I'm trying to intercept a web application which uses a HTTP proxy (basic HTTP auth password protected) to access its resources.
In Fiddler options, there is a setting for manual proxy configuration. But in that field, I can only define the proxy address and port. I need to define an username/password combination for upstream proxy.
Is there any way to do this?
Your scenario is a bit unclear. Clients should automatically prompt for proxy credentials when a HTTP/407 is received, although many don't.
If your question is: "How can I add a Proxy-Authorization header to all requests that pass through Fiddler?" then it's pretty simple.
Rules > Customize Rules > Scroll to OnBeforeRequest and add:
if (!oSession.isHTTPS)
{
oSession.oRequest["Proxy-Authorization"] = "Basic dXNlcm5hbWU6cGFzc3dvcmQ=";
}
Where dXNlcm5hbWU6cGFzc3dvcmQ= is the base64-encoded version of the "username:password" string. You can use Fiddler's Tools > TextWizard to base64-encode a string.