Closes what the security review left as an accepted risk: with the
spam filter down - it is attached fail-open - nothing removes verdict
headers a sender wrote himself, and the inbound journal recorded his
verdict as the filter's. The owner asked whether rspamd could add a
header that proves the others are ours (2026-10-12) and agreed to a
signature: a fixed mark could be written by a sender as easily as the
verdict.
- inbound.FilterKey: a key of 32 random bytes, 64 hex characters, in
/data/antispam/filter.key, made on the first start with the relay
on. 0644 like the list files beside it: the filter is another user
in another container.
- deploy/antispam/rspamd/milter_headers.conf: a custom routine reads
the key through the directory the sidecar already mounts and adds
X-SelfPost-Filter, the HMAC-SHA256 of the message's queue id, hex;
a header of that name from the sender is removed.
- internal/milter/inbound.go: the verdict headers are kept as they
are read, and at end-of-message - when the queue id is known - they
count only if the signature fits (FilterSignature, hmac.Equal). No
key, no queue id, no signature or a wrong one: the row is that of
mail nobody judged (accept, selfpost, no detail).
The signature goes on to the upstream with the mail and is of no use
there: it fits one queue id.
Tried in a throwaway rspamd 3.11 container on the test server before
the Go side was written: the routine loads, signs
(2aa1ca6d... for key 0123...cdef and queue id 4A1B2C3D5E, the value
python's hmac gives and TestFilterSignatureIsWhatTheFilterWrites
pins), removes a sender's header of the same name, and adds nothing
without a queue id.
Tests: signed, unsigned, made-up, another message's, another key's,
the sender's followed by the real one, the next message on the
connection, no queue id; the key made once, read after, replaced when
damaged, left alone by the lists. e2e: filter.key is there, 644, 65
bytes.
docs/security.md: the accepted risk is replaced by a narrower one (the
key is readable on the host); Mail path describes the signature.
Checked on the test server, uncommitted tree: go vet ./... ok,
go test ./... ok (22 packages), make e2e ok.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Step 3/12 of docs/plans/inbound-antispam-panel.md.
The panel serves a second milter while the inbound relay is on
(internal/milter/inbound.go, socket INBOUND_JOURNAL_MILTER_SOCKET,
default /run/selfpost/inbound-journal.sock). build/postfix-config.sh
puts it last on port 25, after the optional antispam milter, with
default_action=accept; postfix-wrapper.sh waits for its socket. At
end-of-message it writes one inbound_spam_log row per recipient at an
inbound domain - other port-25 recipients (DMARC report addresses) get
none - and always accepts. With a filter attached it reads the verdict
from X-Rspamd-Action and X-Spamd-Result (action, score, up to six
weighted symbols, only what the patterns recognise, 255 bytes); without
one those headers are the sender's and are not read. The plan named
X-Rspamd-Score, which rspamd does not have.
The antispam compose fragment mounts rspamd/milter_headers.conf into
the sidecar so that it adds the two headers (checked in rspamd 3.11:
both routines exist and remove a sender's own headers of those names).
Store: InsertInboundSpam, IsInboundDomain.
Checked on the test server, uncommitted tree: go vet ./... ok,
go test ./... ok (22 packages), make e2e ok - the relayed message of
inbound_relay_delivers_and_refuses has its journal row (accept by
selfpost) and the DMARC report sent through the same listener has none.
Not yet seen in a real run: a verdict written by rspamd reaching this
milter. The e2e stand has no filter, and the test server's only inbound
domain has no upstream, so it accepts no mail.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
mixandClaude Opus 5.5