SSL pinning a signed cert on iOS

Question

You hear a lot about how you can use SSL pinning to increase the security in your app. I was always under the assumption that SSL pinning is only helpful if you use a self-signed cert. Are there benefits of SSL pinning when using a cert signed by a cert authority, or does the CA make that unnecessary?

Answer 1

SSL/TLS has some issues, and the root of those issues is the CA system. By default, your browser trusts a bunch of CAs. Other than a few basic checks such as hostname, etc, the real power of the SSL comes from the verification of the certificate via the intermediaries until you get to a certificate that you trust.

Certificate pinning specifies that you will only trust a given certificate for a given web site. That is to say, if you received a "valid" certificate signed by a different CA (even if you trusted that CA) you would not trust it for this site.

In short, the fact that the CA has signed a cert does not reduce the need for certificate pinning.

For example: Companies sometimes intercept SSL. To do this, they actually serve the client a different SSL certificate that is valid for the host that the client is trying to access. Because they can insert any certificate into the trusted root (windows via GPO) they can insert their certificate instead of the actual one issued by the host. In this scenario, the SSL interception is transparent to the user. They are still being issued a "valid" certificate that is signed by a CA they trust (whether they know it or not). If you had been using certificate pinning, the new cert would be rejected.

There are cons to certificate pinning, mostly around the management of the cert as it expires etc.

See this link for more info.