Rule: 

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Sid: 
111-3

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Summary: 
This event is generated when the pre-processor stream4
detects network traffic that may constitute an attack.

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Impact: 
Unknown. This may be an IDS evasion attempt.

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Detailed Information:
The pre-processor stream4 has detected a TCP session that contains
a retransmission of data that has not already been acknowleged. This
may be an attempt to evade any monitoring IDS.

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Affected Systems:
	All systems

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Attack Scenarios: 
An attacker could supply two packets containing different data, one with
a malicious payload destined for a vulnerable host and the other with a
benign payload meant for the IDS. The second packet may disguise itself
in the session as retransmitted data.

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Ease of Attack: 
Simple. Tools such as fragroute contain this functionality.

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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.

Ensure the system is up to date with any appropriate vendor supplied patches.

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Contributors:
Martin Roesch <roesch@sourcefire.com>
Sourcefire Vulnerability Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>

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Additional References:

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