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Re: Question about xls trigger
From: SnortFan <SnortFan () yahoo com>
Date: Fri, 28 Mar 2014 13:39:15 -0400

If the file magic rule will detect the actual file, why is the string match rule needed? 

Thanks,
Ed

Sent from a mobile device. 

On Mar 28, 2014, at 1:24 PM, "Joel Esler (jesler)" <jesler () cisco com> wrote:

The first one will set it based off of the extension, however, if the extension is changed, the file magic rule will 
detect the actual file and set the Flowbit.  


--
Joel Esler
Sent from my iPhone

On Mar 28, 2014, at 12:46, "SnortFan" <SnortFan () yahoo com> wrote:

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

Sent from a mobile device. 
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