Cross-site scripting (sometimes referred to as
XSS) vulnerabilities occur when an attacker uses a web application to
send malicious code, generally in the form of a script, to a
different end user. These flaws are quite widespread and occur
anywhere a web application uses input from a user in the output it
generates without validating it.
type of variable that comes from a user or comes from a place where
you do not control needs to be validated.
An attacker can use cross site scripting 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
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 server, 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.
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
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.
A4.2 Environments Affected
All web servers, application servers, and web
application environments are susceptible to cross site scripting.
A4.3 Examples and References
A4.4 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
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.
A4.5 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.
If you are displaying user supplied input, the
data should be displayed by a function that either escapes or
converts the data into appropriate HTML.
Encoding user supplied data can defeat XSS
vulnerabilities by preventing inserted scripts from being transmitted
to users in an executable form. Applications can gain significant
characters in all generated output to the appropriate HTML entity
Many languages contain functions which will do
these conversions for you. Be sure you understand how they work and
exactly which characters they convert.
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. OWASP
has also released CodeSeeker, an application level firewall. To learn
more about this type of vulnerability, the OWASP WebGoat training
program has lessons specifically on Cross Site Scripting and data