Wednesday, January 21, 2009

Understanding Hidden Threats: Corrupted Software Files

Cyber Security Tip ST06-006
Understanding Hidden Threats: Corrupted Software Files

Click here for a web version of the article with links to the other documents mentioned.

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. These
vulnerabilities may allow attackers to insert and execute malicious
scripts or code, sometimes without being 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
recognizes and protects 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, your legitimate contacts
may unknowingly send you an infected file.
* 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 desktop 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 it.
* 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 need. In email clients, turn off the option to
automatically download attachments.

Related information

* Securing Your Web Browser
* Recovering from Viruses, Worms, and Trojan Horses
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Author: Mindi McDowell
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Produced 2006 by US-CERT, a government organization.

Note: This tip was previously published and is being re-distributed
to increase awareness.

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