a variety of problems for the end user that range in severity from an annoyance to complete account compromise. The most severe XSS attacks involve disclosure of the user’s session cookie, allowing an attacker to hijack the user’s session and take over the account. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirect the user to some other page or site, or modify presentation of content. An XSS vulnerability allowing an attacker to modify a press release or news item could affect a company’s stock price or lessen consumer confidence. An XSS vulnerability on a pharmaceutical site could allow an attacker to modify dosage information resulting in an overdose. For more information on these types of attacks see Content_Spoofing.
How to Determine If You Are Vulnerable
How to Protect Yourself
The primary defenses against XSS are described in the OWASP XSS Prevention Cheat Sheet.
Alternate XSS Syntax
XSS using Script in Attributes
XSS attacks may be conducted without using <script></script> tags. Other tags will do exactly the same thing, for example:
<body onload=alert('test1')> or other attributes like: onmouseover, onerror, onmouseover <b onmouseover=alert('Wufff!')>click me!</b> onerror <img src="http://url.to.file.which/does.not.exist" onerror=alert(document.cookie);>
XSS using Script Via Encoded URI Schemes
If we need to hide against web application filters we may try to encode string characters, e.g.: a=A (UTF-8) and use it in IMG tag:
There are many different UTF-8 encoding notations what give us even more possibilities.
XSS using code encoding
We may encode our script in base64 and place it in META tag. This way we get rid of alert() totally. More information about this method can be found in RFC 2397.
<META HTTP-EQUIV="refresh" CONTENT="0;url=data:text/html;base64,PHNjcmlwdD5hbGVydCgndGVzdDMnKTwvc2NyaXB0Pg">
Example 1 : Cookie Grabber
If the application doesn't validate the input data, the attacker can easily steal a cookie from an authenticated user. All the attacker has to do is to place the following code in any posted input(ie: message boards, private messages, user profiles):
The above code will pass an escaped content of the cookie (according to RFC content must be escaped before sending it via HTTP protocol with GET method) to the evil.php script in "cakemonster" variable. The attacker then checks the results of his evil.php script (a cookie grabber script will usually write the cookie to a file) and use it.
Error Page Example
Let's assume that we have an error page, which is handling requests for a non existing pages, a classic 404 error page. We may use the code below as an example to inform user about what specific page is missing:
<html> <body> <? php print "Not found: " . urldecode($_SERVER["REQUEST_URI"]); ?> </body> </html>
Let's see how it works:
In response we get:
Not found: /file_which_not_exist
Now we will try to force the error page to include our code:
The result is:
We have successfully injected the code, our XSS! What does it mean? For example, that we may use this flaw to try to steal a user's session cookie.