Tried multiple browsers but always receive from booking.com when inspecting traffic:

HTTP/1.1 403 Forbidden
Server: CloudFront
Date: Mon, 19 Oct 2026 19:55:37 GMT
Content-Type: text/html
Content-Length: 919
Connection: keep-alive
X-Cache: Error from cloudfront
Via: 1.1 fc20c1482245d494719df97cc96116e0.cloudfront.net (CloudFront)
X-Amz-Cf-Pop: PHX52-P3
X-Amz-Cf-Id: _nmq3w2vKtiW70wXoSzzLJMBavMsrHTE7odnkCrEVoQgzrdV_iQ7iA==
Strict-Transport-Security: max-age=63072000; includeSubDomains; preload
Accept-CH: ECT

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML><HEAD><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<TITLE>ERROR: The request could not be satisfied</TITLE>
</HEAD><BODY>
<H1>403 ERROR</H1>
<H2>The request could not be satisfied.</H2>
<HR noshade size="1px">
Request blocked.
We can't connect to the server for this app or website at this time. There might be too much traffic or a configuration error. Try again later, or contact the app or website owner.
<BR clear="all">
If you provide content to customers through CloudFront, you can find steps to troubleshoot and help prevent this error by reviewing the CloudFront documentation.
<BR clear="all">
<HR noshade size="1px">
<PRE>
Generated by cloudfront (CloudFront)
Request ID: _nmq3w2vKtiW70wXoSzzLJMBavMsrHTE7odnkCrEVoQgzrdV_iQ7iA==
</PRE>
<ADDRESS>
</ADDRESS>
</BODY></HTML>

Any work around?

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?

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?

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

Hi, i get this error on any site any browser, 

when i looked at the certificate list, i see that the certificate is expired:

i just reinstalled the last version, but the problem persists

is there any way to fix that ?

I need Fiddler to capture my call from nodejs. 

I call https://domain from nodejs like this

  import { ProxyAgent, fetch } from 'undici';
  const dispatcher = new ProxyAgent('http://127.0.0.1:8888' )
   var r2 = await fetch('https://domain', {
	method: 'POST',
	dispatcher,
	headers: {	
                 ...}
	  },
    body:  ...
  }
  );
  var r2data = await r2.json();

I can do the above call to http://domain, but when I changed to https://domain, I got the below error, any idea how do I handle this? thanks