Rule:
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Sid:
1667
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Summary:
This event indicates that a cross-site scripting attack using the "img 
src=javascript" vulnerability is being attempted, or a potential 
attacker is testing your site to determine if it is vulnerable.

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Impact:
Successful cross-site scripting attacks generally target the users of 
your web site.   Attackers can potentially gain access to your users 
cookies or session ids, allowing the attacker to impersonate your 
user.   They could also set up elaborate fake logon screens to steal 
user names and passwords.

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Detailed Information:
Whenever a web application accepts input and then uses that input as 
part of the HTML of a new page without filtering, the application is 
vulnerable to cross-site scripting.  The traditional means of exploiting
this is to embed a "<SCRIPT>" tag into the input.   However, as many 
applications now look for this attack vector, exploitation of the 
ability to use "IMG SRC=javascript:" to embed javascript without the 
script tag is becoming more common.

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Attack Scenarios:
The most common avenue of attack is for the attacker to send an HTML 
formatted email to the victim.  The email will contain a link to a 
specially crafted URL which contains the exploit.   When the victim 
clicks on the link, they are directed to the vulnerable web site and the
attack code is executed by their browser.

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Ease of Attack:
Moderately Easy.  Exploit code exists to automate attacks against users 
of some widely deployed web applications which are known to be 
vulnerable.  Finding vulnerabilities in other, including proprietary, 
web applications is fairly trivial and existing exploit code could 
easily be modified to take advantage of newly discovered 
vulnerabilities.

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False Positives:
Web pages that legimately include the "IMG SRC=javascript:" directive 
could trigger this alert under certain circumstances.

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False Negatives:
None known.

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Corrective Action:
Determine if your web application is actually vulnerable to this 
attack. If it is and the application is not of your own design, 
contact the authors or vendor and see if there is a patch or newer 
version.   If the application is proprietary to you or your company, 
ensure that it properly validates input.

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Contributors:
Sourcefire Research Team
Brian Caswell <bmc@sourcefire.com>
Snort documentation contributed by Kevin Peuhkurinen

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Additional References:

iDefense
http://www.idefense.com/idpapers/XSS.pdf

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