Fiddler blocks some HTTPS connections like Dropbox

Question

I work on a project which make ajax query to a webservice so I use fiddler to see JSON responses.

But I have encounter troubles using Fiddler. When I launch it on my laptop, Dropbox can't synchronize my files anymore but I can debug my ajax requests. The real problem is when I use Fiddler on my desktop computer, all my requests to my WebService are blocked. My WebService runs on localhost.

I don't understand how it works, can you help me?

Answer 1

Dropbox connections don't work because that application uses a feature called "Certificate Pinning" that reject's Fiddler's HTTPS interception certificate. Why this happens is discussed in the Fiddler book, but you can configure Fiddler not to decrypt dropbox.exe's connections which resolves the issue.

To avoid blocking DropBox App traffic while Fiddler is running, you can use Tools > Fiddler Options > HTTPS to either only decrypt Browser traffic or you can configure Fiddler not to decrypt traffic to *.dropbox.com.

The issue with your "WebService" is almost certainly completely unrelated. You need to be far more specific for anyone to help: What is the client? What is the service written in? What do you see in Fiddler when this happens?