Alternate XSS Syntax

Revision as of 07:11, 26 May 2009 by Deleted user (Talk | contribs)

Jump to: navigation, search

[ live tennis updates australian open ] [ africa urbanization ] [ automotive trim screws ] [ pc cillin antivirus updates ] [ mccaffrey antivirus ] [ digital auto compass ] [ auto tradder canada ] page [ crack for panda titanium antivirus 2005 ] [ downhill adventures south africa ] [ water coolers australia ] url [ asian diva girl lili ] [ asia disabled regional society ] [ norton antivirus 2005 serial keys ] coastlines of australia [ openantivirus ] [ nod32 antivirus system ] [ by design auto burlington ] top [ sa vs australia latest score ] [ norton antivirus 2006 keygen ] [ australia wear work ] [ asian dominatrixs ] [ av antivirus free ] [ australia bulldozers heavy in machinery ] [ pc cillin 2000 antivirus ] [ australia open home page ] [ african location tribe tribe ] [ what are 3 great african americans in arts ] [ pc cillan antivirus ] [ ] [ center.antivirusoverride security window ] [ win antivirus pro reviews ] [ avg+antivirus+free ] [ hypoplasia dictionary ] [ norton antivirus download rebate ] [ auto insurance massachusetts online ] [ asian appetizer ] [ asia carrera movie download ] http [ antivirus trialware download ] [ automation home screen touch ] [ australian currency ] [ travel agents for asia and australia in toronto ] [ mtn group south africa ] antivirus stop sign time difference uk australia [ african furniture inspired ]

This page was marked to be reviewed for deletion.

#REDIRECT Cross-site_Scripting_(XSS)

Last revision (mm/dd/yy): 05/26/2009

Related Security Activities

Description of Cross-site Scripting Vulnerabilities

See the OWASP article on Cross-site Scripting (XSS) Vulnerabilities.

How to Avoid Cross-site scripting Vulnerabilities

See the OWASP Development Guide article on Phishing.

See the OWASP Development Guide article on Data Validation.

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.


Cross-site Scripting (XSS) attacks have to pass two hurdles. First they must get through the application without being filtered, validated, or encoded in a way that prevents them from executing. Second, they much be carefully crafted so that they seamlessly insert their payload into the HTML document so that it will run when loaded by the browser.

Cross Site Scripting is not just <script>alert('XSS');</script>. Because of JavaScript and HTML flexibility and their interpretation by the web browsers, it's possible to achieve the same goal in many different ways. These attacks extend the basic XSS attacks with a number of alternate encodings to help them pass through the application.

In effect we may try to bypass more or less successful input data filtering methods. Some JavaScript and HTML constructions after encoding are correctly interpreted by some browsers, nonetheless it often varies on the web browser version, and others are not.

If we want to use popular <script> tags anyway, we may try to bypass filtering replacing given characters with their equivalents:

From To
<    &lt;
>    &gt;
(    &#40;
)    &#41;
#    &#35;
&    &amp;
"    &quot;

In this case:

 <script>alert('y0u ar3 0wn3d!');</script>

would be replaced with:

&lt;script&gt;alert&#40;'y0u ar3 0wn3d!'&#41;&lt;/script&gt;

In most contexts this encoded string will not execute. However there are some environments, particularly Ajax and XML processing engines, that will automatically decode these encoded characters and allow the attack to execute.

We don't need to do replacement at all, we may get the same effect in many different ways.

Risk Factors


XSS using Script in Attributes

XSS attacks may be conducted without using <script></script> tags. Other tags will do exacly the same thing, e.g.:

<body onload=alert('test1')>

or other attribites like: onmouseover, onerror.


<b onmouseover=alert('Wufff!')>click me!</b>


<img src="" 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 tag:

<IMG SRC=j&#X41vascript:alert('test2')>

There are many different UTF-8 encoding notations what 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"

These (just a little modified by me) and others examples can be found on, which is a true encyclopedia of the alternate XSS syntax attack.

Related Threat Agents

  • TBD

Related Attacks

Related Vulnerabilities

Related Controls

  • HTML Entity Encoding
  • Use whitelists and if possible specify detailed format of the expected output data.