[Bro] Bro 2.2 File Extraction (RHEL 6.5)

Jonathon Wright jonathon.s.wright at gmail.com
Thu Aug 7 17:16:17 PDT 2014

This is awesome information, I've been busy "playing" with my extract.bro
and scripts to see what kind of process I can come up with. Thanks again
for all the assistance!!

On Wed, Aug 6, 2014 at 6:14 PM, Seth Hall <seth at icir.org> wrote:

> On Aug 6, 2014, at 10:27 PM, Jonathon Wright <jonathon.s.wright at gmail.com>
> wrote:
> > The problem is, I won't know what file/md5_value to compare it too since
> I wont know the original filename. Hope that makes sense.
> If you're running Bro with broctl, you will already have hashes (md5 and
> sha1) for every file transferred in your files.log.
> > For example, if a user downloads something.exe (via http), bro will
> create a HTTP-blahblah file name. My problem at that point, is how do I
> know what the user tried to download, was it "notepad.exe" or
> "maliciousIntent.exe"? I will only have a directory full of HTTP-blahblah
> names, correct? That was where I was trying to go. Perhaps I misunderstood
> your response and you already answered me?
> # Look at the extracted files.
> $ ls ./extract_files
>         extract-HTTP-FsRNbD323oiMhWA761
> # Look at the line in files.log that maps to that file.
> $ grep extract-HTTP-FsRNbD323oiMhWA761 files.log
> 1407384770.727269       FsRNbD323oiMhWA761
> CuTpVT1LB2eQP0eMP4      HTTP    0       EXTRACT application/x-dosexec   -
>     1.151308        49152   49152   0       0       F       -       -
> -       -       extract-HTTP-FsRNbD323oiMhWA761
> # Look for the HTTP request that maps to that file.
> $ grep FsRNbD323oiMhWA761 http.log
> 1407384770.568614       CuTpVT1LB2eQP0eMP4 1066 80
>      1       GET /lprx.php       -       -       0
> 49152200        OK      -       -       -       (empty) -       -       -
>     -       -       FsRNbD323oiMhWA761      application/x-dosexec
> You can see in that example that the best file name we could have possibly
> hoped to extract for that connection would be "lprx.php" which I don't
> think is what you want.  That is real traffic (with modified field data)
> from a compromised host downloading an update to the malware installed on
> it.
> > If so, apolgies, but I still seem to be missing the connection of the
> bro created file name when its carved and the actual filename of the exe
> that the user attempted to download.
> Ah, ok.  I can explain a bit more here.  Before arriving at the current
> model, I spent a lot of time thinking about how to flexibly name files.
>  What I realized is that I don't want aspects of the network traffic to be
> able to affect the name of the file being written to disk (by default at
> least, you can do whatever you want in your own scripts).  There could be
> maliciously named files or attempts to play with the path to write into
> sensitive areas of the file system.  By giving the files being written to
> disk names that were totally fabricated by the Bro process we sidestep any
> of these potential issues.  You can use the name of the extracted file to
> then pivot back into the logs.
>   .Seth
> --
> Seth Hall
> International Computer Science Institute
> (Bro) because everyone has a network
> http://www.bro.org/
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