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").