Cross-site Scripting (XSS)

Overview
Cross-site scripting attacks are an instantiation of injection problems, in which malicious scripts are injected into the otherwise benign and trusted web sites.

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.

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 (requires interaction with a web server supporting dynamic content)

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.

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.

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.

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