Rule: 

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Sid: 
119-2

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Summary: 
This event is generated when the pre-processor http_inspect detects network traffic that may constitute an attack.

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Impact: 
Unknown. This may be an attempt to evade an IDS.

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Detailed Information:
This event is generated when double encoded characters are detected in web traffic. This is abnormal behavior and may be an indicator of a possible attack against a vulnerable system.

This may also be an attempt to evade an IDS.

Note: This pre-processor is designed to detect attacks aimed at servers, it needs to be configured to monitor for the servers being protected.  Outbound client traffic may result in a high rate of false positive events.

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Affected Systems:
 Microsoft IIS Servers.

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Attack Scenarios: 
An attacker might double encode the request to the web server, this may then evade an IDS monitoring traffic and could then launch a successful attack without being detected.

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Ease of Attack: 
Simple. Exploits exist.

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False Positives:
None Known.

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False Negatives:
None Known.

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Corrective Action:
Check the target host for signs of compromise.

Apply any appropriate vendor supplied patches.

Upgrade to the latest non-affected version of the software

Use Apache.

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Contributors:
Daniel Roelker <droelker@sourcefire.com> 
Sourcefire Vulnerability Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>

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Additional References:

HTTP IDS Evasions Revisited - Daniel Roelker
http://docs.idsresearch.org/http_ids_evasions.pdf

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