diff --git a/CHANGELOG.md b/CHANGELOG.md
index be257dddf..29bded250 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,13 +1,18 @@
-# 2019-01-xx
+# 2019-01-29
 
-## Running container processes as non-root
+## Running container processes as non-root, without capabilities and read-only
 
 To improve security, this playbook no longer starts container processes as the `root` user.
-
 Most containers were dropping privileges anyway, but we were trusting them with `root` privileges until they would do that.
 Not anymore -- container processes now start as a non-root user (usually `matrix`) from the get-go.
 
-For additional security, various [capabilities are also dropped](https://github.com/projectatomic/atomic-site/issues/203) for all containers.
+For additional security, various capabilities are also dropped (see [why it's important](https://github.com/projectatomic/atomic-site/issues/203)) for all containers.
+
+Additionally, most containers now use a read-only filesystem (see [why it's important](https://www.projectatomic.io/blog/2015/12/making-docker-images-write-only-in-production/)).
+Containers are given write access only to the directories they need to write to.
+
+A minor breaking change is the `matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size` variable having being renamed to `matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb` (note the `_mb` suffix). The new variable expects a number value (e.g. `25M` -> `25`).
+If you weren't customizing this variable, this wouldn't affect you.
 
 
 ## matrix-mailer is now based on Exim, not Postfix
diff --git a/group_vars/matrix-servers b/group_vars/matrix-servers
index 70d632563..7d4beccea 100644
--- a/group_vars/matrix-servers
+++ b/group_vars/matrix-servers
@@ -158,7 +158,7 @@ matrix_nginx_proxy_enabled: true
 
 matrix_nginx_proxy_proxy_matrix_client_api_addr_with_container: "{{ 'matrix-corporal:41080' if matrix_corporal_enabled else 'matrix-synapse:8008' }}"
 matrix_nginx_proxy_proxy_matrix_client_api_addr_sans_container: "{{ 'localhost:41080' if matrix_corporal_enabled else 'localhost:8008' }}"
-matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size: "{{ matrix_synapse_max_upload_size_mb }}M"
+matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb: "{{ matrix_synapse_max_upload_size_mb }}"
 
 matrix_nginx_proxy_proxy_matrix_enabled: true
 matrix_nginx_proxy_proxy_riot_enabled: "{{ matrix_riot_web_enabled }}"
diff --git a/roles/matrix-corporal/templates/systemd/matrix-corporal.service.j2 b/roles/matrix-corporal/templates/systemd/matrix-corporal.service.j2
index 3d8c083fc..4035aa651 100644
--- a/roles/matrix-corporal/templates/systemd/matrix-corporal.service.j2
+++ b/roles/matrix-corporal/templates/systemd/matrix-corporal.service.j2
@@ -13,6 +13,7 @@ ExecStart=/usr/bin/docker run --rm --name matrix-corporal \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
+			--read-only \
 			--network={{ matrix_docker_network }} \
 			{% if matrix_corporal_container_expose_ports %}
 			-p 127.0.0.1:41080:41080 \
diff --git a/roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2 b/roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2
index cf2c096e6..980ca9840 100644
--- a/roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2
+++ b/roles/matrix-coturn/templates/systemd/matrix-coturn.service.j2
@@ -13,6 +13,8 @@ ExecStart=/usr/bin/docker run --rm --name matrix-coturn \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
+			--read-only \
+			--tmpfs=/var/tmp:rw,noexec,nosuid,size=100m \
 			-p 3478:3478 \
 			-p 3478:3478/udp \
 			-p {{ matrix_coturn_turn_udp_min_port }}-{{ matrix_coturn_turn_udp_max_port }}:{{ matrix_coturn_turn_udp_min_port }}-{{ matrix_coturn_turn_udp_max_port }}/udp \
diff --git a/roles/matrix-mailer/templates/systemd/matrix-mailer.service.j2 b/roles/matrix-mailer/templates/systemd/matrix-mailer.service.j2
index d588ac0ef..54beb215c 100644
--- a/roles/matrix-mailer/templates/systemd/matrix-mailer.service.j2
+++ b/roles/matrix-mailer/templates/systemd/matrix-mailer.service.j2
@@ -11,6 +11,8 @@ ExecStart=/usr/bin/docker run --rm --name matrix-mailer \
 			--log-driver=none \
 			--user={{ matrix_mailer_container_user_uid }}:{{ matrix_mailer_container_user_gid }} \
 			--cap-drop=ALL \
+			--read-only \
+			--tmpfs=/var/spool/exim:rw,noexec,nosuid,size=100m \
 			--network={{ matrix_docker_network }} \
 			--env-file={{ matrix_mailer_base_path }}/env-mailer \
 			--hostname={{ hostname_matrix }} \
diff --git a/roles/matrix-mxisd/templates/systemd/matrix-mxisd.service.j2 b/roles/matrix-mxisd/templates/systemd/matrix-mxisd.service.j2
index 1835301a8..de0c06814 100644
--- a/roles/matrix-mxisd/templates/systemd/matrix-mxisd.service.j2
+++ b/roles/matrix-mxisd/templates/systemd/matrix-mxisd.service.j2
@@ -12,17 +12,23 @@ Wants={{ service }}
 Type=simple
 ExecStartPre=-/usr/bin/docker kill matrix-mxisd
 ExecStartPre=-/usr/bin/docker rm matrix-mxisd
+
+# mxisd writes an SQLite shared library (libsqlitejdbc.so) to /tmp and executes it from there,
+# so /tmp needs to be mounted with an exec option.
 ExecStart=/usr/bin/docker run --rm --name matrix-mxisd \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
+			--read-only \
+			--tmpfs=/tmp:rw,exec,nosuid,size=10m \
 			--network={{ matrix_docker_network }} \
 			{% if matrix_mxisd_container_expose_port %}
 			-p 127.0.0.1:8090:8090 \
 			{% endif %}
 			-v {{ matrix_mxisd_config_path }}:/etc/mxisd:ro \
-			-v {{ matrix_mxisd_data_path }}:/var/mxisd \
+			-v {{ matrix_mxisd_data_path }}:/var/mxisd:rw \
 			{{ matrix_mxisd_docker_image }}
+
 ExecStop=-/usr/bin/docker kill matrix-mxisd
 ExecStop=-/usr/bin/docker rm matrix-mxisd
 Restart=always
diff --git a/roles/matrix-nginx-proxy/defaults/main.yml b/roles/matrix-nginx-proxy/defaults/main.yml
index 78b2da123..94a86adc4 100644
--- a/roles/matrix-nginx-proxy/defaults/main.yml
+++ b/roles/matrix-nginx-proxy/defaults/main.yml
@@ -44,7 +44,10 @@ matrix_nginx_proxy_proxy_matrix_identity_api_addr_sans_container: "localhost:809
 matrix_nginx_proxy_proxy_matrix_client_api_addr_with_container: "matrix-synapse:8008"
 matrix_nginx_proxy_proxy_matrix_client_api_addr_sans_container: "localhost:8008"
 # This needs to be equal or higher than the maximum upload size accepted by Synapse.
-matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size: "25M"
+matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb: 25
+
+# The tmpfs at /tmp needs to be large enough to handle multiple concurrent file uploads.
+matrix_nginx_proxy_tmp_directory_size_mb: "{{ matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb * 50 }}"
 
 # A list of strings containing additional configuration blocks to add to the matrix domain's server configuration.
 matrix_nginx_proxy_proxy_matrix_additional_server_configuration_blocks: []
@@ -85,4 +88,4 @@ matrix_ssl_lets_encrypt_support_email: "{{ host_specific_matrix_ssl_lets_encrypt
 
 matrix_ssl_base_path: "{{ matrix_base_data_path }}/ssl"
 matrix_ssl_config_dir_path: "{{ matrix_ssl_base_path }}/config"
-matrix_ssl_log_dir_path: "{{ matrix_ssl_base_path }}/log"
\ No newline at end of file
+matrix_ssl_log_dir_path: "{{ matrix_ssl_base_path }}/log"
diff --git a/roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-synapse.conf.j2 b/roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-synapse.conf.j2
index 3638e3577..016b94145 100644
--- a/roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-synapse.conf.j2
+++ b/roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-synapse.conf.j2
@@ -116,7 +116,7 @@ server {
 		proxy_set_header X-Forwarded-For $remote_addr;
 
 		client_body_buffer_size 25M;
-		client_max_body_size {{ matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size }};
+		client_max_body_size {{ matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb }}M;
 		proxy_max_temp_file_size 0;
 	}
 
diff --git a/roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2 b/roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2
index 810797d75..410d6a01b 100644
--- a/roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2
+++ b/roles/matrix-nginx-proxy/templates/systemd/matrix-nginx-proxy.service.j2
@@ -12,10 +12,13 @@ Wants={{ service }}
 Type=simple
 ExecStartPre=-/usr/bin/docker kill matrix-nginx-proxy
 ExecStartPre=-/usr/bin/docker rm matrix-nginx-proxy
+
 ExecStart=/usr/bin/docker run --rm --name matrix-nginx-proxy \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
+			--read-only \
+			--tmpfs=/tmp:rw,noexec,nosuid,size={{ matrix_nginx_proxy_tmp_directory_size_mb }}m \
 			--network={{ matrix_docker_network }} \
 			-p 80:8080 \
 			-p 443:8443 \
@@ -24,6 +27,7 @@ ExecStart=/usr/bin/docker run --rm --name matrix-nginx-proxy \
 			-v {{ matrix_ssl_config_dir_path }}:{{ matrix_ssl_config_dir_path }}:ro \
 			-v {{ matrix_static_files_base_path }}:{{ matrix_static_files_base_path }}:ro \
 			{{ matrix_nginx_proxy_docker_image }}
+
 ExecStop=-/usr/bin/docker kill matrix-nginx-proxy
 ExecStop=-/usr/bin/docker rm matrix-nginx-proxy
 ExecReload=/usr/bin/docker exec matrix-nginx-proxy /usr/sbin/nginx -s reload
diff --git a/roles/matrix-postgres/tasks/import_postgres.yml b/roles/matrix-postgres/tasks/import_postgres.yml
index 9f1c15c08..67d7f86c5 100644
--- a/roles/matrix-postgres/tasks/import_postgres.yml
+++ b/roles/matrix-postgres/tasks/import_postgres.yml
@@ -73,7 +73,7 @@
 
 - name: Note about Postgres importing alternative
   debug:
-    msg: >
+    msg: >-
       Importing Postgres database using the following command: `{{ matrix_postgres_import_command }}`.
       If this crashes, you can stop Postgres (`systemctl stop matrix-postgres`),
       delete its existing data (`rm -rf {{ matrix_postgres_data_path }}/*`), start it again (`systemctl start matrix-postgres`)
diff --git a/roles/matrix-postgres/templates/systemd/matrix-postgres.service.j2 b/roles/matrix-postgres/templates/systemd/matrix-postgres.service.j2
index 89b5abb89..0d2b9fcf0 100644
--- a/roles/matrix-postgres/templates/systemd/matrix-postgres.service.j2
+++ b/roles/matrix-postgres/templates/systemd/matrix-postgres.service.j2
@@ -11,9 +11,12 @@ ExecStart=/usr/bin/docker run --rm --name matrix-postgres \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
+			--read-only \
+			--tmpfs=/tmp:rw,noexec,nosuid,size=100m \
+			--tmpfs=/run/postgresql:rw,noexec,nosuid,size=100m \
 			--network={{ matrix_docker_network }} \
 			--env-file={{ matrix_postgres_base_path }}/env-postgres-server \
-			-v {{ matrix_postgres_data_path }}:/var/lib/postgresql/data \
+			-v {{ matrix_postgres_data_path }}:/var/lib/postgresql/data:rw \
 			-v /etc/passwd:/etc/passwd:ro \
 			{{ matrix_postgres_docker_image_to_use }}
 ExecStop=-/usr/bin/docker stop matrix-postgres
diff --git a/roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2 b/roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2
index e79294891..5cd697746 100644
--- a/roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2
+++ b/roles/matrix-riot-web/templates/systemd/matrix-riot-web.service.j2
@@ -13,6 +13,8 @@ ExecStart=/usr/bin/docker run --rm --name matrix-riot-web \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
+			--read-only \
+			--tmpfs=/tmp:rw,noexec,nosuid,size=10m \
 			-v {{ matrix_riot_web_data_path }}/nginx.conf:/etc/nginx/nginx.conf:ro \
 			-v /dev/null:/etc/nginx/conf.d/default.conf:ro \
 			-v {{ matrix_riot_web_data_path }}/config.json:/etc/riot-web/config.json:ro \
diff --git a/roles/matrix-synapse/defaults/main.yml b/roles/matrix-synapse/defaults/main.yml
index a0c42b038..cc3eed178 100644
--- a/roles/matrix-synapse/defaults/main.yml
+++ b/roles/matrix-synapse/defaults/main.yml
@@ -39,6 +39,9 @@ matrix_synapse_max_upload_size_mb: 10
 matrix_synapse_max_log_file_size_mb: 100
 matrix_synapse_max_log_files_count: 10
 
+# The tmpfs at /tmp needs to be large enough to handle multiple concurrent file uploads.
+matrix_synapse_tmp_directory_size_mb: "{{ matrix_synapse_max_upload_size_mb * 50 }}"
+
 # Log levels
 # Possible options are defined here https://docs.python.org/3/library/logging.html#logging-levels
 # warning: setting log level to DEBUG will make synapse log sensitive information such
@@ -187,4 +190,4 @@ matrix_mautrix_whatsapp_enabled: false
 
 matrix_mautrix_whatsapp_docker_image: "tulir/mautrix-whatsapp:latest"
 
-matrix_mautrix_whatsapp_base_path: "{{ matrix_base_data_path }}/mautrix-whatsapp"
\ No newline at end of file
+matrix_mautrix_whatsapp_base_path: "{{ matrix_base_data_path }}/mautrix-whatsapp"
diff --git a/roles/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2 b/roles/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2
index 79b3478c2..a369190d0 100644
--- a/roles/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2
+++ b/roles/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2
@@ -18,11 +18,14 @@ ExecStartPre=-/usr/bin/docker rm matrix-synapse
 # we'd write files to the local filesystem and fusermount will complain.
 ExecStartPre=/bin/sleep 5
 {% endif %}
+
 ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
 			--cap-drop=ALL \
 			--entrypoint=python \
+			--read-only \
+			--tmpfs=/tmp:rw,noexec,nosuid,size={{ matrix_synapse_tmp_directory_size_mb }}m \
 			--network={{ matrix_docker_network }} \
 			-e SYNAPSE_CACHE_FACTOR={{ matrix_synapse_cache_factor }} \
 			{% if matrix_synapse_federation_enabled %}
@@ -31,14 +34,15 @@ ExecStart=/usr/bin/docker run --rm --name matrix-synapse \
 			{% if matrix_synapse_container_expose_client_server_api_port %}
 			-p 127.0.0.1:8008:8008 \
 			{% endif %}
-			-v {{ matrix_synapse_config_dir_path }}:/data \
-			-v {{ matrix_synapse_run_path }}:/matrix-run \
+			-v {{ matrix_synapse_config_dir_path }}:/data:ro \
+			-v {{ matrix_synapse_run_path }}:/matrix-run:rw \
 			-v {{ matrix_synapse_base_path }}/storage:/matrix-media-store-parent:slave \
 			{% for volume in matrix_synapse_container_additional_volumes %}
 			-v {{ volume.src }}:{{ volume.dst }}:{{ volume.options }} \
 			{% endfor %}
 			{{ matrix_synapse_docker_image }} \
 			-m synapse.app.homeserver -c /data/homeserver.yaml
+
 ExecStop=-/usr/bin/docker kill matrix-synapse
 ExecStop=-/usr/bin/docker rm matrix-synapse
 Restart=always