ASA Basic Access-Lists

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Access-lists are commonly used to implement traffic filtering in the ASA firewalls. By default, ASA permits sessions to be initiated from a higher security level interface to a lower security level interface. When traffic is allowed one way, a session state is created in the state/conn table and return traffic will be automatically allowed back in; thus, access-lists or security-level does not apply to the return traffic:

When an ACL is applied inbound on an interface, all inbound traffic is subject to the ACL rules, and the default security-level rule no longer applies; when an ACL is applied globally, all inbound traffic from all interfaces is subject to the global ACL, and the security-level rules no longer apply.