Blank Pages Cause By Fiddler?

[Shaylin]

One of my clients has Fiddler installed and running while they utilize our web application. They said that when Fiddler is running, they will run into a blank page right away. They said if they restart their computer, the blank page issue will resolve itself. They said this started happening once Fiddler was installed. Any ideas?

[Nick Iliev]

Hello Shaylin,

 

While Fiddler Classic runs and captures, it is set as a system HTTP/HTTPS proxy. However, when Fiddler is turned off, it is immediately unset as a proxy, and no permanent changes can affect the usual behavior. So, when Fiddler is turned off, your client should be able to use any web resources just as they would if Fiddler were never installed. A restart of the PC is unnecessary as FIddler Classic (when turned off) is not changing the network behavior of the host OS.

It is hard to pinpoint what might be causing the issue - as discussed in our previous thread, it might be related to a specific TLS protocol version or cipher (that is used in your web page but is unsupported in the client's host OS), or to a third-party security tool /administrative policy that changes the behavior of Fiddler (for example, that prevents the proper setting/unsetting of the system proxy).

One way to deep dive into the issue is to extract as much as possible information for the client environment (OS, used browser, installed security tooling, exact steps to reproduce, whether it is reproducible with another endpoint, etc.) and then also send us the information so that we could try to reproduce the issue on our side. Alternatively, you can ask them to install a trial version of Fiddler Everywhere, reproduce the issue, and send the Fiddler Everywhere application log files.

 

Regards,
Nick Iliev
Progress Telerik