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National Cyber Alert System
Cyber Security Tip ST04-010
Using Caution with Email Attachments
While email attachments are a popular and convenient way to send documents,
they are also a common source of viruses. Use caution when opening
attachments, even if they appear to have been sent by someone you know.
Why can email attachments be dangerous?
Some of the characteristics that make email attachments convenient and
popular are also the ones that make them a common tool for attackers:
* Email is easily circulated - Forwarding email is so simple that viruses
can quickly infect many machines. Most viruses don't even require users
to forward the emailâ??they scan a users' computer for email addresses and
automatically send the infected message to all of the addresses they
find. Attackers take advantage of the reality that most users will
automatically trust and open any message that comes from someone they
know.
* Email programs try to address all users' needs - Almost any type of file
can be attached to an email message, so attackers have more freedom with
the types of viruses they can send.
* Email programs offer many "user-friendly" features - Some email programs
have the option to automatically download email attachments, which
immediately exposes your computer to any viruses within the attachments.
What steps can you take to protect yourself and others in your address book?
* Be wary of unsolicited attachments, even from people you know - Just
because an email message looks like it came from your mom, grandma, or
boss doesn't mean that it did. Many viruses can "spoof" the return
address, making it look like the message came from someone else. If you
can, check with the person who supposedly sent the message to make sure
it's legitimate before opening any attachments. This includes email
messages that appear to be from your ISP or software vendor and claim to
include patches or anti-virus software. ISPs and software vendors do not
send patches or software in email.
* Keep software up to date - Install software patches so that attackers
can't take advantage of known problems or vulnerabilities (see
Understanding Patches for more information). Many operating systems
offer automatic updates. If this option is available, you should enable
it.
* Trust your instincts - If an email or email attachment seems suspicious,
don't open it, even if your anti-virus software indicates that the
message is clean. Attackers are constantly releasing new viruses, and
the anti-virus software might not have the signature. At the very least,
contact the person who supposedly sent the message to make sure it's
legitimate before you open the attachment. However, especially in the
case of forwards, even messages sent by a legitimate sender might
contain a virus. If something about the email or the attachment makes
you uncomfortable, there may be a good reason. Don't let your curiosity
put your computer at risk.
* Save and scan any attachments before opening them - If you have to open
an attachment before you can verify the source, take the following
steps:
1. Be sure the signatures in your anti-virus software are up to date
(see Understanding Anti-Virus Software for more information).
2. Save the file to your computer or a disk.
3. Manually scan the file using your anti-virus software.
4. If the file is clean and doesn't seem suspicious, go ahead and open
it.
* Turn off the option to automatically download attachments - To simplify
the process of reading email, many email programs offer the feature to
automatically download attachments. Check your settings to see if your
software offers the option, and make sure to disable it.
* Consider creating separate accounts on your computer - Most operating
systems give you the option of creating multiple user accounts with
different privileges. Consider reading your email on an account with
restricted privileges. Some viruses need "administrator" privileges to
infect a computer.
* Apply additional security practices - You may be able to filter certain
types of attachments through your email software (see Reducing Spam) or
a firewall (see Understanding Firewalls).
_________________________________________________________________
Both the National Cyber Security Alliance and US-CERT have identified this
topic as one of the top tips for home users.
_________________________________________________________________
Authors: Mindi McDowell, Allen Householder
_________________________________________________________________
Copyright 2004, 2009 Carnegie Mellon University. Terms of use
US-CERT
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