What’s New in Fiddler 4.6.2

TLDR? – Get the newest Fiddler here.

It’s been just over two months since the last significant release, and Fiddler (and v2.6.2.0) are now available.

As always, the latest build includes a slew of bugfixes and minor tweaks, as well as a number of features described in this post.

Default Certificate Generator Changed

Changes coming to certificate validation in browsers and other clients mean that certificates generated by makecert.exe (previously Fiddler’s default generator) will soon be rejected. To address this problem, the default certificate generator on Windows 7 and later has been changed to CertEnroll. (Windows XP and Vista users should consider installing the similar CertMaker Addon).

Unfortunately, if you’re upgrading from an earlier version of Fiddler which used a different certificate generator, you may need to explicitly reset Fiddler’s certificates. Doing so is simple:

  1. Click Tools > Fiddler Options.
  2. Click the HTTPS tab.
  3. Ensure that the text says Certificates generated by CertEnroll engine.
  4. Click Actions > Reset Certificates. This may take a minute.
  5. Accept all prompts
  6. If you are using Fiddler to capture secure traffic from a mobile device or Firefox, you will need to remove the old Fiddler root certificate from that device (or Firefox) and install the newly-generated Fiddler certificate.

If necessary, you can read more about resetting Fiddler’s Certificates or read more about Fiddler’s Certificate Generators.

SAZ Repair

From time to time, users have asked for help with Fiddler Session Archive files (.SAZ or .RAZ files) that are corrupt, either because they are incomplete (e.g. power failed) or they were mangled by an incomplete download or a failing disk drive.

Fiddler 4.6.2 includes a new feature that can recover data from corrupt Session Archive files. If the Session Archive fails to load due to corruption, you’ll be prompted to attempt a repair of the file. Data recovered from the SAZ file will be stored in a new archive and loaded for display.

Notably, this feature may also be useful to recover corrupt .zip, .docx, .xlsx, .pptx, etc files that have nothing to do with Fiddler; give it a try!

FiddlerHook Removed

This release removes the FiddlerHook extension for Firefox. Mozilla is changing their add-on model for Firefox extensions. Short-term, Firefox requires that extensions be signed (and Mozilla has declined to sign FiddlerHook) and over the next year, Mozilla will be removing the XUL Overlay extension model upon which FiddlerHook was based.

Fortunately, you don’t really need FiddlerHook to use Fiddler with Firefox. For HTTP traffic, it will often “just work” and for HTTPS traffic, only minor configuration updates are needed. You can read this post for tips on using Fiddler with Firefox.

Decryption Control

Previously, Fiddler UI only allowed you to exempt certain hosts from HTTPS decryption; if you wanted to only decrypt from a small number of hosts, you were forced to use the script engine. That limitation has been removed via a new option on the Tools > Fiddler Options > HTTPS tab. Simply click the link to toggle between exclusion and inclusion:


Extensibility Improvements

This release adds a number of improvements to Fiddler’s extensibility, from both FiddlerScript and .NET extensions.


You can now add buttons to Fiddler’s toolbar in a supported way. Simply add a new BindUIButton attribute to a static method in your FiddlerScript file; the string argument is the caption with which to label the button.

For instance:

    BindUIButton("Copy HAR")


toolbar button

Toolbar buttons are added to the left of the toolbar in the order opposite of their listing within the FiddlerScript. Adding images is not supported from FiddlerScript but you can use Unicode Emoji symbols if you’d like.

Fiddler extensions may add to the toolbar using the static method FiddlerToolbar.AddToolstripItem() and may remove entries using .RemoveToolstripItem().

Export to String

In many cases, you may wish to generate a string representing one or more Sessions in either HTTPArchive (HAR) or cURL format. While you can manually use File > Export to generate files of either format, you can now skip the middle-man and export to these types in memory. To do so, simply set the ExportToString option and do not set a Filename option. After the DoExport command completes, the output is found in the OutputAsString option.

For instance, you can add the following to your FiddlerScript:

BindUIButton("Copy HAR")
ContextAction("Copy HAR")
public static function doCopyHar(arrSess: Session[])
var oExportOptions = FiddlerObject.createDictionary();

// If you’d prefer to save to a file, set the Filename instead
//oExportOptions.Add(“Filename”, “C:\\users\\lawrence\\desktop\\out1.har”);

oExportOptions.Add(“ExportToString”, “true”);
oExportOptions.Add(“MaxBinaryBodyLength”, 1000000);
oExportOptions.Add(“MaxTextBodyLength”, 1000000);

FiddlerApplication.DoExport(“HTTPArchive v1.2”, arrSess, oExportOptions, null);
var sOutput: String = oExportOptions[“OutputAsString”];

FiddlerApplication.UI.SetStatusText(“Copied Sessions as HAR”);

…and Fiddler will add a toolbar button and context menu command that copies the Selected Sessions to the clipboard in HAR format. (Tip: You can choose Edit > Paste As Sessions in Fiddler to create new Sessions based on HAR text that you’ve copied from browser tools.)

Alternatively, you can add a similar command to copy the Selected Sessions as a set of cURL commands:

BindUIButton("Copy cURL")
ContextAction("Copy cURL")
public static function doCopyCurl(arrSess: Session[])
var oExportOptions = FiddlerObject.createDictionary();

// If you’d prefer to save to a file, set the Filename instead
//oExportOptions.Add(“Filename”, “C:\\users\\lawrence\\desktop\\out1.bat”);

oExportOptions.Add(“ExportToString”, “true”);

  FiddlerApplication.DoExport("cURL Script", arrSess, oExportOptions, null);
var sOutput: String = oExportOptions["OutputAsString"];

FiddlerApplication.UI.SetStatusText(“Copied Sessions as cURL”);

Invoking on the UI Thread

Fiddler processes Sessions on background threads, but you should only ever manipulate Fiddler’s UI using the UI thread. Only a few of Fiddler’s UI calls are thread-safe; if you’re not sure, your script should use the new FiddlerObject.uiInvoke or FiddlerObject.uiInvokeAsync methods to avoid crashing or corrupting the user-interface.

Load Extensions at Runtime

To support some exciting new work from the community, Fiddler now has the ability to load additional Extensions and Inspectors at runtime; this enables building of more complex add-on systems atop Fiddler’s existing system. To use these APIs, invoke any of these four methods from the UI thread:

FiddlerApplication.oExtensions.InstantiateInspectorsFromPath(string sPathToInspectors)
FiddlerApplication.oExtensions.InstantiateExtensionsFromPath(string sPathToExtensions)
FiddlerApplication.oExtensions.InstantiateExtensionsInFile(FileInfo oFile, bool bWriteToLog, bool bRethrowExceptions)
FiddlerApplication.oExtensions.InstantiateExtensionByType(Type typeExtension, bool bWriteToLog)

Thank You!

Lastly, I’d like to thank everyone for all of your support over the last twelve years, as Fiddler has evolved from a side project to a fully-supported debugging platform used around the world. I’m excited to see where Telerik takes Fiddler next, and while I’ll be keeping plenty busy in my new job, I expect I’ll remain involved in the Fiddler community (updating the book, and haunting the forum) for quite some time.

Wishing you all the best in 2016 and beyond!

-Eric Lawrence

Published by ericlaw

Impatient optimist. Dad. Author/speaker. Created Fiddler & SlickRun. PM @ Microsoft 2001-2012, and 2018-2022, working on Office, IE, and Edge. Now a SWE on Microsoft Defender Web Protection. My words are my own, I do not speak for any other entity.

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