How to Install New Cisco ASA 5505 Firewall Password Revcovery

Password Recovery Procedure

To recover passwords for the ASA, perform the following steps:

Step 1 Connect to the ASA console port according to the instructions in "Accessing the Command-Line Interface" section.

Step 2 Power off the ASA, and then power it on.

Step 3 After startup, press the Escape key when you are prompted to enter ROMMON mode.

Step 4 To update the configuration register value, enter the following command:

rommon #1> confreg 0x41

Update Config Register (0x41) in NVRAM...

Step 5 To set the ASA to ignore the startup configuration, enter the following command:

rommon #1> confreg

The ASA displays the current configuration register value, and asks whether you want to change it:

Current Configuration Register: 0x00000041

Configuration Summary: 

  boot default image from Flash

  ignore system configuration

Do you wish to change this configuration? y/n [n]: y

Step 6 Record the current configuration register value, so you can restore it later.

Step 7 At the prompt, enter Y to change the value.

The ASA prompts you for new values.

Step 8 Accept the default values for all settings. At the prompt, enter Y.

Step 9 Reload the ASA by entering the following command:

rommon #2> boot

Launching BootLoader...

Boot configuration file contains 1 entry.

Loading disk0:/asa800-226-k8.bin... Booting...Loading...

The ASA loads the default configuration instead of the startup configuration.

Step 10 Access the privileged EXEC mode by entering the following command:

hostname> enable

Step 11 When prompted for the password, press Enter.

The password is blank.

Source : 

https://supportforums.cisco.com/document/79016/asa-password-recovery#Password_Recovery_Procedure