Rule:

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Sid:
1839

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Summary:
This event is generated when a cross-site scripting attack 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 (either via the URL or the 
POST method) 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. The code following the tag is then 
executed by the victim's browser.

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Affected Systems:
Many older versions of web server software are affected, as are numerous
web applications.

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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 <SCRIPT> tag could generate this 
event 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>
Nigel Houghton <nigel.houghton@sourcefire.com>
Snort documentation contributed by Kevin Peuhkurinen

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Additional References:

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