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Re: blocking Google Desktop
From: Michael Holstein <michael.holstein () csuohio edu>
Date: Mon, 13 Feb 2006 12:34:19 -0500

First, I made a mistake in the version number. The current/new one is version 3 (the one that uploads your data to Google)

I've been experimenting with Snort sigs to detect this.

Google Desktop uses a unique user-agent (I got a tip about this from another user at full-disclosure -- thanks Charles!) :

User-Agent: Mozilla/4.0 (compatible; Google Desktop)

So here is a snort sig for that ...

alert tcp $HOME_NET any -> $EXTERNAL_NET 80 (msg: "BLEEDING-EDGE Google Desktop User-Agent Detected"; flow: to_server,established; content:"User-Agent\: Mozilla/4.0 (compatible\; Google Desktop)"; nocase; classtype: policy-violation; sid: 3000001; rev:1; )

That sig would at least let you know who's using it, but blocking that traffic wouldn't do anything except prevent the RSS feeds (news, weather, etc) from loading.

Now, for the file-specific stuff, since that's all done over SSL to google.com :

Upon examining the SSL/TLS session setup, I wrote this one to flag the certificate Google is using (from Thwarte). This will probably change when they change/renew their certificates.

alert tcp $EXTERNAL_NET 443 -> $HOME_NET 1024:65535 (msg: "BLEEDING-EDGE Google SSL key exchange"; flow: from_server,established; content:"|30 36 30 36 30 37 32 32 31 32 35 34 5A 30 68 31|"; rawbytes; content:"|77 77 77 2E 67 6F 6F 67 6C 65 2E 63 6F 6D|"; rawbytes; classtype:policy-violation; sid: 3000002; rev:1; )

Note that this also flags logons to gmail.

The fetches with the "Google Desktop" user-agent happen first when the application is started -- then you get the SSL setup for any new data to be uploaded to Google's servers.

Unfortunately, the dynamic/activate stuff in snort dosen't let you do an "alert" action after an activate -- because it's designed to just dump the next (n) packets. If there was a good way to chain the two rules together -- to say "after seeing 1, do REACT on #2" you could reliably kill any SSL/TLS sessions from somebody running Google Desktop, thus preventing the upload of anything.


Michael Holstein CISSP GCIA
Cleveland State University
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