adduser -d /some/path user

Match User user
ForceCommand internal-sftp
PasswordAuthentication yes
ChrootDirectory /some/path
PermitTunnel no
AllowAgentForwarding no
AllowTcpForwarding no
X11Forwarding no

chown root.root /some/
chown user.user /some/path

lftp -p 22 sftp://user@server
Password:
lftp [email protected]:~> ls
Fatal error: Host key verification failed.

ssh user@server should fix it