[Solved] [Error] [Fiddler] The server [domain] requested a client certificate, but no client certificate was available.

[Feylin]

Hey everyone,

I’m hitting a bit of a wall with mTLS in Fiddler Everywhere. My logs are showing the following error repeatedly: "[Error] [Fiddler] The server [domain] requested a client certificate, but no client certificate was available."

The Situation:

• I have my client certificate ready (as a .pfx, no password).

• My setup involves several subdomains, specifically:

  • appdata.mydomain.com

  • apps-boost-global.mydomain.com

• They all use the SAME certificate.

The Problem: I can’t find a clear, definitive path on where to save these files so Fiddler Everywhere actually "sees" and picks them up. I've seen mentions of Documents\FiddlerCore, .fiddler\ClientCerts, and others, but none seem to trigger the handshake correctly.

My Questions:

1. What is the exact directory path Fiddler Everywhere uses for client certificates in the latest version?

2. Since I’m using the same cert for multiple domains, do I need to duplicate and rename the file for every single subdomain (e.g., hostname.pfx), or is there a way to map one cert to multiple hosts in the UI/Settings?

3. Does the file extension matter (should I use .pfx, .p12, or rename to .cer)?

   
   

[Nick Iliev]

Hello Feylin,

 

The so-called Mutual TLS (the usage of both server and client certificates) is not yet supported in Fiddler Everywhere , so there is no option to provide a client certificate. The issue was logged as a feature request through the following public feedback thread, which you can upvote to raise its priority:

https://feedback.telerik.com/fiddler-everywhere/1624585-fiddler-everywhere-client-certificate-authentication 

 

Regards,
Nick Iliev
Progress Telerik

[Feylin]

Hey Nick,

thanks for the reply. Yearh I assume Burpsuite is the only Tool supporting this for now. Right now it seems focus on Fiddler is on LLMs :)

[Nick Iliev]

Indeed, we aim to deliver more features on current hot topics, but we also regularly seek and triage client feedback. I will let you know if there is new information related to the support for client certifiacates.