Malicious code is not always hidden in web page scripts or unusual file
formats. Attackers may corrupt types of files that you would recognize and
typically consider safe, so you should take precautions when opening files
from other people.

What types of files can attackers corrupt?

An attacker may be able to insert malicious code into any file, including
common file types that you would normally consider safe. These files may
include documents created with word processing software, spreadsheets, or
image  files. After corrupting the file, an attacker may distribute it
through email or post it to a web site. Depending on the type of malicious
code, you may infect your computer by just opening the file.

When corrupting files, attackers often take advantage of vulnerabilities
that they discover in the software that is used to create or open the file.
These vulnerabilities may allow attackers to insert and execute malicious
scripts  or  code,  and  they  are  not always detected. Sometimes the
vulnerability involves a combination of certain files (such as a particular
piece of software running on a particular operating system) or only affects
certain versions of a software program.

What problems can malicious files cause?

There are various types of malicious code, including viruses, worms, and
Trojan horses (see Why is Cyber Security a Problem? for more information).
However, the range of consequences varies even within these categories. The
malicious code may be designed to perform one or more functions, including
* interfering with your computer’s ability to process information by
consuming  memory  or  bandwidth  (causing your computer to become
significantly slower or even “freeze”)
* installing, altering, or deleting files on your computer
* giving the attacker access to your computer
* using  your  computer to attack other computers (see Understanding
Denial-of-Service Attacks for more information)

How can you protect yourself?

* Use and maintain anti-virus software – Anti-virus software can often
recognize and protect your computer against most known viruses, so you
may be able to detect and remove the virus before it can do any damage
(see Understanding Anti-Virus Software for more information). Because
attackers are continually writing new viruses, it is important to keep
your definitions up to date.
* Use caution with email attachments – Do not open email attachments that
you were not expecting, especially if they are from people you do not
know. If you decide to open an email attachment, scan it for viruses
first (see Using Caution with Email Attachments for more information).
Not only is it possible for attackers to “spoof” the source of an email
message,  but your legitimate contacts may unknowingly send you an
infected  file.  If  your  email  program  automatically downloads
attachments, check your settings to see if you can disable this feature.
* Be wary of downloadable files on web sites – Avoid downloading files
from sites that you do not trust. If you are getting the files from a
supposedly  secure  site,  look  for  a  web site certificate (see
Understanding Web Site Certificates for more information). If you do
download a file from a web site, consider saving it to your computer and
manually scanning it for viruses before opening it.
* Keep software up to date – Install software patches so that attackers
cannot  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
* Take advantage of security settings – Check the security settings of
your  email  client  and your web browser (see Evaluating Your Web
Browser’s Security Settings for more information). Apply the highest
level of security available that still gives you the functionality you

Related information

* Securing Your Web Browser
* Recovering from Viruses, Worms, and Trojan Horses

Author: Mindi McDowell

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