Receiving this error: 

[2025-09-25 16:50:27:054] [Error] [Fiddler] !SecureClientPipeDirect failed: Org.BouncyCastle.Tls.TlsFatalAlertReceived illegal_parameter(47) for pipe (CN=*.facebook.com, O=Progress Telerik Fiddler, OU=Created by http://www.fiddler2.com)

I have a user that reported issues trying to use "Fiddler Everywhere" after upgrading their OS:

"I updated from windows 10 to windows 11...Fiddler Everywhere was working last week when I was using on Windows 10. After upgrading, I tried to use it again today and cannot establish a remote connection to monitor traffic from a mobile device. I noticed when I first opened Fiddler I received a windows firewall notification. The option to "allow" was greyed out and I could only close the prompt. I am assuming the problem with connecting is related to the firewall but these seem to be managed by the organization. Please help allow traffic through Fiddler to assist in my debugging workflows."

I don't use Fiddler, and my team doesn't have internal documentation on troubleshooting Fiddler.  I enclosed some screen captures of what the user has sent us.  He is an Administrator on his Windows laptop, but I noticed some duplicate entries in his Windows Firewall that are grayed out, likely to Group Policy.

Any assistance would be greatly appreciated.

Hi,
I'm unable to access DuckDuckGo with Fiddler Everywhere. I can connect to DuckDuckGo without Fiddler. I think it's because DuckDuckGo detects Fiddler. Other possibilities are that the Fiddler certificate also supports unsafe TLS 1.0 and TLS 1.1, and their server ignores the request, but I'm not sure. Can you connect to DuckDuckGo, or is this issue exclusive to me? Thanks

This is what I received in the response. 

Delete my account

I need to capture URLs for Oracle SmartView logins. I saw some video for fiddler classic to setup. but can you help me to do the same set up ln enterprise.

https://youtu.be/syUoIw8N2WQ

pls provide the steps for enterprise:

 

When I try to analyse the requests on Yahoo Finance, I see different results when requesting from a browser with Fiddler turned on compared to the same request when Fiddler is turned off. I also tried different capture methods (System Proxy, Network Capture and Browser), but the same occurs. If I use this url:

 

https://query1.finance.yahoo.com/v8/finance/chart/IVV?events=capitalGain%7Cdiv%7Csplit&formatted=true&includeAdjustedClose=true&interval=1d&period1=0&period2=1747180800&symbol=IVV&userYfid=true&lang=en-US&region=US

 

Opening this url in a Chrome tab with Fiddler turned off, everything works fine (HTTP 200 response). Wih Fiddler turned on, however, the site responds with a HTTP 429 code (Too Many Requests).

 

Why this is happening?

Following guide online to swap out the old max speed with the new max speed but i'm using a mac whereas most people used fiddler classic on windows and i'm not sure whether I put the values in the correct places within the rule.  Anyone spot my noob mistake ?  Seems like many people had success so I know it's possible and my phone with the app making the API calls is connected properly to Fiddler that's not the problem. Please any help is so appreciated ! Thanks so much

websocket message garbled

Hello, for some tests, I'd like to use Fiddler to route through the corporate proxy network and, in a specific case, process requests/responses directly with mitmproxy scripts (because Fiddler Everywhere doesn't yet offer the same functionality as FC).

So I connect Fiddler to mitmproxy by setting the gateway proxy to mitmproxy listening at localhost:9090.

But it seems that Fiddler doesn't respect this setting. It always routes through the internet normally.
Even if I intentionally configure the wrong proxy port, the connection persists.

What should I do? Maybe I got wrong something??

 

I'm using Fiddler in Linux and already added certificates of Fiddler and mitmproxy to the system (actually i'm using the same one).

I keep getting this warning and as soon I click, I get it back. I think one of the security apps is monitoring and locking down the proxy server setting in Windows. I am running Fiddler as an admin and manually setting the proxy server but my setting is instantly removed.
It seems I can't use network sniffers that depend on certain proxy server settings.

Does Fiddler Anywhere suffer from the same issue?