Cyber Security Tip ST05-010
Understanding Web Site Certificates
You may have been exposed to web site, or host, certificates if you
have ever clicked on the padlock in your browser or, when visiting a
web site, have been presented with a dialog box claiming that there is
an error with the name or date on the certificate. Understanding what
these certificates are may help you protect your privacy.
What are web site certificates?
If an organization wants to have a secure web site that uses
encryption, it needs to obtain a site, or host, certificate. Some
steps you can take to help determine if a site uses encryption are to
look for a closed padlock in the status bar at the bottom of your
browser window and to look for "https:" rather than "http:" in the URL
(see Protecting Your Privacy for more information). By making sure a
web site encrypts your information and has a valid certificate, you
can help protect yourself against attackers who create malicious sites
to gather your information. You want to make sure you know where your
information is going before you submit anything (see Avoiding Social
Engineering and Phishing Attacks for more information).
If a web site has a valid certificate, it means that a certificate
authority has taken steps to verify that the web address actually
belongs to that organization. When you type a URL or follow a link to
a secure web site, your browser will check the certificate for the
1. the web site address matches the address on the certificate
2. the certificate is signed by a certificate authority that the
browser recognizes as a "trusted" authority
Can you trust a certificate?
The level of trust you put in a certificate is connected to how much
you trust the organization and the certificate authority. If the web
address matches the address on the certificate, the certificate is
signed by a trusted certificate authority, and the date is valid, you
can be more confident that the site you want to visit is actually the
site that you are visiting. However, unless you personally verify that
certificate's unique fingerprint by calling the organization directly,
there is no way to be absolutely sure.
When you trust a certificate, you are essentially trusting the
certificate authority to verify the organization's identity for you.
However, it is important to realize that certificate authorities vary
in how strict they are about validating all of the information in the
requests and about making sure that their data is secure. By default,
your browser contains a list of more than 100 trusted certificate
authorities. That means that, by extension, you are trusting all of
those certificate authorities to properly verify and validate the
information. Before submitting any personal information, you may want
to look at the certificate.
How do you check a certificate?
There are two ways to verify a web site's certificate in Internet
Explorer or Mozilla. One option is to click on the padlock in the
status bar of your browser window. However, your browser may not
display the status bar by default. Also, attackers may be able to
create malicious web sites that fake a padlock icon and display a
false dialog window if you click that icon. A more secure way to find
information about the certificate is to look for the certificate
feature in the menu options. This information may be under the file
properties or the security option within the page information. You
will get a dialog box with information about the certificate,
including the following:
* who issued the certificate - You should make sure that the issuer
is a legitimate, trusted certificate authority (you may see names
like VeriSign, thawte, or Entrust). Some organizations also have
their own certificate authorities that they use to issue
certificates to internal sites such as intranets.
* who the certificate is issued to - The certificate should be
issued to the organization who owns the web site. Do not trust the
certificate if the name on the certificate does not match the name
of the organization or person you expect.
* expiration date - Most certificates are issued for one or two
years. One exception is the certificate for the certificate
authority itself, which, because of the amount of involvement
necessary to distribute the information to all of the
organizations who hold its certificates, may be ten years. Be wary
of organizations with certificates that are valid for longer than
two years or with certificates that have expired.
When visiting a web site, you may have been presented with a dialog
box that claims that there is an error with the site certificate. This
may happen if the name the certificate is registered to does not match
the site name, you have chosen not to trust the company who issued the
certificate, or the certificate has expired. You will usually be
presented with the option to examine the certificate, after which you
can accept the certificate forever, accept it only for that particular
visit, or choose not to accept it. The confusion is sometimes easy to
resolve (perhaps the certificate was issued to a particular department
within the organization rather than the name on file). If you are
unsure whether the certificate is valid or question the security of
the site, do not submit personal information. Even if the information
first so that you know what is being done with that information (see
Protecting Your Privacy for more information).
Authors: Mindi McDowell, Matt Lytle
Produced 2005 by US-CERT, a government organization.
Note: This tip was previously published and is being re-distributed
to increase awareness.
This document can also be found at
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