I am using the following version

Progress Telerik Fiddler Classic v5.0.20253.3311 for .NET 4.6.2

When capturing WebSocket communication with a URL of "ws://10.133.37.150:9999/", the request line in the Inspectors' Headers view is correct as follows:
"GET ws://10.133.37.150:9999/ HTTP/1.1"

However, the URL column in the left pane says "ws://10.133.37.150:9999/", but the correct value is "/".

Also, the Inspectors' Raw tab says
"GET http://10.133.37.150:9999ws://10.133.37.150:9999/ HTTP/1.1", but the correct value is
"GET ws://10.133.37.150:9999/ HTTP/1.1".

I think this is a bug

 

I am using the following version

Progress Telerik Fiddler Classic v5.0.20253.3311 for .NET 4.6.2

After opening a WebSocket, the communication is captured in which the server returns the messages "WebSocket!Message1!" and "WebSocket!Message2!".

With the first WebSocket, these messages are displayed in the "WebSocket" tab as expected.

However, when the exact same interaction is captured again,
these messages are not displayed in the "WebSocket" tab as they are with the second WebSocket.
Instead, they are displayed as the response body in the "Raw" tab of "Inspectors".

Whether the messages are displayed in the "WebSocket" tab or the "Raw" tab of "Inspectors" is random, and there is no visible pattern.

There seems to be a problem with the process of determining the boundary between the WebSocket opening handshake and the WebSocket message exchange.

 

 

 

I'm pretty disappointed in y'all. 

The January 2025 version of Fiddler introduces a pointless "Security" dialog telling the user about what modules Fiddler is loading. I'm sure you thought that this would improve security, but it won't, because the same level of permission needed to add an extension to Fiddler allows the extension to make this dialog not show (base64 is not encryption).

Worse, the dialog is messed up in high-DPI to the point where it's not readable. https://bsky.app/profile/ericlawrence.com/post/3lh2wvht5oc2k

Worse, the dialog's "Always allow" button doesn't do anything when the user is using `-viewer` mode, because when in Viewer mode preferences don't work.

Worse, it looks like you also broke Transcoder extensions (File > Import); after selecting the importer to use, nothing happens.

Please go back to the way things were before. If you'd like to talk to me about Fiddler's Threat Model, I'm happy to help.

It appears that the latest version of Fiddler breaks Fiddler's public interface. The `Fiddler.WebFormats.JSON` interface no longer appears to be available, breaking extensions and scripts that depend upon it.

 

---------------------------
Failed to import NetLog
---------------------------
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 http://www.telerik.com/forums/fiddler.

Could not load type 'JSONParseErrors' from assembly 'Fiddler, Version=5.0.20253.3311, Culture=neutral, PublicKeyToken=null'.

Type: System.TypeLoadException
Source: FiddlerImportNetlog
   at FiddlerImportNetlog.NetlogImporter..ctor(StreamReader oSR, List`1 listSessions, EventHandler`1 evtProgressNotifications)

   at FiddlerImportNetlog.NetLogFormatImport.ImportSessions(String sFormat, Dictionary`2 dictOptions, EventHandler`1 evtProgressNotifications)


Fiddler v5.0.20253.3311 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 10.0.26100.0] 
---------------------------
OK   
---------------------------

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.

Thanks

Fiddler's Cookies inspector doesn't do very much today other than breaking out the Cookie header into a word-wrappable format.

It would be nice if it (and even the Headers inspector) flagged cookies that were sent expired (with an Expires header before the value of the Date header on the server's response). Otherwise it's hard to notice that a cookie being "Set" is actually being "Deleted".

First of all I'll say that I've investigated it deeply.

I used to have fiddler  4 , earlier versions and never updated.

When my computer turned to sleep , I was able to wake it by trying to rdp it.

The connection itself to rdp - woke the computer ( via network card) and the computer awoke ,and then I could connect to it.

this feature is called "wake on link".

After installing fiddler ( latest version) , suddenly my computer won't wake up from sleep when trying to connect to pc.

at first I didn't know what whet wrong and then I started uninstalling latest installed programs

When I uninstalled fiddler , it started waking  the computer

So I don't know what's you've done in the last version , but it f*s some other functionalities.

please fix it.

I'm working on an open-source Fiddler Classic extension to search the Session log (history). I'm not aware of such an extension and the lack of a search feature for the log entries always bothered me.

My current implementation already works, it searches the history listview, but it can only search in the text displayed in the listview itself because additional data is only available in the HistoryItem instances, attached to the Tag property of each list item.

Since HistoryItem is internal, I cannot access this class. I could work around this by using reflection but the internal classes of Fiddler are all obfuscated so, at the very least, my source code would look very strange and unmaintainable.

Please make HistoryItem public so it'd be accessible and un-obfuscated.

Fiddler currently does not validate that the Fiddler Trusted Root Certificate is not expired when generating certificates, and it generates certificates that have an expiration that is after the expiration of the root certificate. These certificates will not work because the browser validates the expiration of each certificate in the chain.

Browsers have a poor error message for this case and will imply that the Site's certificate is expired when it's actually the ROOT that expired.

When it loads the root certificate, Fiddler should verify that it is not expired, and if it is, it should trigger the RESET ALL CERTIFICATES flow to help unblock the user from this situation.

It should also be changed such that the root certificate is valid for MUCH longer than the site certificates (e.g. 5 years for the root) so this is less likely to happen.

(If you look in the forums, users are hitting this problem and they are not sure why or how to fix it.)

If I add a filter to hide something like /teams-modular-packages/ after closing fiddler classic and opening new file filter shows as active but is not working. I need to explicit remove filter and add it again. I would understand that filter wouldn't work accross sessions if filters were removed but they aren't and always showing in bottom left corner. This can be easily reproducible. (in this repro i have imported HAR file, not done a live capture)

 

The error messages pops up on Windows 10 version 1703.  Fiddler installs and runs with not issues on Windows 10 version 1607.

I often have to locally save a lot of responses manually.

My workflow is:
• Open a .saz file

• Search for a particular request

• Save the response locally.

 

For that, I always have to manually click the "Response body is encoded. Click to decode." Button.

Fiddler Classic doesn't have a feature to automatically decode the selected request's response body.

 

So if I don't pay attention, and skip a step, I will store an encoded response body, without ever noticing it. Which can cause trouble later, since these files are then sent to my customer. And the customer could randomly check the files.

 

I need a toggle in Fiddler, that automatically decodes the selected request's response body.

Today, Fiddler exposes these two events to handle scenarios where the user is saving or loading a SAZ file.

        /// fires just before a SAZ file is saved
        public static event EventHandler<WriteSAZEventArgs> OnSaveSAZ;

        /// fires just after a SAZ file is loaded
        public static event EventHandler<ReadSAZEventArgs> OnLoadSAZ;

Equivalent event handlers should be created for the scenario where a user is Importing content into the Sessions list (e.g. using NetLog import or HAR import, etc). Otherwise, developers must undertake cumbersome workarounds to detect that a list of Sessions has been created/loaded from a file import, if, say, they wish to perform processing on those imported Sessions (adding custom properties or changing the display properties in the Session list).

Would really appreciate a proper machine based installation again, user-based installs are difficult to manage in corporate/enterprise environments & the psuedo machine install of redirecting install folder & creating new shortcuts isn't great, especially if as you mention yourself extensions wont work.

I understand the advantage of not needing admin rights to install programs, but surely most of the targeted audience for this application would either A) have admin rights, or B) be in a managed environment with deployment software in use (and potentially white-listing/App Control software preventing unauthorized apps to run anyway)

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.

Thanks.

I would be nice if Fiddler could decrypt zstandard compressed requests.

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)

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

https://portswigger.net/research/the-curl-quirk-that-exposed-burp-suite-amp-google-chrome

 

BasicFormats.dll - cURLExport.cs

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";

           }

The .NET Framework has added support for TLS/1.3.

We should do the work to enable TLS/1.3 in Fiddler (it's very little additional work to add "Tls1.3" to the options dialog and the underlying code).