Nigel, Removed isc.incidents.org reference since it is no longer active.
Rule:

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Sid:
1889

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Summary:
This event is generated when a web server infected by the slapper worm attempts to send traffic via a communication channel. 

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Impact:
Remote access and potentially denial of service.  A slapper worm infection indicates a successful compromise of the host.  A communication channel established between infected hosts can be used as a vehicle for a distributed denial of service attack of a target host or network.

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Detailed Information:
The Apache/mod_ssl worm, also known as slapper, exploits a vulnerability associated with certain versions of OpenSSL.  Once a host has been infected by the worm, the worm then attempts to establish a communication channel using UDP port 2002 (both source and destination) to the infecting host.  This communication channel is used to create a network for infected hosts to communicate with each other to identify other infected hosts and to deliver attack instructions for other sites.

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Affected Systems:
Linux hosts running Apache with mod_ssl using SSLv2-enabled OpenSSL 0.9.6d or earlier on Intel x86 architectures.

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Attack Scenarios:
The communication channel created by the slapper worm allows infected hosts to receive direction from other infected hosts.  This can be used, for instance, to coordinate a DDoS attack.

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Ease of Attack:
Simple.  Exploit code exists. 

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False Positives:
None Known.

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False Negatives:
It has been observed that the port number for the communication channel may vary.  Ports 1978 and 4156 have also been seen.

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Corrective Action:
Apply the appropriate patch or upgrade to the most current version of OpenSSL.

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Contributors:
Original rule writer unknown.
Sourcefire Research Team
Judy Novak <judy.novak@sourcefire.com>

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Additional References:

CERT
http://www.cert.org/advisories/CA-2002-27.html

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