antispam: rspamd 4.2.2-0, no capabilities, only the milter on the network (audit S4)

Security audit 2026-10-17, finding S4 (low), stage 4. The sidecar ran
rspamd 3.11.1 (December 2024) with default capabilities; upstream is 4.2.2
(2026-10-10), and the line between fixed a heap overflow in
util.lower_utf8, a use-after-free on reload and parser crashes - rspamd
reads every byte a stranger sends to port 25.

A read-only reading of rspamd 3.11.1 and 4.2.2 and of rspamd-docker
v3.11.1+3 and v4.2.2+0, side by side (sources in dev/upstream), found every
function SelfPost relies on unchanged: milter_headers (skip_wanted, the
custom routine, remove 0, create_specific_keyed, get_queue_id from {i}),
the X-Spamd-Result text, multimap (from/smtp, ip, the reject pre-result,
file-map rereading), the image's uid 11333 and CMD. It also found that the
image binds the normal worker (which takes client-named file and shm
inputs) and the controller (default password "q1") on every address,
reachable from any container on the compose network - so:

- image rspamd/rspamd:4.2.2-0 (the one build of 4.2.2; the others float);
- cap_drop: [ALL], no-new-privileges (rspamd starts unprivileged, ports
  above 1024, nothing setuid);
- RSPAMD_LOCAL_ADDR=localhost keeps every worker on loopback, and a new
  antispam/rspamd/worker-proxy.inc puts the proxy alone on *:11332. rspamd
  expands its template syntax in comments too - the first version of that
  comment quoted it and the config failed to load; it quotes none now.

Checked in a throwaway container on the test server with the three files
and the fragment's options (dev/server/rspamd-try.sh): proxy on any
address, 11333/11334 on loopback; from a private address a sender's own
verdict headers removed and rspamd's added; X-SelfPost-Filter equal to
openssl's HMAC-SHA256 of the queue id; a denied address refused with the
deny list's message; healthy. The e2e has no rspamd; the live sidecar is
checked at the rollout. Citations re-pointed to 4.2.2.

Docs: guide § Inbound relay (the third file; copy antispam/ again);
CHANGELOG; plan.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/antispam/rspamd/milter_headers.conf

@@ -8,7 +8,7 @@
 # rspamd removes headers of these names that a sender wrote himself before it
 # adds its own - for every client, because of the two lines below. Without
 # them it would do neither for a client it counts as local or one that
-# authenticated: rspamd 3.11.1, src/plugins/lua/milter_headers.lua,
+# authenticated: rspamd 4.2.2 (as 3.11.1), src/plugins/lua/milter_headers.lua,
 # skip_wanted() - skip_local and skip_authenticated are on by default, and
 # "local" is loopback, a unix socket, or an address in local_addrs, the
 # private ranges (src/lua/lua_ip.c, lua_ip_is_local(); conf/options.inc). The
@@ -20,7 +20,9 @@
 # message carrying its own X-Spamd-Result and X-Rspamd-Action): without the
 # two lines, from 10.0.0.5 or as an authenticated user, the milter reply
 # removed neither header, added neither, and added the signature; with
-# them, both removed and rspamd's own added, as from a public address.
+# them, both removed and rspamd's own added, as from a public address. The
+# same again on 2026-10-17 in rspamd/rspamd:4.2.2-0 (from 10.0.0.5), and the
+# signature equal to HMAC-SHA256 of the queue id computed with openssl.
 #
 # Without this file the journal still lists accepted mail, with no verdict
 # next to it.
@@ -34,7 +36,7 @@
 # signature travels on with the mail and is of no use to whoever sees it: it
 # fits that one queue id.
 #
-# rspamd 3.11.1, src/plugins/lua/milter_headers.lua: a custom routine is called
+# rspamd 4.2.2 (as 3.11.1), src/plugins/lua/milter_headers.lua: a custom routine is called
 # as function(task, common_meta) and returns error, headers to add, headers to
 # remove, metadata; a remove value of 0 removes every header of that name
 # (docs.rspamd.com/modules/milter_headers, "Custom routines").

deploy/antispam/rspamd/multimap.conf

@@ -3,7 +3,7 @@
 # The allow and deny lists kept in the panel (Inbound > Filter lists). SelfPost
 # writes them as six files under data/antispam/, which the fragment mounts at
 # /etc/rspamd/selfpost; rspamd rereads a file when it changes, no restart.
-# (rspamd 3.11.1, src/libserver/maps/map.c: a file map is watched with libev's
+# (rspamd 4.2.2 (as 3.11.1), src/libserver/maps/map.c: a file map is watched with libev's
 # ev_stat - inotify where the kernel has it, else a stat poll every
 # map_watch_interval x map_file_watch_multiplier, 5min x 0.1, conf/options.inc;
 # docs.rspamd.com/configuration/maps asks for an atomic rename, which
@@ -14,7 +14,7 @@
 # allowed one is not accepted outright - rspamd would then let whichever of
 # the two rules ran first decide - but given a weight no spam score
 # outweighs, which a refusal by the deny list still overrides. (rspamd
-# 3.11.1, src/plugins/lua/multimap.lua: every rule is registered as a
+# 4.2.2 (as 3.11.1), src/plugins/lua/multimap.lua: every rule is registered as a
 # "normal" symbol, one with an action with the "passthrough" augmentation;
 # no "prefilter" setting is read. Measured with this file in a throwaway
 # rspamd/rspamd:3.11.1-3, 2026-10-15: every mix of an allow and a deny entry
@@ -29,7 +29,7 @@
 # bounce whose From: domain is on the deny list was refused by default and
 # is not with this setting; a sender on the list is refused either way.
 #
-# A domain entry is that domain, not its subdomains. (rspamd 3.11.1,
+# A domain entry is that domain, not its subdomains. (rspamd 4.2.2 (as 3.11.1),
 # src/libserver/maps/map_helpers.c: a hash map is looked up by the whole key,
 # case-insensitively - rspamd_icase_hash, rspamd_lc_cmp; "email:domain" gives
 # the address's domain part, src/plugins/lua/multimap.lua, apply_addr_filter().)

deploy/antispam/rspamd/worker-proxy.inc

@@ -0,0 +1,18 @@
+# Mounted into the sidecar as /etc/rspamd/local.d/worker-proxy.inc.
+#
+# The milter - the proxy worker - is the one listener SelfPost needs on the
+# compose network. The fragment sets RSPAMD_LOCAL_ADDR=localhost, which keeps
+# every worker on the container's loopback: the normal worker (11333), whose
+# scan accepts a client-named file or shared-memory input, and the controller
+# (11334), whose default password is the well-known "q1" (rspamd 4.2.2,
+# conf/rspamd.conf: each worker's bind_socket is the env variable LOCAL_ADDR,
+# localhost by default, and its port; conf/worker-controller.inc: password =
+# "q1"; the rspamd-docker v4.2.2+0 Dockerfile sets RSPAMD_LOCAL_ADDR=* for all
+# three). This line puts the proxy alone back on every address: a local.d
+# scalar at priority 1 replaces the default one (contrib/libucl/ucl_parser.c,
+# UCL_DUPLICATE_MERGE). The proxy still reaches the normal worker on
+# localhost, as before (security audit 2026-10-17, S4).
+#
+# rspamd expands its template syntax in these files, comments included:
+# quote none of it here.
+bind_socket = "*:11332";