Anatomy of a Certificate Error

The most important step in diagnosing a specific security error involves determining what the error is telling you. There are a few things that can cause certificate errors, and what you do depends entirely on what is causing the error to begin with. Once you know what the error is telling you, it becomes much easier to figure out what you need to do next.

Getting the Message

One of the more concise and effective Certificate Errors is the one delivered by Outlook. An image of it is below.



Note the numbers 1, 2, and 3. These don’t normally show up on the error because I put them there for reference. In case you were looking at your own error. At any rate, the numbers are sitting next to three possible kinds of errors you can get with a certificate.

For this particular error, you’ll note that there is a red X next to number 3. That X points out that one of the Validity checks run against the certificate failed. Specifically, the name I used to access the server doesn’t match either the Common Name on the certificate or any of the Subject Alternate Names. This is probably the most common certificate error you’ll see.

The four Checks

Every time you access a website that is secured with SSL, there are four checks the computer you use runs to verify that the certificate is valid. The reason for these checks is explained in my article on Digital Certificates. The four checks are as follows, and match the numbering in the image above.

  1. Was the Certificate issued by a known and trusted Certificate Authority?
  2. Is the current date within the period of time the Certificate is valid?
  3. Does the host name used to access the server match any of the host names defined by the certificate?
  4. Has the Certificate been Revoked? (Wait, there’s no 4 on the image! Don’t worry, I’ll explain later.)

If any of these checks fails, you’ll get a certificate error. Note that this *does not mean* that the data you’re trying to encrypt isn’t going to be encrypted. Any time you use SSL or TLS, you’re data will be encrypted whether the certificate is valid or not. However, if any of the checks fail it is much more likely that someone could decrypt the data you encrypt. Here’s why, based on each of the possible certificate errors.

Was the Certificate issued by a known and trusted Certificate Authority?

Certificate Authorities are servers that are designed specifically to generate digital certificates. Anyone on the planet can create a Certificate Authority server if they want to (and know how to). If you have your own Certificate Authority, you can create a certificate that matches any Common Name you want and use that certificate to interject yourself into any secure transmission and read the data without anyone knowing, but only if the client computer *trusts* your Certificate Authority.

Normally, most computer Operating Systems and Web Browsers have a list of CAs that are trusted right out of the box. These include CAs owned by companies like Godaddy, Entrust, and Network Solutions. So unless you happen to gain control of the Certificate Authority owned by these companies and defined by the Root CA Certificate installed on the OSes of every computer in the world, your CA is probably not going to be trusted without a lot of extra work.

If you see a certificate error that warns the Certificate Authority isn’t trusted, it means the Certificate was issues by a *private* CA. You can instruct your computer to trust the CA if you want, but if you are using a site that normally has no certificate error and this error suddenly shows up one day, there’s a good chance your data is being intercepted and redirected.

As an IT Professional, if you see this error when accessing a system under your control, there are two solutions.

  1. Request a new certificate from a trusted, Third Party Root CA provider.
  2. Install the Root CA certificate as a Trusted Third Party Root CA in the OS.

#1 requires significantly less effort to accomplish because it means you don’t have to actually install the certificate on your users’ computers, phones, or other devices.

Is the current date within the period of time the Certificate is valid?

Certificates are only valid for a set period of time. Most certificates are valid from 1 – 3 years from the time they are first generated, depending on options used during certificate generation. Certificate Validity periods are meant to ensure that only a limited amount of time is available for a certificate’s Private Key to be discovered.

The possibility of a brute force attack successfully discoverying the Private Key in use is astronomically small, and the time to run a full brute force attack against modern Certificates is in the million year period. But as we progress technologically, the time required reduces exponentially. If a certificate was generated in, say, 1991 using the DES algorithm, it would have taken thousands of years to crack it with normal computing resources. Today, it would take less than an hour.

Having a certificate validity period ensures that technology doesn’t outpace the security of the certificate. Having a validity period between 1 and 3 years is the general recommendation for certificates these days. If you run across a certificate that has an expiration date that is more than 2-3 years in the past, I highly recommend not using the site that uses that Certificate.

If a server you control has this error, you need to generate and install a new certificate on the server. This is the only possible solution to this error.

Does the host name used to access the server match any of the host names defined by the certificate?

This error is always caused by attempts to access a server using a URL that uses a host name not included on the certificate. For instance, let’s say a web server has a certificate that defines the host name as If you attempt to reach that server using, you’ll get a certificate error.

This check is meant to ensure that the server we are communicating with is the one we *want* to communicate with. If the server we want to talk to is using a valid third-party certificate, we can be significantly more certain that the server we’re talking to is the one we want to talk to and that no one is attempting to spy on the data we send if this check comes back okay. If not, it’s important to check the information listed on the certificate to verify that we’re talking to the right server.

For IT Professionals, there are two definite solutions for this error.

  1. Generate a certificate that matches the Host Name you want people to use to access your server. If there is a need for multiple names, get a SAN cert that includes all host names or a Wildcard cert that is valid for any host name at a specific domain (Wilcard certificates are generated with a common name of * and will be considered valid for any value that you want to replace the *. This is slightly less secure since it can be used on any number of servers, but the security difference is minimal. Be certain to verify that the web server you are using fully supports wildcard certificates before obtaining one. IIS supports them, as do the vast majority of Microsoft solutions, though some may require additional setup.)
  2. Create a DNS record for a host name that matches the certificate and point it to the web server.
  3. Note: There are some applications that use HTTPS that may have specific host name requirements, and may require multiple host names to function properly (Exchange Autodiscover for example). Be aware that this type of certificate error will always occur unless you have a certificate that matches *all* the necessary host names or have made sufficient configuration changes to allow things to work properly with a single host name.

Has the Certificate been Revoked?

This is actually a very unusual error that you will not see often. I don’t have a picture of one of these to show you, since it takes a good bit of effort to force the error to occur. Certificate Revocation is not particularly common, and was developed to combat the possibility of a certificate being compromised. A certificate is considered compromised when an unauthorized entity obtains a copy of the certificate’s Private Key.

If this happens, or if the certificate is reissued for any reason (For instance, if you want to change the common name, modify the list of Subject Alternate Names, or make any other changes), the certificate is listed in a Certificate Revocation List (CRL) that is published by the server that originally generated the certificate. A CRL is just a simple web page that a web browser or other application that checks certificate validity will go to and check to determine if the certificate is still valid. If the certificate is listed in the CRL, many applications are designed to absolutely refuse further communication with the server using that certificate (Web browsers specifically). Servers using revoked certificates are always considered to be compromised, and it is always a good idea to avoid using servers with revoked certificates. Basically, if you see this error, *DO NOT CONTINUE!*.

Brown’s Anatomy

So that’s it for Certificate errors. The 4 checks are designed primarily to keep your data safe, so make sure you are aware of what you’re walking into when you see these errors. As a regular joe, non-IT person, you’re pretty likely to run into these errors, and knowing what they mean will help you determine if it’s a good idea to keep going or not. For IT people, you are going to see these errors a lot, no matter what, and knowing what they mean will help you fix them.


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