UPDATE! Also works for revision E1
So, let’s say you have this router, the DIR-100 revision D1, I did. Being bored, I went the login screen but since the password was not the default I was unable login. So I cranked open Chromes fantastic development tools.
Now, I sent random values to try to capture some traffic and lo and behold there was a several requests made to /cliget.cgi?cmd=$… . The client-side code was loading configuration-data using XHR, including a partial MD5 of the password! Trying more values I discovered that if I requested /cliget.cgi?cmd=$, I would get the entire configuration file, including the username and password! The router was giving away admin access to everyone with access to the login page (LAN, or if remote admin is enabled LAN+WAN).
After watching a bit of WarGames (the 80’s flick that thaught us that every system has a help function) I got this funky idea, that would happen if I requested help from the router? Apparently, a lot, you get a list of every command in the system, including amongst other dhcps (lists all DHCP-clients), mem, reboot and GDB (supposedly not GNU GDB). One of the first thing i tried executing, was gdb which crashed the system, it might be that it triggers a debugger Connected to the internal 3-pin serial port. mem however is quite a different beast, it’s a memory monitor like debug.com of DOS-past. That’s right: you can read and write memory on the device without authentification.
In addition to all this, I have it from credible sources that the router is plagued by the wide-spread UPnP portmap-from-wan bug that was detected, which means that an attacker may use the router for connection-bouncing and/or presumably use the UPnP bug to gain access to the internal admin-panel.
I will however remark that there has been some mention of the cliget.cgi and cli.cgi APIs before http://forum.nag.ru/forum/index.php?showtopic=58050&st=20.
So, as any responsible man would do, I attempted to contact D-Link, to no avail. D-Link has also made it difficult to track down firmware upgrades for the router.
I ask that anyone handling this information does so with care, and that D-Link makes sufficient arrangements for their unprotected customers.