I understand my mistake… I was using a user without the adequate privilege. Switching to a user with the proper privilege allowed me to change the passwords.
deploy:/home/ubuntu# confd_cli -u security-admin -C
<snip>
deploy(config)#aaa authentication users user
Possible completions:
Login name of the user admin oper range security-admin
deploy(config)#aaa authentication users user oper ?
Possible completions:
gid Group Identifier
homedir Absolute path to user's home directory
password
ssh_keydir Absolute path to directory where user's ssh keys may be found
uid User Identifier
<cr>
deploy(config)#aaa authentication users user oper
deploy(config-user-oper)#password
(<hash digest string>): ***