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 email—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).

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Both the National Cyber Security Alliance and US-CERT have identified
this topic as one of the top tips for home users.

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Authors: Mindi McDowell, Allen Householder

Questions or comments? Leave us a comment below

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Copyright 2004, 2009 Carnegie Mellon University.

Original Post by US-CERT