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Cross-Site Scripting (XSS) attacks are a type of injection, in which
malicious scripts are injected into otherwise benign and trusted
websites. 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. For more details on the different types of XSS
flaws, see:
Types of Cross-Site Scripting
.
How to Avoid Cross-site scripting Vulnerabilities
XSS (Cross Site Scripting) Prevention Cheat Sheet
DOM based XSS Prevention Cheat Sheet
OWASP Development Guide
article on Data Validation
OWASP Development Guide
article on Phishing
How to Review Code for Cross-site scripting Vulnerabilities
See the
OWASP Code Review Guide
.
How to Test for Cross-site scripting Vulnerabilities
See the latest
OWASP Testing Guide
article on how to
test for the various kinds of XSS vulnerabilities.
Testing_for_Reflected_Cross_site_scripting
Testing_for_Stored_Cross_site_scripting
Testing_for_DOM-based_Cross_site_scripting
Description
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.
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.
Reflected and Stored XSS Attacks
XSS attacks can generally be categorized into two categories: reflected
and stored. There is a third, much less well-known type of XSS attack
called
DOM Based XSS
that is discussed
separately
here
.
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 website.
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-I XSS (the attack is carried out
through a single request / response cycle).
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-II XSS.
Blind Cross-site Scripting
Blind Cross-site Scripting is a form of persistent XSS. It generally occurs when the attacker’s payload saved on the server and reflected back to the victim from the backend application. For example in feedback forms, an attacker can submit the malicious payload using the form, and once the backend user/admin of the application will open the attacker’s submitted form via the backend application, the attacker’s payload will get executed.
Blind Cross-site Scripting is hard to confirm in the real-world scenario but one of the best tools for this is XSS Hunter.
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 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, redirecting the user to some other page or site, or 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. For more information on these types of attacks see
Content_Spoofing
.
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 primary defenses against XSS are described in the
OWASP XSS Prevention Cheat
Sheet
.
Also, it’s crucial that you turn off HTTP TRACE support on all web
servers. An attacker can steal cookie data via Javascript even when
document.cookie is disabled or not supported by the client. This attack
is mounted when a user posts a malicious script to a forum so when
another user clicks the link, an asynchronous HTTP Trace call is
triggered which collects the user’s 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 web
servers.
The
OWASP ESAPI project
has produced a set of
reusable security components in several languages, including validation
and escaping routines to prevent parameter tampering and the injection
of XSS attacks. In addition, the
OWASP WebGoat Project
training
application has lessons on Cross-Site Scripting and data encoding.
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/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=&\#X41
(UTF-8) and use it in
IMG
tags:
<IMG SRC=jAvascript:alert('test2')>
There are many different UTF-8 encoding notations that 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">
These and others examples can be found at the OWASP XSS Filter Evasion Cheat Sheet which is a true
encyclopedia of the alternate XSS syntax attack.
Examples
Cross-site scripting attacks may occur anywhere that possibly malicious
users are allowed to post unregulated material to a trusted website for
the consumption of other valid users.
The most common example can be found in bulletin-board websites 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
websites 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):
