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Certificate Pinning in mobile apps

What is certificate pinning:
Certificate pinning is an countermeasure to any attempt of MITM (Man in the middle) attacks where an attacker tries to inject his own certificate to sniff the traffic between the client and the server.

Why cert pinning is required:
The question here is why it's required for mobile apps, why not for browsers? The reason is mobile apps are very obscure and very difficult to get about what's going behind the scene. The end user does not get any warning from the app about any possible malicious activities going on. However in case of a browsers on desktops the user is presented with various warnings about TLS such as domain name not matching etc. This alerts the user to sense that something is not right there. And he has option to accept/ reject it , while in mobile apps it's not the case.

How to do it:
Generally it's done on code level, where the developer binds (pins) a certificate with the app. It's sort of hard coding the cert name, CA, domain name etc, so that the app does only connect if all the above are correct and not accept any other certificate even if there's a bit of difference.

Best practices:

  • Pinning the CA: Not a good idea. Many developers think that if they have pinned the app to a particular CA name, it's protected. It's true to some extent , but not entirely. Getting a CA certificate is not a big task for malicious attackers. The downside of this approach is that the app will connect to ANY certificate signed by the CA, thus defeating the purpose of certificate pinning.
  • Pinning the Intermediate CAs: Same as above so again not so effective
  • Pinning public key of the actual leaf (end) certificate: This is the most secure way of implementing the pinning the app. Rather than pinning CA/ Intermediate CA certificates, its recommended to pinning to the actual leaf certificate. Every certificate as a public key associated with the private key which could be pinned with the app. This approach makes sure that the app trusts the certificate which we pinned, not the the one which the attacker presents.


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