Last Updated: 24 Dec 2023 07:22 by Xriuk
Created on: 03 Jul 2023 07:28
Type: Bug Report
Fiddler Classic duplicates headers

Related issue: https://github.com/aws/aws-sdk-net/issues/2567

When sending multiple requests to the same domain, sometimes Fiddler alters the headers (in this case by duplicating the user-agent one), and in this case it causes a fail because a precomputed signature of the request does not match.

Posted on: 24 Dec 2023 07:22
No custom FiddlerScript was created
Posted on: 24 Dec 2023 02:57
There's nothing in Fiddler that could cause this on its own, but it could easily happen if you had written some buggy FiddlerScript that attempted to manipulate the User-Agent header. Have you looked at your script (Rules > Customize Rules) and/or checked for any browser extensions. Does this happen when loading arbitrary pages (e.g. bayden.com/ua.aspx) in your browser?
Nick Iliev
Posted on: 03 Jul 2023 11:21

Hey Xriuk,


Thank you for your feedback and observation of the unwanted behavior.

While we cannot yet reproduce the issue on our side, you can work around it by creating an AutoResponder rule that explicitly sets the User-Agent header with the specific value that AWS expects (so that it won't receive a double value).

Of course, you can set the rule only for specific AWS endpoints and not for the captured traffic (as shown above with the asterisk symbol).


Nick Iliev
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