There seems to be a problem with Firefox version 36.0 onwards with the Fiddler HTTPS decryption option.  The instructions at http://www.telerik.com/blogs/configuring-firefox-for-fiddler used to work for adding the Fiddler root certificate so that Firefox will trust the certificates that Fiddler issues.  This is also the only way to trust Fiddler for HSTS (HTTP Strict Transport Security) sites because Firefox doesn't allow adding regular "exceptions" for those sites.

After updating Firefox, I found that the Fiddler root CA no longer works to connect to the HTTPS websites.  Firefox gives a "ssl_error_bad_cert_domain" error.  An example of what this looks like is:

www.google.com uses an invalid security certificate. The certificate is only valid for *.google.com (Error code: ssl_error_bad_cert_domain)

 

So this appears to be failing to match the wildcard domain (*.google.com) to the website (www.google.com) and rejecting the certificate as a result.

I tried downgrading back to different Firefox versions, and found that 35.0.1 is the last one that seems to work properly for the Fiddler HTTPS decryption, with 36.0 breaking this functionality.

I first reported this issue on Mozilla support: https://support.mozilla.org/en-US/questions/1090724

 

Is this a known bug / break in compatibility?  Has anyone reported this issue yet and is there a known workaround?

Claude Desktop will not let me enter an MCP server with our https

Jay

Columns I find useful like the timestamp and duration are missing from the default, so I have to add them when I open a HAR. Since version 8.0.0 I have started to get a dialog box when I try to close Fiddler Anywhere:

Title: You have unsaved Changes!
Are you sure you want to quit?
You have unsaved changes that will be lost in Filters.

I have not added filters to this HAR file, and I have closed all other HAR files and all other saved sessions.

I never actually want to save anything related to the files I open. I don't care if any filters I've added are unsaved, I don't want to be prompted. I just want the program to close when I click the close button.

Is there any way to disable this dialog?

hi, i am new to fiddler and i only use it to set up some private servers for games, and it ends up logging data from all my active apps even chrome, is there a way to stop fiddler from touching chrome/other apps while it only works with my games? even when i am on discord i have a hard time on calls because fiddler is on and i have to turn it off to make the voice call work smoothly without issues, is there a way i can block fiddler from interfering with discord?

Hello Fiddler team,

I am writing to request clarification on how commercial use of Fiddler Classic is defined following the licensing change effective 3 August 2026.

We have used Fiddler Classic for many years in software and API testing, troubleshooting, and network traffic analysis. We are currently reviewing legacy deployments and images that may still contain the product and planning the appropriate migration and compliance steps.

Could you please clarify how commercial use is assessed in the following situations?

• Does the presence of Fiddler Classic on an EC2 instance or AMI, by itself, constitute commercial use?

• Or does commercial use apply only when Fiddler Classic is launched and actively used, such as to capture API calls or other network traffic?

• How should an unused legacy AMI that still contains the installation be treated?

• What licensing or compliance steps do you recommend for deployments or images that may still contain Fiddler Classic after 17 September 2026?

A clear explanation of the applicable criteria would help us determine the appropriate migration and compliance steps. Thank you for your assistance. We look forward to your guidance.

Hi Telerik -

I need to capture a the web requests that go to an OAUTH token endpoint in order to troubleshoot a bug.

The app seldom refreshes tokens, and normally it works fine, but sometimes it gets stuck in a loop where it does it over and over and over. 

Currently, if I use the "Keep #### Sessions" combobox, once it gets stuck, I get only the last #### sessions which is not what I want.  I want the first ones so I can see what might have been different that caused it to get stuck in a loop.

Alternatively, I can keep all sessions but that crashes the PC because it uses all the RAM before I notice it when into the loop.

It would be great if, next to the "Keep: #### Sessions" drop down, there was an option to "Stop Capture after this limit."

Can you please add this feature?

I am using Fiddler Classic strictly for its FiddlerScript capabilities, and I have just learned that the Progress team no longer have any plans to add this awesome functionality to Fiddler Everywhere. This is very sad news for me, as I feel that probably Fiddler's most appreciated feature is dropped all together. I have just installed the Everywhere version, checked out its Rules tab, but I am most positive that all those hard-coded rules will not even come close to the functionality and the simplicity provided by a scripting language. The Rules tab in Everywhere may be a more easier and intuitive option for most users, but this comes at the price of loosing the flexibility and the powerful tools that only a script can provide. The web is becoming more and more complex with each day, and in order for Fiddler to stay on top of it, I believe it should give its users access to the most powerful tools available. Employing scripting capabilities is certainly one of it, if not the best one! Why restrict that?!

I would like to take this opportunity to kindly ask the design team to re-evaluate this decision of dropping this future. It sets Fiddler apart from all the other software out there, and dropping it all together would be a major downgrade for Fiddler, in my opinion. Having a Rules tab for those in need of a quick set-up, and preserving the current scripting capabilities for those looking for powerful functionality seems like the best way to go for me. With these two foundations, I have no doubt Fiddler will continue to stay relevant and grow in the years ahead.

Anyhow, in case no re-evaluation will be made and approved, I would like to know if you have any plans of dropping support/updates for Fiddler Classic in the future. This would be a second blow to all the 10 year+ Fiddler users.

I hope both versions will be offered to us for many years to come, no matter how much the web will be changing. So, please confirm if we can at least rely on Fiddler Classic, as some believe it is only a matter of time until it will be deprecated (including me). For the time being, hopefully the plans to add support for HTTP/2 and TLS 1.3 will indeed materialize (for Fiddler Classic).

If others feel like me on this whole issue, please do comment. Thank you!

Alex

Hello,

i try to read a capfile created by zscaler. But Fiddler gives a error. 
Any idea how to solve that?

Regards,

 

René

Hello,

i have a pcap file exported from zscaler. when reading it into fiddler classic the error attached in the screenshot is displayed.

how can i solve it?

 

regards,

 

René

 

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)?