Hi everyone, I’ve been trying to understand how MiTM setups like a transparent proxy work.
Obviously, the use-case here is in a personal scope: I’d like to inspect the traffic of some of my machines. I am aware that Squid can be a transparent proxy, and some might use the Burp Suite to analyse network traffic.
Could someone explain the basic networking and the concept of certificates in this scenario? I feel like I don’t understand how certificates are used well enough.
For example: I realise that if someone inserts a root certificate in the certificate store of an OS, the machine trusts said CA, thus allowing encrypted traffic from the machine to be decrypted. However, say the machine was trying to access Amazon; won’t Amazon have its own certificate? I don’t know how I’m confused about such a simple matter. Would really appreciate your help!
A decrypting proxy acts like a local certificate authority and issues a certificate on behalf of the site to create one encrypted connection between the client and proxy while creating a second on with the original site on the other side.
The client will get an error (or depending on the site and browser may refuse to connect at all in the case of pinned and preloaded certificates) indicating that they’re connecting to a site with a certificate from an untrusted issued unless they’ve been configured to trust the CA held on the proxy.
The ‘break’ between the client side connection and the server side connection will be in plaintext and can be examined through internal tools or sent to external ones typically through ICAP.
I see. Which would mean that in cases where the application/OS can be made to blindly trust the “fake” CA (by inserting a root certificate into the certificate store, like in Android), the proxy can simply send the certificate and the client will believe that the certificate comes from Amazon. The certificate list can be refreshed by flushing the cache, yes?
Thanks for the explanation
There are some cases where this would not work by the way. It’s called certificate pinning and it’s basically when an application comes with the trusted certificate for a host built-in. Even if you were to override it with a root certificate in the certificate store, the app simply wouldn’t use it.
Shouldn’t flushing the cache mitigate this problem?
No, not at all. The request never hits the cache. The certificate is stored within the app and all internet communication is specifically pinned to said certificate. It doesn’t even ask your certificate store.