Need More Info
Last Updated: 12 Jul 2023 07:10 by Xiaoming
Created on: 08 Jun 2022 06:12
Type: Bug Report
Not able to "Exempt All" "AppContainer Loopback Exemption" in fiddler classic

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.


Posted on: 12 Jul 2023 07:10

I may know how to solve this problem, although it has been six months since the creation of this post. After selecting all the check boxes, there will be an application that prevents you from using such changes, such as the "EarTrumpet" application on my device. Selecting this application will cause an error. You can first check the check boxes one by one and make them effective, gradually identifying which application on your computer is causing this error.

This comment is translated by machine from other languages and hoping it could make a sense. Have a good day.

Posted on: 01 Jul 2022 19:35
Looking at the source of the Windows function, ERROR_INVALID_PARAMETER (0x57) can be returned for various AppContainer SID values based on other characteristics of the applications in question. The suggestion to specifically select the exact AppContainers you need to exempt is indeed your best workaround.
Posted on: 13 Jun 2022 17:42


Which version of Windows are you running? We have tested loopback exemption with the latest version of Fiddler Classic with both Win10 and Win11.

The tool that Fiddler uses for loopback exemptions has not been updated in the latest Fiddler release and is the same as previous versions. You can run it directly from the Fiddler installation directory - "%localappdata%\Programs\Fiddler\EnableLoopback.exe"

First I suggest that you try manually selecting the applications you want exempt instead of selecting all. If that does not work, check if the application you have selected has values in all data columns populated (especially the AC SID column).


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Posted on: 08 Jun 2022 06:14
By the way, this happened only on the latest Fiddler and I never faced this issue on older versions. And fiddler does not have repo of old version which sucks since new versions always have issues and breaks things before they become stable. So this policy of not keeping old versions does not make sense.