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/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().)