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Trying to replicate "Client-side desync" vulnerability

Tim | Last updated: Aug 22, 2022 08:40PM UTC

Hello, I am using the current version of Burp Suite Pro, and have a couple of sites which are reporting (tentative) issues with Client-side desync vulnerabilities. I have read James Kettle's blog post, so think I have the general idea, but the two requests and their responses in the issue don't appear to contain what is being described in the advisory. It describes a POST request where the body is another GET request, and the Content-Length is being ignored to slide the second request in. However, the requests shown are just two separate requests, with their expected responses. Basically trying to understand how to replicate, so that I can confirm the issue as real (or false positive), demo to the client and so on. Cheers!

Liam, PortSwigger Agent | Last updated: Aug 23, 2022 07:56AM UTC

Hi Tim. Thanks for your message. Would it be possible to provide us with screenshots of the Issue detail and requests and responses? If so, you can email us via support@portswigger.net.

Tim | Last updated: Aug 23, 2022 03:43PM UTC

Thanks Liam, will do. Cheers, Tim

Tim | Last updated: Aug 23, 2022 03:47PM UTC

Actually, rather than screenshots that will have a hard time capturing details, I can export the issue directly. I'll send that first, can always take screenshots too if preferred.

Liam, PortSwigger Agent | Last updated: Aug 23, 2022 09:22PM UTC

Thanks Tim, we'll follow up in the email thread.

117AJ | Last updated: Oct 14, 2022 09:03PM UTC