Asante FM2008 10/100 Ethernet switch backdoor login

[Joe Philipps <secfocus () joe philipps us>]

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.