思科官方文檔說明：
To recover from the loss of passwords, perform the following steps:
Step 1 Connect to the security appliance console port by super terminal
Step 2 Power off the security appliance, and then power it on.
Step 3 During the startup messages, press the Escape key when prompted to enter ROMMON.
Step 4 To set the security appliance to ignore the startup configuration at reload, enter the following command:
rommon #1> confreg ( 0x41)在ROMMON模式下改寄存值爲:0x41,按BOOT重啓設備）
The security appliance displays the current configuration register value, and asks if you want to change
the value:
Current Configuration Register: 0x00000011
Configuration Summary:
boot TFTP image, boot default image from Flash on netboot failure
Do you wish to change this configuration? y/n [n]:
Step 5 Record your current configuration register value, so you can restore it later.
Step 6 At the prompt, enter Y to change the value.
The security appliance prompts you for new values.
Step 7 Accept the default values for all settings, except for the “disable system configuration?” value; at that
prompt, enter Y.
Step 8 Reload the security appliance by entering the following command:
rommon #2> boot
The security appliance loads a default configuration instead of the startup configuration.
Step 9 Enter privileged EXEC mode by entering the following command:
hostname> enable
Step 10 When prompted for the password, press Return.
The password is blank.
Step 11 Load the startup configuration by entering the following command:
hostname# copy startup-config running-config
Step 12 Enter global configuration mode by entering the following command:
hostname# configure terminal
Step 13 Change the passwords in the configuration by entering the following commands, as necessary:
hostname(config)# password cisco （配置密碼）
hostname(config)# enable password cisco（配置enable密碼）
hostname(config)# username name password cisco（配置用戶及密碼）
Step 14 Change the configuration register to load the startup configuration at the next reload by entering the
following command:
hostname(config)# config-register value （更改回寄存值爲：0x11)
Where value is the configuration register value you noted in Step 5. 0x1 is the default configuration
register.
Step 15 Save the new passwords to the startup configuration by entering the following command:
hostname(config)# copy running-config startup-config（保存配置重啓即可恢復密碼）
That's a completely procedure of restoring lost password on ASA5500 security appliance.