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To Allow or Not Allow E-Mail Attachments?
t0pper311 asks: "I work for a pretty large utility company in the midwest and of course, security is a big concern. We use Trend Micro as a mail gateway to basically scan for virii and strip off most attachments like executables or VB script. Now with the Sobig.E virus on the loose, we need to ask ourselves if we should be blocking ZIP files. We got lucky this time and were not effected, but what about next time? What are other companies doing? If you do block ZIP files, how do you give the people who need to sends files the ability to do so? Do you allow any attachments at all?"
Pretty simple really.
Given that most users love to download crap via hotmail etc. , lets hope you have a virus scanner on their PC too.
From what I can gather from the virus information, it's not an Outlook virus. It's a Windows virus that propogates through its own SMTP routines, harvesting email addresses from a variety of local files. In Outlook it requires the user to extract the executable and run it, just like any other mail client.
10 years ago, on BBS (bulletin board system), every time someone uploaded something, the system automatically unpacked the { zip | rar | arj } on a temp directory. Then the content of the archive were automatically checked for virii with *MANY* anti-virus like MacAfee, FProt and MSAV (if the BBS were DOS-based). If the archive passed the test, it was made available to download by other user. Then, the temp directory was cleaned.
Actually, the virus he talks about only works through social engineering. You have to manually open the zip file and click the .exe file.
The chroot command should help.
Here's what I did with postfix.
/^(Content-(Type|Disposition):.*|\s*(file)?)name=( "[^"]*|\S*)\.(ade|adp|bas|shm|cmd|com|dll|hlp|js|j se|exe|com|chm|hta|jse|reg|shb|shs|vbe|vbs|vxd|scr |pif|bat|lnk|dll|vbs|js|mp*)\b/ REJECT
/^(Content-(Type|Disposition):.*|\s*(file)?)name=( \S*your_details)\.(zip)\b/ REJECT
in my main.cf created a line that says
body_checks = pcre:/etc/postfix/extensions
then created a file called extensions that looks like this:
The first line (yes it's all one line) blocks all executable files from entering the server. The second line block the only version of sobig that we received. Actually we received 2 modifications...one attachment was called your_details.zip, and the other was 'your_details.zip the ' allowed it to get around the filter, hence the wildcard.
The key is, to inform your users over and voer not to open things from people they don't know or aren't expecting. If you start blocking zip's you might as well block all attachments.
/* oops I accidentally made a comment, sorry */
Sobig.E came out before the virus scanners had signature updates. When viruses spread so fast these days about all you can do is push your email through MessageLabs who have never let a virus through to a customer due to their custom AV scanner which uses heuristics instead of signatures.
Your point about not relying on any one point of access is well taken though - all entry points need to be protected in one way or another.
Matt. Want XML + Apache + Stylesheets? Get AxKit.
Here's what we do:
1. Use Symantec Antivirus for SMTP Gateways 3.1. Blocks spam by subject, sender, multiple RBLs and heuristic antispam and has whitelist support. Scans for viruses and attachments can be deleted by filename with wildcards. I block anything that is executable in a Windows environment (since we use Windows, Exchange and Outlook -- no flames, please). Any file deleted gets a .txt file added to the message stating that <filename> is not allowed for security reasons. This means that if anyone needs to send a .exe, .cmd, .bat, .vb?, .cpl, .dll, and a number of others must first call so I can temporarily disable the deletion.
2. Use another company's antivirus on the mail servers. We use Sybari with multiple scan engines. This saved us this past week when the new FortNight.E managed to get past SAV for SMTP because it didn't detect it yet and it was essentially a script embedded in an html. (I'd love to strip them, too, but too many legitimate emails come through as html.) Sybari caught it after Symantec missed it.
3. Use another antivirus package for clients and servers. We use SAV Corporate edition with a master server setup so that one server d/l's updates from Symantec nightly or when I force it. Each remote location's server d/l's from that server nightly or when I force it. Each workstation d/l's from their location's server every 4 hours.
Since starting this practice, we've had a total of 2 viruses make it into our network. One was on a laptop that, for some strange reason never got antivirus installed and it was infected at the user's home. The virus never got further than that, but it took a while to discover where the virus alerts were coming from when it attempted to infect other machines. The other one had a corrupt install of the desktop antivirus and the end user didn't let us know that something didn't look right on his client. He then fell for the e-card virus (Go to this URL to download the greeting card X sent you.). Again, never got further than this one workstation. This is all the infections we've seen in over 2 years. Not bad for a 1500 user network.
But why is the rum gone?