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Sophos advises how to combat Blaster

  • 12 August, 2003 17:03

<p>Some generic advice on how to combat Blaster – Paul Ducklin, Head of Technology, Asia Pacific, Sophos Anti-Virus.</p>
<p>1) Update Windows to patch the vulnerability exploited by W32/Blaster-A. The worm uses RPC ("remote procedure call"), which is supposed to allow remote processes to call well-defined services on your computer. But the worm exploits an RPC bug which allows it to take total control of your PC. This is important, because this bug leaves you vulnerable to more than just virus attack. See Microsoft's website for further info.</p>
<p>2) Block port 135 at your firewall. It is extremely unlikely that you need to allow users on the internet to make RPC calls into your network. If you really do need to do this, consider changing your IT infrastructure so you don't, and then block the port!</p>
<p>3) As a Blaster-specific fix, delete or rename the file TFTP.EXE (in the SYSTEM32 directory). You probably don't need to run TFTP servers for users to access (see 4), as they are notoriously insecure, so you are unlikely to need a TFTP client on your workstations. Since this particular virus relies on being able to run TFTP.EXE in order to propagate, a computer without this file can't download the virus onto itself.</p>
<p>4) As a general safety feature, but also as a Blaster-specific fix, block port 69 (TFTP) wherever you can -- particularly on workstations. TFTP servers are rarely used for anything but network booting, and are notorious for security problems because TFTP does not provide any authentication mechanism. So port 69 should almost never be open. Since Blaster relies on listening on port 69 in order to receive connections from the TFTP.EXE program on PCs it is trying to infect, any PC with port 69 blocked can't spread the virus (though it can get infected, but see 3).</p>
<p>5) Block port 69 in and out at your firewall (see 4). Blocking outbound prevents you embarrassing yourself by attacking other people's networks via TFTP.</p>
<p>6) In fact, if you have ports 135 and 69 open at your firewall, you probably have an overly-liberal regimen. Remember: at the packet level, you should block *everything*. Then, and only then, should you open up ports which you know you need.</p>
<p>7) Whilst you're about it, ensure that ports 137, 139 and 445 (used by Windows file sharing) are blocked inbound from the internet. These are exploited by a range of other viruses. If you think you need to allow just anyone on the internet access to files on your network, then you should probably think again (try thinking about a VPN). Likewise for the ports used by Microsoft SQL Server (1433 and 1434). You don't intend your company databases to be queried directly off the internet, do you?</p>
<p>8) Yes, you should update your anti-virus. But note that W32/Blaster-A doesn't spread by email so an absence of detections at your mail gateway does not mean you can give yourself the "all clear".</p>
Paul Ducklin ( is available for comment:
+61 407 320 515 (mobile)
+61 2 9409 9100 (tel)
+61 2 9409 9191 (fax)</p>
<p>Sophos's press contact at Gotley Nix Evans is:
Michael Henderson (
+61 2 9957 5555 (tel)
+61 413 054 738 (mobile)
+61 2 9957 5575 (fax)</p>

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