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Lab: Host validation bypass via connection state attack


This lab is vulnerable to routing-based SSRF via the Host header. Although the front-end server may initially appear to perform robust validation of the Host header, it makes assumptions about all requests on a connection based on the first request it receives.

To solve the lab, exploit this behavior to access an internal admin panel located at, then delete the user carlos.


Solving this lab requires features first released in Burp Suite 2022.8.1.

This lab is based on real-world vulnerabilities discovered by PortSwigger Research. For more details, check out Browser-Powered Desync Attacks: A New Frontier in HTTP Request Smuggling.


  1. Send the GET / request to Burp Repeater.

  2. Make the following adjustments:

    • Change the path to /admin.

    • Change Host header to

  3. Send the request. Observe that you are simply redirected to the homepage.

  4. Duplicate the tab, then add both tabs to a new group.

  5. Select the first tab and make the following adjustments:

    • Change the path back to /.

    • Change the Host header back to YOUR-LAB-ID.h1-web-security-academy.net.

  6. Using the drop-down menu next to the Send button, change the send mode to Send group in sequence (single connection).

  7. Change the Connection header to keep-alive.

  8. Send the sequence and check the responses. Observe that the second request has successfully accessed the admin panel.

  9. Study the response and observe that the admin panel contains an HTML form for deleting a given user. Make a note of the following details:

    • The action attribute (/admin/delete)

    • The name of the input (username)

    • The csrf token.

  10. On the second tab in your group, use these details to replicate the request that would be issued when submitting the form. The result should look something like this:

    POST /admin/delete HTTP/1.1 Host: Cookie: _lab=YOUR-LAB-COOKIE; session=YOUR-SESSION-COOKIE Content-Type: x-www-form-urlencoded Content-Length: CORRECT csrf=YOUR-CSRF-TOKEN&username=carlos
  11. Send the requests in sequence down a single connection to solve the lab.