Before selling or discarding an old computer, or throwing away a disk or CD, you naturally make sure that you’ve copied all of the files you need. You’ve probably also attempted to delete your personal files so that other people aren’t able to access them. However, unless you have taken the proper steps to make sure the hard drive, disk, or CD is erased, people may still be able to resurrect those files.

Where do deleted files go?

When you delete a file, depending on your operating system and your settings, it may be transferred to your trash or recycle bin. This “holding area” essentially protects you from yourself—if you accidentally delete a file, you can easily restore it. However, you may have experienced the panic that results from emptying the trash bin prematurely or having a file seem to disappear on its own. The good news is that even though it may be difficult to locate, the file is probably still somewhere on your machine. The bad news is that even though you think you’ve deleted a file, an attacker or other unauthorized person may be able to retrieve it.

What are the risks?

Think of the information you have saved on your computer. Is there banking or credit card account information? Tax returns? Passwords? Medical or other personal data? Personal photos? Sensitive corporate information? How much would someone be able to find out about you or your company by looking through your computer files?

Depending on what kind of information an attacker can find, he or she may be able to use it maliciously. You may become a victim of identity theft. Another possibility is that the information could be used in a social engineering attack. Attackers may use information they find about you or an organization you’re affiliated with to appear to be legitimate and gain access to sensitive data (see Avoiding Social Engineering and Phishing Attacks for more information).

Can you erase files by reformatting?

Reformatting your hard drive or CD may superficially delete the files, but the information is still buried somewhere. Unless those areas of the disk are effectively overwritten with new content, it is still possible that knowledgeable attackers may be able to access the information.
How can you be sure that your information is completely erased?

Some people use extreme measures to make sure their information is destroyed, but these measures can be dangerous and may not be completely successful. Your best option is to investigate software programs and hardware devices that claim to erase your hard drive or CD. Even so, these programs and devices have varying levels of effectiveness. When choosing a software program to perform this task, look for the following characteristics:

• Data is written multiple times - It is important to make sure that not only is the information erased, but new data is written over it. By adding multiple layers of data, the program makes it difficult for an attacker to “peel away” the new layer. Three to seven passes is fairly standard and should be sufficient.
• Use of random data - Using random data instead of easily identifiable patterns makes it harder for attackers to determine the pattern and discover the original information underneath.
• Use of zeros in the final layer - Regardless of how many times the program overwrites the data, look for programs that use all zeros in the last layer. This adds an additional level of security.

While many of these programs assume that you want to erase an entire disk, there are programs that give you the option to erase and overwrite individual files.

An effective way to ruin a CD or DVD is to wrap it in a paper towel and shatter it. However, there are also hardware devices that erase CDs or DVDs by destroying their surface. Some of these devices actually shred the media itself, while others puncture the writable surface with a pattern of holes. If you decide to use one of these devices, compare the various features and prices to determine which option best suits your needs.

You may think that you are anonymous as you browse web sites, but pieces of information about you are always left behind. You can reduce the amount of information revealed about you by visiting legitimate sites, checking privacy policies, and minimizing the amount of personal information you provide.

What information is collected?

When you visit a web site, a certain amount of information is automatically sent to the site. This information may include the following:

• IP address - Each computer on the internet is assigned a specific, unique IP (internet protocol) address. Your computer may have a static IP address or a dynamic IP address. If you have a static IP address, it never changes. However, some ISPs own a block of addresses and assign an open one each time you connect to the internet—this is a dynamic IP address. You can determine your computer’s IP address at any given time by visiting www.showmyip.com
• Domain name - The internet is divided into domains, and every user’s account is associated with one of those domains. You can identify the domain by looking at the end of URL; for example, .edu indicates an educational institution, .gov indicates a US government agency, .org refers to organization, and .com is for commercial use. Many countries also have specific domain names. The list of active domain names is available from the Internet Assigned Numbers Authority (IANA).
• Software details - It may be possible for an organization to determine which browser, including the version, that you used to access its site. The organization may also be able to determine what operating system your computer is running.
• Page visits - Information about which pages you visited, how long you stayed on a given page, and whether you came to the site from a search engine is often available to the organization operating the web site.

If a web site uses cookies, the organization may be able to collect even more information, such as your browsing patterns, which include other sites you’ve visited. If the site you’re vising is malicious, files on your computer, as well as passwords stored in the temporary memory, may be at risk.
How is this information used?

Generally, organizations use the information that is gathered automatically for legitimate purposes, such as generating statistics about their sites. By analyzing the statistics, the organizations can better understand the popularity of the site and which areas of content are being accessed the most. They may be able to use this information to modify the site to better support the behavior of the people visiting it.

Another way to apply information gathered about users is marketing. If the site uses cookies to determine other sites or pages you have visited, it may use this information to advertise certain products. The products may be on the same site or may be offered by partner sites.

However, some sites may collect your information for malicious purposes. If attackers are able to access files, passwords, or personal information on your computer, they may be able to use this data to their advantage. The attackers may be able to steal your identity, using and abusing your personal information for financial gain. A common practice is for attackers to use this type of information once or twice, then sell or trade it to other people. The attackers profit from the sale or trade, and increasing the number of transactions makes it more difficult to trace any activity back to them. The attackers may also alter the security settings on your computer so that they can access and use your computer for other malicious activity.
Are you exposing any other personal information?

While using cookies may be one method for gathering information, the easiest way for attackers to get access to personal information is to ask for it. By representing a malicious site as a legitimate one, attackers may be able to convince you to give them your address, credit card information, social security number, or other personal data (see Avoiding Social Engineering and Phishing Attacks for more information).
How can you limit the amount of information collected about you?

• Be careful supplying personal information - Unless you trust a site, don’t give your address, password, or credit card information. Look for indications that the site uses SSL to encrypt your information (see Protecting Your Privacy for more information). Although some sites require you to supply your social security number (e.g., sites associated with financial transactions such as loans or credit cards), be especially wary of providing this information online.
• Limit cookies - If an attacker can access your computer, he or she may be able to find personal data stored in cookies. You may not realize the extent of the information stored on your computer until it is too late. However, you can limit the use of cookies (see Browsing Safely: Understanding Active Content and Cookies for more information).
• Browse safely - Be careful which web sites you visit; if it seems suspicious, leave the site. Also make sure to take precautions by increasing your security settings (see Evaluating Your Web Browser’s Security Settings for more information), keeping your virus definitions up to date (see Understanding Anti-Virus Software for more information), and scanning your computer for spyware (see Recognizing and Avoiding Spyware for more information).