Asante FM2008 10/100 Ethernet switch backdoor login
The Asante FM2008 is an 8 port managed Ethernet 10/100 switch. It may be
managed, like many others in its device class, by Telnet, by serial port, by
HTTP, or by SNMP. Also like most similar devices, the serial port, HTTP, and
Telnet access methods require one to provide username/password credentials.
The firmware version "v01.06" has three UIs: Web browser, character cell
terminal (Telnet/serial port) with cursor positioning (hereafter just called
"terminal"), and CLI. The access control model of that firmware shares one set
of user-configurable credentials between the serial port, HTTP, and Telnet
access methods. The "normal" terminal interface limits the "username" and
"password" to eight characters each. The Web browser access method does not
appear to have such limitations. The "alternate," or CLI, interface can be
accessed via Telnet or the serial port by entering "superuser" for the username
and "asante" for the password. This CLI is not documented in the User Manual,
but one of the CLI commands is "help" which provides help (in English) of the
available commands and their parameters. There is no command I can find to
alter this set of credentials directly (although one of the memory address or
port alteration commands may be able to do this). The backdoor cr
edentials do not seem to be valid for the HTTP access method.
Another separate problem that could be considered a vulnerability is that
configuration backups (initiated via the Web browser interface and accomplished
in conjunction with a TFTP server) are not obscured (let alone encrypted) in
any way. This is how I discovered this backdoor set of credentials: examining
a TFTP dump of the switch's configuration.
If one simply edits the strings "superuser" and "asante" in the dump file and
restores the switch configuration via TFTP, upon switch restart the
configuration checksum is invalid and the firmware reverts to factory defaults.
If someone can figure out how to generate a proper checksum and insert it
into this TFTP file (and restore it to the switch), the backdoor might be
mitigated.
Although only a workaround and in my opinion not a fix, since the Web browser
interface does not place (known) limitations on the username length, the
user-configured username can be entered as "superuser" and the password can be
of one's choosing. (The terminal interface limits one to entering only
"superuse" on the configuration screen for this purpose. The switch does not
limit the number of characters to 8 when authenticating however.) The behavior
of the firmware is such that this would supercede the backdoor credentials,
thus making the backdoor and the CLI inaccessible.
The SNMP access method of course has its own community string authorization and
access control mechanisms, which is not covered in this message.
Since Asante has been made aware of this problem since August and has provided
no update or communication of any kind since September, I felt it has been an
awfully long time to fix what seems to be a simple firmware problem, and that
any users of this switch should know about this and implement the "superuser
password override" workaround outlined above ASAP.