diff --git a/doc/source/configuration/openbao.rst b/doc/source/configuration/openbao.rst
index e83185019d..32dd871e7a 100644
--- a/doc/source/configuration/openbao.rst
+++ b/doc/source/configuration/openbao.rst
@@ -460,6 +460,8 @@ Enable the required TLS variables in kayobe and kolla
 
       kayobe overcloud host command run --command "systemctl restart kolla-nova_compute-container.service" --become --show-output -l compute
 
+.. _openbao-pulp-tls:
+
 Pulp TLS
 ========
 
diff --git a/doc/source/configuration/stackhpc-mixin-environments.rst b/doc/source/configuration/stackhpc-mixin-environments.rst
index fa8950e8b7..bc8ed55900 100644
--- a/doc/source/configuration/stackhpc-mixin-environments.rst
+++ b/doc/source/configuration/stackhpc-mixin-environments.rst
@@ -33,3 +33,10 @@ baremetal-policy
 ----------------
 
 .. include:: ../../../etc/kayobe/environments/baremetal-policy/README.rst
+
+.. _mixin-security:
+
+security
+--------
+
+.. include:: ../../../etc/kayobe/environments/security/README.rst
diff --git a/etc/kayobe/environments/security/README.rst b/etc/kayobe/environments/security/README.rst
new file mode 100644
index 0000000000..b753aea27a
--- /dev/null
+++ b/etc/kayobe/environments/security/README.rst
@@ -0,0 +1,96 @@
+Security stack
+~~~~~~~~~~~~~~
+
+Mixin environment that applies a security baseline to new and existing
+deployments. It bundles several security-related configuration modules into a
+single, opinionated environment:
+
+* **Firewall** — firewalld is enabled on all host types (controllers, compute,
+  storage, monitoring, infrastructure VMs, seed, and seed hypervisor) using the
+  standardised StackHPC firewalld zones and rules. Kolla Ansible is configured
+  to open ports in firewalld for services on the public API network. See
+  :ref:`firewall` for details.
+
+* **CIS benchmark hardening** — the
+  ``stackhpc_enable_cis_benchmark_hardening_hook`` flag is set to ``true``,
+  which means the CIS hardening playbooks run automatically as part of
+  ``kayobe * host configure``. See :doc:`security-hardening` for details.
+
+* **Walled garden** — a Squid caching proxy is enabled on the seed, and
+  overcloud hosts are configured to route HTTP/HTTPS traffic through it. NTP is
+  sourced from the seed node. Network connectivity checks are redirected to
+  ``localhost`` so they pass in environments without external Internet access.
+  See :doc:`walled-garden` for background.
+
+* **Pulp TLS** — TLS is enabled for the local Pulp server. Certificates must be
+  provided before deploying Pulp. See `Prerequisites`_ below.
+
+Prerequisites
+^^^^^^^^^^^^^
+
+Before activating this environment, ensure the following requirements are met.
+
+Pulp TLS certificates
+"""""""""""""""""""""
+
+TLS is enabled for Pulp. Certificates must be generated and configured before
+deploying Pulp. See :ref:`openbao-pulp-tls` for the full procedure.
+
+Pulp stack user password
+""""""""""""""""""""""""
+
+The ``pulp_stack_password`` variable is mandatory and must be set before
+running any Pulp-related playbooks. Define it in a secrets file or via the
+environment (never commit it to source control):
+
+.. code-block:: yaml
+   :caption: $KAYOBE_CONFIG_PATH/$KAYOBE_ENVIRONMENT/secrets.yml
+
+   pulp_stack_password: <your-password>
+
+Firewall network zones
+""""""""""""""""""""""
+
+Every network in ``networks.yml`` must have a zone defined. The standard
+approach is to assign the internal network zone to ``trusted`` and every other
+zone to the name of the network. See the :ref:`firewall` documentation and
+``etc/kayobe/environments/ci-multinode/networks.yml`` for a practical example.
+
+Consuming this environment
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Add the ``security`` environment to your ``.kayobe-environment`` file:
+
+.. code-block:: yaml
+   :caption: $KAYOBE_CONFIG_PATH/$KAYOBE_ENVIRONMENT/.kayobe-environment
+
+   dependencies:
+     - security
+
+Apply host configuration to enable the firewall and CIS hardening across all
+host types:
+
+.. code-block:: console
+
+   kayobe seed hypervisor host configure -t network,firewall
+   kayobe seed host configure -t network,firewall
+   kayobe infra vm host configure -t network,firewall
+   kayobe overcloud host configure -t network,firewall
+
+.. note::
+
+   Applying the firewall for the first time carries a risk of locking yourself
+   out of hosts. Read the safety guidance in :ref:`firewall` — in particular
+   the sections on using the ``firewalld-watchdog.yml`` playbook and applying
+   controller changes one at a time — before proceeding.
+
+.. note::
+
+   CIS hardening may require a reboot to take full effect. The CIS roles will
+   warn you when this is necessary.
+
+Redeploy Pulp to pick up the TLS and credential changes:
+
+.. code-block:: console
+
+   kayobe seed service deploy -t seed-deploy-containers -kt none
diff --git a/etc/kayobe/environments/security/compute.yml b/etc/kayobe/environments/security/compute.yml
new file mode 100644
index 0000000000..286c863045
--- /dev/null
+++ b/etc/kayobe/environments/security/compute.yml
@@ -0,0 +1,23 @@
+---
+###############################################################################
+# Compute node firewalld configuration.
+
+# Whether to install and enable firewalld.
+compute_firewalld_enabled: true
+
+# A list of zones to create. Each item is a dict containing a 'zone' item.
+compute_firewalld_zones: "{{ stackhpc_firewalld_zones }}"
+
+# A firewalld zone to set as the default. Default is unset, in which case
+# the default zone will not be changed.
+# Predefined zones are listed here:
+# https://firewalld.org/documentation/zone/predefined-zones.html
+compute_firewalld_default_zone: trusted
+
+# A list of firewall rules to apply. Each item is a dict containing
+# arguments to pass to the firewalld module. Arguments are omitted if not
+# provided, with the following exceptions:
+# - offline: true
+# - permanent: true
+# - state: enabled
+compute_firewalld_rules: "{{ stackhpc_firewalld_rules }}"
diff --git a/etc/kayobe/environments/security/controllers.yml b/etc/kayobe/environments/security/controllers.yml
new file mode 100644
index 0000000000..1a02bddef2
--- /dev/null
+++ b/etc/kayobe/environments/security/controllers.yml
@@ -0,0 +1,23 @@
+---
+###############################################################################
+# Controller node firewalld configuration.
+
+# Whether to install and enable firewalld.
+controller_firewalld_enabled: true
+
+# A list of zones to create. Each item is a dict containing a 'zone' item.
+controller_firewalld_zones: "{{ stackhpc_firewalld_zones }}"
+
+# A firewalld zone to set as the default. Default is unset, in which case
+# the default zone will not be changed.
+# Predefined zones are listed here:
+# https://firewalld.org/documentation/zone/predefined-zones.html
+controller_firewalld_default_zone: trusted
+
+# A list of firewall rules to apply. Each item is a dict containing
+# arguments to pass to the firewalld module. Arguments are omitted if not
+# provided, with the following exceptions:
+# - offline: true
+# - permanent: true
+# - state: enabled
+controller_firewalld_rules: "{{ stackhpc_firewalld_rules }}"
diff --git a/etc/kayobe/environments/security/infra-vms.yml b/etc/kayobe/environments/security/infra-vms.yml
new file mode 100644
index 0000000000..11f5ae3956
--- /dev/null
+++ b/etc/kayobe/environments/security/infra-vms.yml
@@ -0,0 +1,23 @@
+---
+###############################################################################
+# Infrastructure VM node firewalld configuration
+
+# Whether to install and enable firewalld.
+infra_vm_firewalld_enabled: true
+
+# A list of zones to create. Each item is a dict containing a 'zone' item.
+infra_vm_firewalld_zones: "{{ stackhpc_firewalld_zones }}"
+
+# A firewalld zone to set as the default. Default is unset, in which case
+# the default zone will not be changed.
+# Predefined zones are listed here:
+# https://firewalld.org/documentation/zone/predefined-zones.html
+infra_vm_firewalld_default_zone: trusted
+
+# A list of firewall rules to apply. Each item is a dict containing
+# arguments to pass to the firewalld module. Arguments are omitted if not
+# provided, with the following exceptions:
+# - offline: true
+# - permanent: true
+# - state: enabled
+infra_vm_firewalld_rules: "{{ stackhpc_firewalld_rules }}"
diff --git a/etc/kayobe/environments/security/inventory/group_vars/overcloud/proxy.yml b/etc/kayobe/environments/security/inventory/group_vars/overcloud/proxy.yml
new file mode 100644
index 0000000000..bab048b5fd
--- /dev/null
+++ b/etc/kayobe/environments/security/inventory/group_vars/overcloud/proxy.yml
@@ -0,0 +1,32 @@
+---
+# HTTP proxy URL (format: http(s)://[user:password@]proxy_name:port). By
+# default no proxy is used.
+http_proxy: "http://{{ lookup('vars', admin_oc_net_name ~ '_ips')[groups.seed.0] }}:3128"
+
+# HTTPS proxy URL (format: http(s)://[user:password@]proxy_name:port). By
+# default no proxy is used.
+https_proxy: "{{ http_proxy }}"
+
+# List of domains, hostnames, IP addresses and networks for which no proxy is
+# used. Defaults to ["127.0.0.1", "localhost", "{{ ('http://' ~
+# docker_registry) | urlsplit('hostname') }}"] if docker_registry is set, or
+# ["127.0.0.1", "localhost"] otherwise. This is configured only if either
+# http_proxy or https_proxy is set.
+no_proxy:
+  - "127.0.0.1"
+  - "localhost"
+  - "{{ ('http://' ~ docker_registry) | urlsplit('hostname') if docker_registry else '' }}"
+  - "{{ lookup('vars', admin_oc_net_name ~ '_ips')[groups.seed.0] }}"
+  - "{{ lookup('vars', admin_oc_net_name ~ '_ips')[inventory_hostname] }}"
+  - "{{ lookup('vars', internal_net_name ~ '_ips')[groups.controllers.0] }}"
+  - "{{ kolla_external_fqdn }}"
+  - "{{ kolla_internal_fqdn }}"
+
+# PyPI proxy URL (format: http(s)://[user:password@]proxy_name:port)
+pip_proxy: "{{ https_proxy }}"
+
+# Apt proxy URL for HTTP. Default is empty (no proxy).
+apt_proxy_http: "{{ http_proxy }}"
+
+# Apt proxy URL for HTTPS. Default is {{ apt_proxy_http }}.
+apt_proxy_https: "{{ https_proxy }}"
diff --git a/etc/kayobe/environments/security/inventory/group_vars/overcloud/time.yml b/etc/kayobe/environments/security/inventory/group_vars/overcloud/time.yml
new file mode 100644
index 0000000000..dec9761369
--- /dev/null
+++ b/etc/kayobe/environments/security/inventory/group_vars/overcloud/time.yml
@@ -0,0 +1,7 @@
+---
+# NTP services for overcloud hosts
+# During early initialisation we use the seed Node
+# Following deployment we include the OpenStack VIP
+
+chrony_ntp_servers:
+  - server: "{{ lookup('vars', admin_oc_net_name ~ '_ips')[groups.seed.0] }}"
diff --git a/etc/kayobe/environments/security/inventory/hosts b/etc/kayobe/environments/security/inventory/hosts
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/etc/kayobe/environments/security/kolla/globals.yml b/etc/kayobe/environments/security/kolla/globals.yml
new file mode 100644
index 0000000000..ae457b6a60
--- /dev/null
+++ b/etc/kayobe/environments/security/kolla/globals.yml
@@ -0,0 +1,3 @@
+---
+enable_external_api_firewalld: true
+external_api_firewalld_zone: "{{ public_net_name | net_zone }}"
diff --git a/etc/kayobe/environments/security/monitoring.yml b/etc/kayobe/environments/security/monitoring.yml
new file mode 100644
index 0000000000..7057bb8fe9
--- /dev/null
+++ b/etc/kayobe/environments/security/monitoring.yml
@@ -0,0 +1,23 @@
+---
+###############################################################################
+# Monitoring node firewalld configuration.
+
+# Whether to install and enable firewalld.
+monitoring_firewalld_enabled: true
+
+# A list of zones to create. Each item is a dict containing a 'zone' item.
+monitoring_firewalld_zones: "{{ stackhpc_firewalld_zones }}"
+
+# A firewalld zone to set as the default. Default is unset, in which case
+# the default zone will not be changed.
+# Predefined zones are listed here:
+# https://firewalld.org/documentation/zone/predefined-zones.html
+monitoring_firewalld_default_zone: trusted
+
+# A list of firewall rules to apply. Each item is a dict containing
+# arguments to pass to the firewalld module. Arguments are omitted if not
+# provided, with the following exceptions:
+# - offline: true
+# - permanent: true
+# - state: enabled
+monitoring_firewalld_rules: "{{ stackhpc_firewalld_rules }}"
diff --git a/etc/kayobe/environments/security/networks.yml b/etc/kayobe/environments/security/networks.yml
new file mode 100644
index 0000000000..77f91c5d02
--- /dev/null
+++ b/etc/kayobe/environments/security/networks.yml
@@ -0,0 +1,11 @@
+---
+# Kayobe network configuration.
+
+###############################################################################
+# Network connectivity check configuration.
+
+# External IP address to check. Default is 8.8.8.8.
+nc_external_ip: 127.0.0.1
+
+# External hostname to check. Default is google.com.
+nc_external_hostname: localhost
diff --git a/etc/kayobe/environments/security/pulp.yml b/etc/kayobe/environments/security/pulp.yml
new file mode 100644
index 0000000000..70e071d3d8
--- /dev/null
+++ b/etc/kayobe/environments/security/pulp.yml
@@ -0,0 +1,19 @@
+---
+###############################################################################
+# Local Pulp server configuration
+
+# Whether to enable TLS for Pulp.
+pulp_enable_tls: true
+
+# Path to a TLS certificate to use when TLS is enabled.
+pulp_cert_path: "{{ kayobe_env_config_path ~ '/pulp/certificates/pulp.crt' if pulp_enable_tls | bool else '' }}"
+
+# Path to a TLS key to use when TLS is enabled.
+pulp_key_path: "{{ kayobe_env_config_path ~ '/pulp/certificates/pulp.key' if pulp_enable_tls | bool else '' }}"
+
+###############################################################################
+# Local Pulp access credentials
+
+# Credentials for non-admin user within Pulp.
+pulp_stack_username: stack
+pulp_stack_password: "{{ notdefined | mandatory(msg='Please set a password for the stack pulp user') }}"
diff --git a/etc/kayobe/environments/security/seed-hypervisor.yml b/etc/kayobe/environments/security/seed-hypervisor.yml
new file mode 100644
index 0000000000..4aa9b534b7
--- /dev/null
+++ b/etc/kayobe/environments/security/seed-hypervisor.yml
@@ -0,0 +1,33 @@
+---
+###############################################################################
+# Seed hypervisor node firewalld configuration.
+
+# Whether to install and enable firewalld.
+seed_hypervisor_firewalld_enabled: true
+
+# A list of zones to create. Each item is a dict containing a 'zone' item.
+seed_hypervisor_firewalld_zones: "{{ stackhpc_firewalld_zones }}"
+
+# A firewalld zone to set as the default. Default is unset, in which case
+# the default zone will not be changed.
+# Predefined zones are listed here:
+# https://firewalld.org/documentation/zone/predefined-zones.html
+seed_hypervisor_firewalld_default_zone: trusted
+
+# A list of firewall rules to apply. Each item is a dict containing
+# arguments to pass to the firewalld module. Arguments are omitted if not
+# provided, with the following exceptions:
+# - offline: true
+# - permanent: true
+# - state: enabled
+seed_hypervisor_firewalld_rules: "{{ stackhpc_firewalld_rules }}"
+
+###############################################################################
+# Seed hypervisor sysctl configuration.
+
+# Dict of sysctl parameters to set.
+seed_hypervisor_sysctl_parameters:
+  # By default this is 1, which causes layer 2 traffic flowing through Linux
+  # bridges to pass through iptables. This blocks traffic from VMs (seed, wazuh) to
+  # the Internet.
+  net.bridge.bridge-nf-call-iptables: 0
diff --git a/etc/kayobe/environments/security/seed.yml b/etc/kayobe/environments/security/seed.yml
new file mode 100644
index 0000000000..939ded7d9b
--- /dev/null
+++ b/etc/kayobe/environments/security/seed.yml
@@ -0,0 +1,30 @@
+---
+###############################################################################
+# Seed node firewalld configuration.
+
+# Whether to install and enable firewalld.
+seed_firewalld_enabled: true
+
+# A list of zones to create. Each item is a dict containing a 'zone' item.
+seed_firewalld_zones: "{{ stackhpc_firewalld_zones }}"
+
+# A firewalld zone to set as the default. Default is unset, in which case
+# the default zone will not be changed.
+# Predefined zones are listed here:
+# https://firewalld.org/documentation/zone/predefined-zones.html
+seed_firewalld_default_zone: trusted
+
+# A list of firewall rules to apply. Each item is a dict containing
+# arguments to pass to the firewalld module. Arguments are omitted if not
+# provided, with the following exceptions:
+# - offline: true
+# - permanent: true
+# - state: enabled
+seed_firewalld_rules: "{{ stackhpc_firewalld_rules }}"
+
+###############################################################################
+# Seed node additional containers configuration
+
+# Seed container running a Squid caching proxy. This can be used to proxy
+# HTTP(S) requests from control plane hosts.
+seed_squid_container_enabled: true
diff --git a/etc/kayobe/environments/security/stackhpc.yml b/etc/kayobe/environments/security/stackhpc.yml
new file mode 100644
index 0000000000..0b612b6a04
--- /dev/null
+++ b/etc/kayobe/environments/security/stackhpc.yml
@@ -0,0 +1,6 @@
+---
+###############################################################################
+# Feature flags
+
+# Whether or not to run CIS benchmark hardening playbooks. Default is false.
+stackhpc_enable_cis_benchmark_hardening_hook: true
diff --git a/etc/kayobe/environments/security/storage.yml b/etc/kayobe/environments/security/storage.yml
new file mode 100644
index 0000000000..63c94d56a2
--- /dev/null
+++ b/etc/kayobe/environments/security/storage.yml
@@ -0,0 +1,23 @@
+---
+###############################################################################
+# Storage node firewalld configuration.
+
+# Whether to install and enable firewalld.
+storage_firewalld_enabled: true
+
+# A list of zones to create. Each item is a dict containing a 'zone' item.
+storage_firewalld_zones: "{{ stackhpc_firewalld_zones }}"
+
+# A firewalld zone to set as the default. Default is unset, in which case
+# the default zone will not be changed.
+# Predefined zones are listed here:
+# https://firewalld.org/documentation/zone/predefined-zones.html
+storage_firewalld_default_zone: trusted
+
+# A list of firewall rules to apply. Each item is a dict containing
+# arguments to pass to the firewalld module. Arguments are omitted if not
+# provided, with the following exceptions:
+# - offline: true
+# - permanent: true
+# - state: enabled
+storage_firewalld_rules: "{{ stackhpc_firewalld_rules }}"
diff --git a/releasenotes/notes/add-security-mixin-9ca2c2b29364b427.yaml b/releasenotes/notes/add-security-mixin-9ca2c2b29364b427.yaml
new file mode 100644
index 0000000000..e553ba02c1
--- /dev/null
+++ b/releasenotes/notes/add-security-mixin-9ca2c2b29364b427.yaml
@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Introduce the security enviroment mixin which aims to provide an easy
+    approach for bringing a deployment inline with security baseline and
+    best practices.