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authorGuilhem Moulin <guilhem@fripost.org>2021-01-26 12:39:10 +0100
committerGuilhem Moulin <guilhem@fripost.org>2021-01-26 13:35:40 +0100
commit44100bab38d32596392a3bc7199b4daa202b4032 (patch)
tree3007929247752cb2957caaa4cd3d8938f39e0f34 /roles/out
parentdb5431e95fc6bc998169b272b30b5998798b56c1 (diff)
Postfix: pin key material to our MX:es for fripost.org and its subdomains.
This solves an issue where an attacker would strip the STARTTLS keyword from the EHLO response, thereby preventing connection upgrade; or spoof DNS responses to route outgoing messages to an attacker-controlled SMTPd, thereby allowing message MiTM'ing. With key material pinning in place, smtp(8postfix) immediately aborts the connection (before the MAIL command) and places the message into the deferred queue instead: postfix-out/smtp[NNN]: … dsn=4.7.5, status=undeliverable (Server certificate not verified) This applies to the smarthost as well as for verification probes on the Mail Submission Agent. Placing message into the deferred queue might yield denial of service, but we argue that it's better than a privacy leak. This only covers *internal messages* (from Fripost to Fripost) though: only messages with ‘fripost.org’ (or a subdomain of such) as recipient domain. Other domains, even those using mx[12].fripost.org as MX, are not covered. A scalable solution for arbitrary domains would involve either DANE and TLSA records, or MTA-STS [RFC8461]. Regardless, there is some merit in hardcoding our internal policy (when the client and server are both under our control) in the configuration. It for instance enables us to harden TLS ciphers and protocols, and makes the verification logic independent of DNS.
Diffstat (limited to 'roles/out')
-rw-r--r--roles/out/tasks/main.yml13
-rw-r--r--roles/out/templates/etc/postfix/main.cf.j25
-rw-r--r--roles/out/templates/etc/postfix/smtp_tls_policy.j212
3 files changed, 29 insertions, 1 deletions
diff --git a/roles/out/tasks/main.yml b/roles/out/tasks/main.yml
index 48c162a..7a297f1 100644
--- a/roles/out/tasks/main.yml
+++ b/roles/out/tasks/main.yml
@@ -28,6 +28,19 @@
owner=root group=root
mode=0644
+- name: Copy the SMTP TLS policy maps
+ template: src=etc/postfix/smtp_tls_policy.j2
+ dest=/etc/postfix-{{ postfix_instance[inst].name }}/smtp_tls_policy
+ owner=root group=root
+ mode=0644
+
+- name: Compile the SMTP TLS policy maps
+ postmap: cmd=postmap src=/etc/postfix-{{ postfix_instance[inst].name }}/smtp_tls_policy db=lmdb
+ owner=root group=root
+ mode=0644
+ notify:
+ - Reload Postfix
+
- meta: flush_handlers
- name: Start Postfix
diff --git a/roles/out/templates/etc/postfix/main.cf.j2 b/roles/out/templates/etc/postfix/main.cf.j2
index c05d9a5..f8aa55a 100644
--- a/roles/out/templates/etc/postfix/main.cf.j2
+++ b/roles/out/templates/etc/postfix/main.cf.j2
@@ -56,7 +56,10 @@ smtp_tls_protocols = !SSLv2, !SSLv3
smtp_tls_note_starttls_offer = yes
smtp_tls_session_cache_database = lmdb:$data_directory/smtp_tls_session_cache
-smtpd_tls_security_level = none
+smtp_tls_fingerprint_digest = sha256
+smtp_tls_policy_maps = lmdb:$config_directory/smtp_tls_policy
+
+smtpd_tls_security_level = none
strict_rfc821_envelopes = yes
smtpd_delay_reject = yes
diff --git a/roles/out/templates/etc/postfix/smtp_tls_policy.j2 b/roles/out/templates/etc/postfix/smtp_tls_policy.j2
new file mode 100644
index 0000000..7722dc8
--- /dev/null
+++ b/roles/out/templates/etc/postfix/smtp_tls_policy.j2
@@ -0,0 +1,12 @@
+# Lookup table matching next-hop destinations to TLS security policies;
+# this allows pining the key material for chosen recipient domains.
+#
+# {{ ansible_managed }}
+# Do NOT edit this file directly!
+{% for nexthop in ['fripost.org','.fripost.org'] %}
+
+{{ nexthop }} fingerprint ciphers=high protocols=!SSLv2:!SSLv3:!TLSv1:!TLSv1.1
+{% for h in groups.MX | sort %}
+ match={{ lookup('pipe', 'openssl pkey -pubin -outform DER <"certs/public/mx'+(hostvars[h].mxno | default('') | string)+'.fripost.org.pub" | openssl dgst -sha256 -c | sed "s/[^=]*=\s*//"') }}
+{% endfor %}
+{% endfor %}