March Update broke Fiddler Public API

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Last Updated: 01 Apr 2025 20:38 by Eric

Eric

Created on: 31 Mar 2025 15:12

Type: Bug Report

March Update broke Fiddler Public API

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.

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Failed to import NetLog
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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]
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OK
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2 comments

Eric

Posted on: 01 Apr 2025 20:38

While I'm very skeptical about the claim of a security problem (the code in question is managed C# and performs simple string processing), even if it was necessary to remove it, the correct way to do so would be to expose the existing WebFormats.JSON public interface with a different backing implementation.

As it stands now, you've broken an interface that's ~ a decade old, and this is really crummy.

ADMIN

Nick Iliev

Posted on: 01 Apr 2025 07:53

Hello Eric,

The extension has been removed from the latest version of Fiddler Classic because it was identified as a potential security risk during our assessment of the tool. Fiddler Classic now uses the .NET JSON parser instead of relying on a third-party dependency.

Regards,
Nick Iliev
Progress Telerik

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