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Question about xls trigger
From: SnortFan <SnortFan () yahoo com>
Date: Fri, 28 Mar 2014 12:44:35 -0400

Hi All,

I'm seeing a lot of false positives with Excel files and I think the problem has to do with the way flowbits sets .xls 
files. Both SID 15463 and 19166 set 'file.xls', however it seems that 15463 is unnecessary considering 19166. Under 
what circumstances would 15463 be effective while 19166 fails? Are there any reasons to keep both rules active rather 
than suppressing 15463?

SID 15463
tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"FILE-IDENTIFY Microsoft Office Excel file download request"; 
flow:to_server,established; content:".xls"; fast_pattern:only; http_uri; pcre:"/\x2exls([\?\x5c\x2f]|$)/smiU"; 
flowbits:set,file.xls; flowbits:noalert; metadata:service http; reference:url,en.wikipedia.org/wiki/.xlsFile_formats; 
classtype:misc-activity; sid:15463; rev:16;)

SID 19166
tcp $EXTERNAL_NET $FILE_DATA_PORTS -> $HOME_NET any (msg:"FILE-IDENTIFY Microsoft Office Excel file magic detected"; 
flow:to_client,established; file_data; content:"|D0 CF 11 E0|"; depth:4; 
content:"W|00|o|00|r|00|k|00|b|00|o|00|o|00|k|00|"; fast_pattern:only; flowbits:set,file.xls; flowbits:noalert; 
metadata:service ftp-data, service http, service imap, service pop3; classtype:misc-activity; sid:19166; rev:13;)

I'm using ips_policy=security in my pulledpork.  

Thanks,
Ed

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