Last Updated: 16 Apr 2024 06:45 by ADMIN

Hi Team,

We wanted to use Fiddler Classic and when we send for Security Scanning, its flagged as Malicious. From your end, do you have confirmation like it would be false positive. Attached the screenshot where it was flagged as Malicious.


Last Updated: 29 Mar 2024 02:49 by Ryuu

For legacy reasons, Fiddler logs the message "HTTPSLint> Warning: ClientHello record was {0} bytes long. Some servers have problems with ClientHello's greater than 255 bytes"

This message should be removed because at this point, effectively ALL clienthellos are over 500 bytes and basically all servers are okay with it.

(This message was only relevant around 2014 or so when longer clienthellos started becoming common)

Last Updated: 28 Mar 2024 10:43 by ADMIN
Created by: Eric
Comments: 0
Type: Bug Report

When sending post data, -d option needs to ensure that the data does not start with an @


BasicFormats.dll - cURLExport.cs

Last Updated: 28 Mar 2024 10:42 by ADMIN

As noted in the Fiddler book,

Sessions rerouted from one hostname to another using the Host Remapping tool are rendered with a light blue background in the Web Sessions list. HTTPS Sessions that have been rerouted have the X-IgnoreCertCNMismatch and X-OverrideCertCN Session Flags set to avoid raising “Certificate Name Mismatch” errors.

However, there's a bug. In the HostsFile.cs code, there's are several places that look like:

            if (oS.isTunnel) {

                oS["x-overrideCertCN"] = oS.hostname;
                oS["X-IgnoreCertCNMismatch"] = "HOSTS-Ext";

This usually works for browser traffic going through Fiddler (because the HTTPS handshake is typically conducted on the CONNECT tunnel). However, it doesn't work (and the user is spammed with cert error warnings) if the traffic is sent from Fiddler itself (e.g. via Composer or using the "Reissue requests" context menu item). 

The code should look like this:

            if (oS.isHTTPS || oS.isTunnel) {

                oS["x-overrideCertCN"] = oS.hostname;
                oS["X-IgnoreCertCNMismatch"] = "HOSTS-Ext";


Last Updated: 04 Feb 2024 09:49 by piccolo


I’m developing a .NetFramework extension for Fiddler and am finding an issue with clearing bold, italic, strikethrough on the session text in the session list when using “this.session.RefreshUI()”. I’d like to be able to see these changes occur upon a context menu item click, immediately within Fiddler, without having the reload the sessions or the application. I can see the session flags are removed from the session as expected, but the bold, italic, or strikethrough is not unset.

I’m aware there is an option to Mark, Unmark sessions, but this doesn’t fit integrate closely enough with the extension I am developing or do exactly what I would like.

I seem to have no issues with changing the UI-Backcolor or UI-Color and refreshing for the updates to be immediately seen.

I can set UI-Bold, UI-Italic, UI-Strikethrough, but I cannot unset these with RefreshUI().

Is this a bug? Is the RefreshUI() call not doing something for UI-Bold, UI-Italic & UI-Strikethrough which it does do for UI-BackColor and UI-Color?



Pending Review
Last Updated: 25 Jan 2024 17:38 by Matías

I will be very useful if the font can be configured separately. One font for the composer and results, and the other for the IDE.

If I change the FONT, all the IDE is updated, including the composers, but after RESTARING, the composer has the same font than before

Last Updated: 24 Dec 2023 07:22 by Xriuk
Created by: Xriuk
Comments: 3
Type: Bug Report

Related issue:

When sending multiple requests to the same domain, sometimes Fiddler alters the headers (in this case by duplicating the user-agent one), and in this case it causes a fail because a precomputed signature of the request does not match.

Need More Info
Last Updated: 24 Dec 2023 03:00 by Eric

good morning 

i got error while capturing my app to

=== Windows 10 64bit OS==

===Protocols I have enabled: <client>;ssl2;ssl3;tls1.0;tls1.1;tls1.2

====== Message from Fiddler v5.0.20211.51073  for .NET v4.6.1
HTTP/1.1 200 Connection Established
FiddlerGateway: Direct
Connection: close> HTTPS handshake to (for #17) failed. System.Security.Authentication.AuthenticationException A call to SSPI failed, see inner exception. < The message received was unexpected or badly formatted

Win32 (SChannel) Native Error Code: 0x80090326


Any help ??

Thank you


The fiddler response

A SSLv3-compatible ClientHello handshake was found. Fiddler extracted the parameters below.

Version: 3.3 (TLS/1.2)
Random: 65 58 47 1D 28 AB A2 68 50 33 C7 75 1E A4 8B F5 9D A6 FB A0 F9 D2 54 FA 93 D5 AE 78 E5 26 7D 89
"Time": 26/07/1985 15:35:49
SessionID: empty
status_request OCSP - Implicit Responder
supported_groups x25519 [0x1d], secp256r1 [0x17], secp384r1 [0x18]
ec_point_formats uncompressed [0x0]
signature_algs rsa_pss_rsae_sha256, rsa_pss_rsae_sha384, rsa_pss_rsae_sha512, rsa_pkcs1_sha256, rsa_pkcs1_sha384, rsa_pkcs1_sha1, ecdsa_secp256r1_sha256, ecdsa_secp384r1_sha384, ecdsa_sha1, dsa_sha1, rsa_pkcs1_sha512, ecdsa_secp521r1_sha512
SessionTicket empty
ALPN http/1.1
extended_master_secret empty
renegotiation_info 00


Need More Info
Last Updated: 01 Nov 2023 15:00 by ADMIN

Help menu to check for updates says:

Error retrieving version information

System.IO.InvalidDataException ... Server Resonse Code = 502
    at [encoding error] ...Updater.cs:Line 90

GET /UpdateCheck.aspx?isBeta=False HTTP/1.1 Result 502


v5.0.20211.51073 for .NET 4.6.1
Built: Wednesday, December 15, 2021
64-bit AMD64, VM: 84.0mb, WS: 129.0mb
.NET 4.8 WinNT 10.0.19045.0
Won't Fix
Last Updated: 25 Aug 2023 08:14 by ADMIN

Dear Fiddler developers,


I was recently inspecting the Punch In and Clock Out system of our company. After I captured the POST to send the punch in packages, I try to replayed it.

The replay functions returened a valid request. It is still valid even if I export the session into curl of a bat fil. But I got a issue after I copy the code into a bigger automatic code. The server I sent report me a 543 error "Please check the shifts." And the map API returned a UTF-8 wrong decode.


First I thought that this is because the limitation of both Unix and Windows system, I tried to pipe all the satings into the limitation they desire, 520 as I measured in UNIX. But it did not work.

Later on I watch the debug with the bash -x for about an hour, then I realise that you, the Fiddler program actually somehow turn one % into double %%.

After I replace all the %% with single %, everything works fine.


My first doubt is the ChatGPT. But as I was looking through the chat history, the ansewer is no, The problem is somewhere before I ever peate the code to ChatGPT to run. So I regenerated the whole cURL code from Fiddle, and yes this fault is your issue.


Please keep me updated if you fix that problems. Also, maybe I am ingnorant on this common problem. I am just a noob in the holy I.T. stuffs.



Last Updated: 16 Aug 2023 06:36 by ADMIN

Fiddler's update check will always fail by default because you've disabled TLS/1.0 support on your server and Fiddler's now-very-outdated TLS config only enables TLS/1.0 by default. The user must manually go into Tools > Fiddler Options > HTTPS and change the Supported protocols string to <client>;tls1.2 to fix this problem (and to also fix problems with the Composer and any other Fiddler features that do not pass through traffic through a client that supports later TLS.

This is tracked by oAcceptedServerHTTPSProtocols in Config.cs.

Need More Info
Last Updated: 12 Jul 2023 07:10 by Xiaoming

I am trying to use settings Tools -> Win8 Loopback Exemptions -> Exempt All -> Save Changes. But it is giving me some error and saving those changes. Following is the error.

Failed to set IsolationExempt AppContainers; call returned 0x57

I am blocked due to this error and cannot perform tests on my application. Pls help.


Last Updated: 05 May 2023 07:24 by ADMIN

Fiddler classic uninstall after epic app & valorant online game not working some error as to come valorant error code : val 29

Epic error code : 404

Need More Info
Last Updated: 05 May 2023 07:09 by ADMIN
Created by: Michael
Comments: 1
Type: Bug Report

Fiddler would NOT allow me to save the file.  We tried this multiple times to various file locations. 

I do not have Visual Studio installed on my PC -- so I selected the latest verison in this form because it is a REQUIRED field. 

Fiddler provided the text below. 



Awww, Fiddlesticks!


Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report at


Could not find file 'C:\Users\629243897\Documents\IBMSPend on v85 debug - 2 refreshes after logon.saz'.


Type: System.IO.FileNotFoundException

Source: mscorlib

   at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)


   at System.IO.FileInfo.get_Length()


   at Fiddler.frmViewer.actSaveSessionArchive(String , String
, Session[] ) in C:\Jenkins\Fiddler_Windows\workspace\Fiddler2\Fiddler.Shared\Viewer.cs:line 6808



Fiddler v5.0.20211.51073 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 10.0.22621.0]




Last Updated: 22 Nov 2022 16:49 by Eric

If you open use fiddler for a while (a Day or so), eventually it lock up the UI on the other monitors. It is almost as if there is an invisible dialog over the other screens. Once you kill Fiddler. its all restored again.


We have been Noticing this over the last few months.

Last Updated: 29 Aug 2022 05:36 by ADMIN

Fiddler downloads content to show on its homescreen from HTTP URLs. This is not safe because the content is rendered to the user in a WebView control and thus an attacker on the wire could replace it with malicious code or instructions that could harm the user.

Fiddler should not be using HTTP URLs for anything in this day and age.

Last Updated: 12 Aug 2022 06:32 by ADMIN

document-policy: force-load-at-top

Pending Review
Last Updated: 10 Apr 2022 14:30 by Eu

I want to capture the traffic from Android Phone, and already install Root Certificate as 'System Trusted Credintials' in /system/etc/security/cacerts (renamed like '269953fb.0' by md5).

Accessing the https page, It seems to behave well in a normal application and other browser (like XBrowser, a tiny volume browser, only 1.01MB) ,and which proves that the certificate is installed correctly, but in Chrome it prompts 'NET:ERR_CERT_AUTHORITY_INVALID'. As shown, it seems that Chrome does not associate the web certificate returned by Fiddler with the root certificate already installed on the system, but I use the same installation method in Charles, it behaves well and can browse https pages normally, it seems that Charles generates a different certificate than Fiddler? I used the command

'openssl x509 -inform der -text -in /mnt/d/Users/menEu/Desktop/FiddlerRoot.cer'

to see the difference and it doesn't seem to be a huge difference (but I'm not specialized in this area)

Need More Info
Last Updated: 07 Apr 2022 05:55 by ADMIN

I've tried multiple allocation ratios and nothing solves the problem, the screen allocation ratio is 2560x1600.

Version Windows 10 Home Chinese
Version number 21H1
Installation Date ‎2021/‎12/‎25
OS Build 19043.1586
Experience Windows Feature Experience Pack 120.2212.4170.0

fiddler version :

v5.0.20211.51073 for .NET 4.6.1
Need More Info
Last Updated: 07 Mar 2022 10:17 by ADMIN


I have a connectivity issue when I run Fiddler (Progress Telerik Fiddler Web Debugger). I work behind a corporate proxy server and the server IP is changed every few hours. The change in IP is normally okay, but when I am running Fiddler with the 'Automatically Authenticate' option enabled, then Fiddler shows an error every time the proxy is changed; it shows a yellow error message suggesting "The system proxy was changed. Click to reenable capturing".

Clicking on the error in Fiddler works fine and it reconnects, but I find this frustrating because if i don't realise there was an error in fiddler and don't click it immediately then it affects my connectivity and some activities on my pc start to fail without me realising.

My suggestion is that you should let Fiddler auto-reconnect when this type of proxy change happens and when the 'Automatically Authenticate' option is enabled. I know it can't try to reconnect forever because it will cause an endless cycle for other people who have other types of proxy disconnects, but in my case it would be very helpful to let fiddler at least try once to auto reconnect (for me 1 retry is usually fine and Fiddler is able to connect to the new proxy address, but perhaps you can let the number of retries be a menu option so that users can set the number of retries that they want Fiddler to do after there was a proxy change or proxy error).


Thanks so much for your work, I really like Fiddler!

I hope you can bug fix this or add it as a feature, it will be very helpful!



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