Unable to run Fiddler with Windows Firewall rules applied via GPO

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Fiddler Classic Fiddler Everywhere
Joe
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Joe asked on 02 Feb 2022, 12:52 PM

As title.  If the firewall is disabled, we are able to start Fiddler no problem.  If the firewall is enabled, I get the following popup when it tries to start.

 

What could be the issue here?

----------------------8<-------------------

 

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 http://www.telerik.com/forums/fiddler.

The composition produced a single composition error. The root cause is provided below. Review the CompositionException.Errors property for more detailed information.



1) The specified quota list is internally inconsistent with its descriptor



Resulting in: An exception occurred while trying to create an instance of type 'Telerik.NetworkConnections.NetworkConnectionsManager'.



Resulting in: Cannot activate part 'Telerik.NetworkConnections.NetworkConnectionsManager'.

Element: Telerik.NetworkConnections.NetworkConnectionsManager -->  Telerik.NetworkConnections.NetworkConnectionsManager -->  AssemblyCatalog (Assembly="Telerik.NetworkConnections, Version=0.1.2.0, Culture=neutral, PublicKeyToken=67cb91587178ac5a")



Resulting in: Cannot get export 'Telerik.NetworkConnections.NetworkConnectionsManager (ContractName="Telerik.NetworkConnections.NetworkConnectionsManager")' from part 'Telerik.NetworkConnections.NetworkConnectionsManager'.

Element: Telerik.NetworkConnections.NetworkConnectionsManager (ContractName="Telerik.NetworkConnections.NetworkConnectionsManager") -->  Telerik.NetworkConnections.NetworkConnectionsManager -->  AssemblyCatalog (Assembly="Telerik.NetworkConnections, Version=0.1.2.0, Culture=neutral, PublicKeyToken=67cb91587178ac5a")



Type: System.ComponentModel.Composition.CompositionException
Source: System.ComponentModel.Composition
   at System.ComponentModel.Composition.Hosting.CompositionServices.GetExportedValueFromComposedPart(ImportEngine engine, ComposablePart part, ExportDefinition definition)

   at System.ComponentModel.Composition.Hosting.CatalogExportProvider.GetExportedValue(CatalogPart part, ExportDefinition export, Boolean isSharedPart)

   at System.ComponentModel.Composition.Primitives.Export.get_Value()

   at System.ComponentModel.Composition.ExportServices.GetCastedExportedValue[T](Export export)

   at Fiddler.Proxy.() in C:\Jenkins\Fiddler_Windows\workspace\FiddlerCore\FiddlerCore.Shared\Common\Core\Proxy.cs:line 283

   at Fiddler.Proxy..ctor(Boolean isPrimary, ProxySettings upstreamProxySettings) in C:\Jenkins\Fiddler_Windows\workspace\FiddlerCore\FiddlerCore.Shared\Common\Core\Proxy.cs:line 210

   at Fiddler.frmViewer.‹•(String[] ˆ•) in C:\Jenkins\Fiddler_Windows\workspace\Fiddler2\Fiddler.Shared\Viewer.cs:line 3339

   at Fiddler.frmViewer.‡•(String[] ˆ•) in C:\Jenkins\Fiddler_Windows\workspace\Fiddler2\Fiddler.Shared\Viewer.cs:line 3168


Fiddler v5.0.20211.51073 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 10.0.19042.0]

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Nick Iliev
Telerik team
answered on 03 Feb 2022, 06:21 AM

Hello Joe,

 

The proper approach would be to add Fiddler Classic in the allowlist of your firewall and make sure that it is not closing the Fiddler port and preventing the usage of MITM proxy. No action can be taken from the Fiddler application. You need to contact your network administrator if you don't have full administrative rights for handling the firewall.

Regards,
Nick Iliev
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Joe
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commented on 03 Feb 2022, 07:46 AM

Thanks, I am the network administrator and am trying to get it working for a user.  This is Windows Firewall, I have allowed fiddler.exe (with the correct path) in the Outbound rules to talk any protocol to anything, but it still gives this error when it starts.  If I disable the Outbound rules, it starts fine.
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Nick Iliev
Telerik team
answered on 04 Feb 2022, 08:38 AM

Hi Joe,

 

I've tried to reproduce the issue while creating a local group policy for fiddler.exe, but to no avail (the program is starting and working as expected). Perhaps, some other GPO rules are blocking the execution?

Refer to the attached screenshots to see what I've used to create the rule.

Regards,
Nick Iliev
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