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Show Shellshock the door, 18.10.2014 18:45

Lately the requests trying to exploit the Shellshock vulnerability are getting annoying. Of course my hosts are patched — even before the first such request arrived — and they are using Dash as /bin/sh anyway.
But this does not stop attackers from sending those requests. Some even seem to have programmed a loop which sends request after request even though their exploit is not working.

Since most of the requests are for valid URLs, the webserver just replies with a 200 status code and serves the content. As this gives no indication to the attacker whether his exploit worked or not, he has no reason to remove the host from his target-list and thus continues to send requests.

To break this pattern and signal that the host is not vulnerable to Shellshock, I came up with the nginx config snippet below. It recognizes Shellshock patterns in a request and replies with a '403 Forbidden' status code, thus indicating to an attacker that his request was blocked.

if ( $http_referer ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $http_user_agent ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $http_cookie ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $http_host ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $args ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $content_type ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $remote_user ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $request ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}
if ( $request_body ~ ^\s*\(\s*\)\s*\{ ) { 
        return 403 "Blocked by Shellshock protection (https://blog.x-way.org/Show-Shellshock-the-door).";
}

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