Re: [Full-Disclosure] MSBlast DDoS

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To: full-disclosure@lists.netsys.com
Date: Thu, 14 Aug 2003 13:43:22 +1000

On Thu, 2003-08-14 at 00:08, Benjamin M.A. Robson wrote:
> This is almost right. The packets should go straight to the firewall
> and out to the Internet, unless there is a device (such as the firewall
> itself) performing some sort of NAT redirect for the purposes of a
> transparent proxy. If this is the case then the packets will be sent
> via the proxy, and if they are not properly formed HTTP commands (GET,
> HEAD, etc..) the proxy server should reject them as bogus.

According to eEye, the DOS attack is a SYN flood. Thus, it wouldn't even
get to the stage where HTTP is spoken. In a transparent proxy setup, the
proxy server will suffer the SYN flood and nothing will get out.

In a non-transparent proxy setup the proxy likely wouldn't see the
packets (since msblast.exe isn't looking for proxies). Hence it depends
on whether the Internet firewall mandates HTTP connections to be made
through a proxy, or whether it just allows anybody to make outbound
HTTP connections. In the former case, the firewall will drop the SYN's
(unless the proxy itself is infected), in the latter case you'll SYN
flood windowsupdate.com.

In short, it is a good idea to allow outbound HTTP connections from
proxy servers only (and always has been). It is an even better idea not
to run the proxies on top of Windows.

Cheers
Steffen.

-- 
Steffen Kluge				Tel: +61 2 9887 9449
Security Specialist			Mobile: 0413 486 390
Fujitsu Australia Ltd			Facsimile: +61 2 9887 9458
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• Previous message: CHeeKY: "Re: [Full-Disclosure] Firewalls"
• In reply to: Benjamin M.A. Robson: "Re: [Full-Disclosure] MSBlast DDoS"
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