If you don't care about the current configuration and don't mind reconfiguring from scratch then there is a very easy solution.
Connect to the console port of the sensor.
Power the sensor Off and back On.
During bootup there will be a Grub menu that popsup providing you with 2 options.
The first option boots the application partition.
The second option boots the recovery partition.
By default it will boot the application partition if nothing is entered after a few seconds.
During those few seconds you can interupt the process and select the 2nd boot option.
This will boot the recovery partition which will re-image the application partitions with completely new files and in the process reset the cisco user password back to the default "cisco" password.
This will also erase almost all of the rest of the sensor configuration.
If you need to know the current configuration and don't want to do the re-image, then try contacting Cisco TAC. They may have another option for breaking into the boot process and resetting the password.