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Network Security Snort-Signatures
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[Snort-sigs] False positive/Rule change for SID 1893?

Subject: [Snort-sigs] False positive/Rule change for SID 1893?
Date: Thu, 9 Nov 2006 17:24:00 -0500
Greetings,

I am running a 4 sensor snort site (versions 2.6 and 2.4), and believe that
there is an error in the
snmp rule for SID 1893 (SNMP missing community string).

I've been getting alerts, that upon examination look like false positives.
Here is an example:

*Sample Data:*

length = 90

000 : 30 58 02 01 00 04 07 68 6F 73 74 69 6E 67 A0 4A   0X.....hosting.J

010 : 02 *04 00* E0 7A 20 02 01 00 02 01 00 30 3C 30 0E   ....z ......0<0.

020 : 06 0A 2B 06 01 02 01 02 02 01 0A 02 05 00 30 0E   ..+...........0.

030 : 06 0A 2B 06 01 02 01 02 02 01 10 02 05 00 30 0C   ..+...........0.

040 : 06 08 2B 06 01 02 01 01 03 00 05 00 30 0C 06 08   ..+.........0...

050 : 2B 06 01 02 01 01 05 00 05 00                     +.........


There is a valid community string within the proper place in the SNMP msg header, but snort thinks otherwise...


From the snort rules:

alert udp $EXTERNAL_NET any -> $HOME_NET 161 (msg:"SNMP missing community string attempt"; content:"|04 00|"; depth:15; offset:5; reference:bugtraq,2112; reference:cve,1999-0517; classtype:misc-attack; sid:1893; rev:4;)


My thought is that the "Depth:15" should be "Depth:6" instead? I have implemented this rule change on 1 of my sensors, and am waiting to see if that fixes it.


Thank you for your help or advice, David Glenn
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