port 25 ends DATA only at CRLF.CRLF: smtpd_forbid_bare_newline=normalize (port25-bare-newline)

Postfix 3.7.11 leaves its 2023 SMTP-smuggling defence off (default "no"
before 3.9, src/smtpd/smtpd.c:807), and the inbound relay's smtp/inet
listener did not turn it on: a "." line ended by a bare LF ended DATA and
what followed was read as the next transaction. The listener now sets
normalize (bare_lf_mask_table, smtpd.c:1656; the DATA loop drops such a
"." line, 3649-3652), the setting Postfix recommends and 3.9's default.
The submission ports keep the default (own-mta-builds, section 5).

Tests: e2e inbound_bare_newline_is_one_message sends the smuggling
sequence over raw SMTP (net/smtp's DotWriter would rewrite the bare LF)
and finds one message at the upstream with the smuggled part inside;
port25 config check asserts the parameter on smtp/inet. On the server:
make e2e exit 0 (511 s), go vet + go test 27 packages ok.

Docs: CHANGELOG [Unreleased] Security, security.md, architecture.md,
guide.md; roadmap: port25-bare-newline, base-trixie and
panel-dkim-signing agreed by the owner in this order.

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

CHANGELOG.md

@@ -5,6 +5,21 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
 
 ## [Unreleased]
 
+### Security
+
+- **Port 25 could be made to read a second transaction out of a message's
+  body.** Postfix 3.7 leaves its 2023 SMTP-smuggling defence off
+  (`smtpd_forbid_bare_newline`, default `no` before 3.9), and the inbound
+  relay's listener did not turn it on: a `.` line ended by a bare newline
+  ended DATA, and what followed was read as the next transaction's commands.
+  The listener now sets `normalize` - the setting Postfix recommends and 3.9
+  made the default: DATA ends only at the standard `<CR><LF>.<CR><LF>`, a
+  bare newline elsewhere is read as a line end. Found while reading for
+  [own-mta-builds](docs/own-mta-builds-2026-10-15.md) (§ 5); `test/e2e`
+  sends the sequence and finds one message at the upstream with all of it
+  inside. Only installations with `INBOUND_RELAY_ENABLE=true` or
+  `DMARC_REPORTS_ENABLE=true` have the listener.
+
 ## [2.6.0] - 2026-10-15
 
 Stage 5 of the 2026-10-12 audit: the image and Postfix. The supervisor

build/postfix-config.sh

@@ -643,7 +643,20 @@ if [ "${INBOUND_RELAY_ENABLE}" = "true" ] || [ "${DMARC_REPORTS_ENABLE}" = "true
 	# program name. Only smtpd carries it: cleanup and the rest are other
 	# services and log as postfix/.)
 	postconf -M "smtp/inet=smtp inet n - n - - smtpd"
+	# smtpd_forbid_bare_newline: the 2023 SMTP-smuggling defence, which this
+	# Postfix leaves off. normalize takes a bare <LF> as a line end but ends
+	# DATA only at the standard <CR><LF>.<CR><LF>: a "." line that a bare
+	# <LF> precedes or ends is dropped, not taken as the end of the message
+	# (Postfix 3.7.11, src/smtpd/smtpd.c:807, the default "no" for Postfix
+	# < 3.9; :1656, bare_lf_mask_table, normalize = BARE_LF_FLAG_WANT_STD_EOD;
+	# :3649-3652, the DATA loop; RELEASE_NOTES, "Major changes with Postfix
+	# 3.8.5, 3.7.10": normalize is the recommended setting and 3.9's
+	# default). Its exclusions default to $mynetworks, loopback only here
+	# (above), so no client is spared. The submission ports keep the
+	# default: their smuggled transaction would still be the authenticated
+	# session's, under its bindings and milters (own-mta-builds, § 5).
 	postconf -P \
+		"smtp/inet/smtpd_forbid_bare_newline=normalize" \
 		"smtp/inet/syslog_name=postfix/inbound" \
 		"smtp/inet/smtpd_sasl_auth_enable=no" \
 		"smtp/inet/smtpd_tls_auth_only=no" \

docs/architecture.md

@@ -189,6 +189,11 @@ smtp/inet` removes Debian's stock smtpd). Outbound delivery still uses the
   panel's sender check (§ Panel roles).
 - **Level-1 rate limit** — `smtpd_client_message_rate_limit` /
   `anvil_rate_time_unit` from `RATE_LIMIT_*` env vars; independent of milter.
+- **Port 25 ends DATA only at `<CR><LF>.<CR><LF>`** —
+  `smtpd_forbid_bare_newline=normalize` on the `smtp/inet` listener (Postfix
+  3.7.11, `src/smtpd/smtpd.c`, the DATA loop at 3649: a `.` line next to a
+  bare `<LF>` is dropped, not taken as the end); a bare `<LF>` elsewhere is
+  read as `<CR><LF>`. The submission ports keep Postfix's default.
 - **Chroot disabled** for all services (DNS/TLS inside container).
 - **TLS certs** — read-only mount at `TLS_CERT_FILE` / `TLS_KEY_FILE`, copied
   at start into `/etc/postfix/tls-internal`, owned by root and `0600` as

docs/guide.md

@@ -1010,7 +1010,11 @@ SelfPost never stores the message locally.
 container. Until then there is no inbound-relay `smtp inet` listener, no
 *Inbound* group in the menu, and `/inbound/…` is 404. (Port 25 can still
 listen when only `DMARC_REPORTS_ENABLE=true` — see [DMARC
-reports](#dmarc-reports).) Outbound 465/587 is unchanged.
+reports](#dmarc-reports).) Outbound 465/587 is unchanged. Port 25 ends a
+message only at the standard `<CR><LF>.<CR><LF>` (the SMTP-smuggling
+defence, since `2.6.1`); a sending server that ends DATA with a bare
+newline waits for a reply that does not come, and there is no allow-list
+for such a sender.
 
 **Panel** (`/inbound/domains`): the global role adds and deletes domains. On a
 domain's page, set the upstream host/port and TLS to that hop (opportunistic /

docs/roadmap.md

@@ -42,9 +42,9 @@ in `git log` and [CHANGELOG.md](../CHANGELOG.md).
 | settings-save-row | Server › Settings: the Save button no longer stands alone under a long right column | agreed (2026-10-15) | — | — |
 | dns-record-collapse | A DNS record that is right is shown folded: its name and its tag, the values on request | agreed (2026-10-15) | — | — |
 | own-mta-builds | Analysis: build Postfix and OpenDKIM ourselves, patched, instead of Debian's packages | analysed 2026-10-15: no own builds; three rows proposed instead | — | [own-mta-builds-2026-10-15.md](own-mta-builds-2026-10-15.md) |
-| port25-bare-newline | The inbound relay's port 25 refuses bare newlines in DATA (`smtpd_forbid_bare_newline=normalize`) | candidate (2026-10-15, from own-mta-builds; recommended now, PATCH) | — | — |
+| port25-bare-newline | The inbound relay's port 25 refuses bare newlines in DATA (`smtpd_forbid_bare_newline=normalize`) | agreed (2026-10-15; first of the three, PATCH) | — | — |
-| base-trixie | The image's base from Debian bookworm (LTS since 2026-07-12) to trixie: Postfix 3.10 with the 2026 security batches | candidate (2026-10-15, from own-mta-builds; recommended next, MINOR) | — | — |
+| base-trixie | The image's base from Debian bookworm (LTS since 2026-07-12) to trixie: Postfix 3.10 with the 2026 security batches | agreed (2026-10-15; after port25-bare-newline, MINOR) | — | — |
-| panel-dkim-signing | DKIM signing in the panel's own milter; OpenDKIM, the syslog relay and the parser port removed | candidate (2026-10-15, from own-mta-builds; recommended after base-trixie, MINOR) | — | — |
+| panel-dkim-signing | DKIM signing in the panel's own milter; OpenDKIM, the syslog relay and the parser port removed | agreed (2026-10-15; after base-trixie, MINOR) | — | — |
 | phone-desktop-mode | A phone browser in "desktop site" mode gets the phone navigation | agreed (2026-10-13: after stages 1 and 2 of audit-2026-10-12) | — | — |
 | from-header-binding | Hold a message's `From:` header against the application that sends it | shipped `2.4.0` | 9/9 | [plans/from-header-binding.md](plans/from-header-binding.md) |
 | panel-redesign | Panel redesign (accepted mockups, design contract) | shipped `2.0.0` | 27/27 | [plans/panel-redesign.md](plans/panel-redesign.md) |
@@ -784,8 +784,8 @@ is answered by three rows instead, in this order:
 Postfix 3.7.11 lacks the three 2026 security batches, which carry no CVE
 ids and so will not come), and
 [panel-dkim-signing](#panel-dkim-signing) (signing in the panel's own
-milter; OpenDKIM, the syslog relay and the parser port go). Each needs
+milter; OpenDKIM, the syslog relay and the parser port go). The owner
-the owner's OK.
+agreed to the three and their order on 2026-10-15.
 
 ## port25-bare-newline
 

docs/security.md

@@ -262,7 +262,14 @@ operator can act on it.
   is never accepted with nowhere to send it. Prefer recipient mode `list` to
   refuse unknown addresses at RCPT (no backscatter). OpenDKIM is not attached
   on inbound. An optional antispam milter is inbound-only; default action is
-  fail-open (`accept`).
+  fail-open (`accept`). Since `2.6.1` the listener ends DATA only at the
+  standard `<CR><LF>.<CR><LF>` (`smtpd_forbid_bare_newline=normalize`, the
+  2023 SMTP-smuggling defence, which Postfix 3.7 leaves off by default): a
+  `.` line next to a bare newline is body, and what a smuggler would put
+  behind it is delivered as part of the one message, not read as a second
+  transaction. The submission ports keep the default - there a smuggled
+  transaction is still the authenticated session's, under its sender
+  bindings and milters ([own-mta-builds](own-mta-builds-2026-10-15.md) § 5).
 - **The panel's milters judge every message of a connection.** Postfix
   introduces a client to a milter once per connection, and the milter library
   replaces the milter's session after any reply that is not "continue". Until

test/e2e/main_test.go

@@ -386,6 +386,9 @@ func TestE2E(t *testing.T) {
 	run("inbound_relay_delivers_and_refuses", func(t *testing.T) {
 		testInboundRelay(t, sc)
 	})
+	run("inbound_bare_newline_is_one_message", func(t *testing.T) {
+		testInboundBareNewline(t, sc)
+	})
 }
 
 // trimZoneSuffix strips the "e2e.test" suffix a fully-qualified DKIM record

test/e2e/port25_client.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"net"
 	"net/smtp"
+	"net/textproto"
 	"time"
 )
 
@@ -80,3 +81,53 @@ func waitForPort25Ready(timeout time.Duration) error {
 		return true, nil
 	})
 }
+
+// deliverPort25Raw is deliverPort25 with the DATA bytes written as they
+// stand. net/smtp's data writer would turn a bare LF into CRLF and stuff a
+// lone "." (net/textproto, DotWriter), which is the very input this is for.
+// It returns an error for a reply other than the one each step expects.
+func deliverPort25Raw(from, to string, data []byte) error {
+	conn, err := net.DialTimeout("tcp", port25Addr, 10*time.Second)
+	if err != nil {
+		return fmt.Errorf("dial %s: %w", port25Addr, err)
+	}
+	defer conn.Close()
+	_ = conn.SetDeadline(time.Now().Add(60 * time.Second))
+	tp := textproto.NewConn(conn)
+	expect := func(code int, after string) error {
+		if _, _, err := tp.ReadResponse(code); err != nil {
+			return fmt.Errorf("after %s: %w", after, err)
+		}
+		return nil
+	}
+	if err := expect(220, "connect"); err != nil {
+		return err
+	}
+	for _, step := range []struct {
+		cmd  string
+		code int
+	}{
+		{"HELO mx.stranger.e2e.test", 250},
+		{"MAIL FROM:<" + from + ">", 250},
+		{"RCPT TO:<" + to + ">", 250},
+		{"DATA", 354},
+	} {
+		if err := tp.PrintfLine("%s", step.cmd); err != nil {
+			return fmt.Errorf("%s: %w", step.cmd, err)
+		}
+		if err := expect(step.code, step.cmd); err != nil {
+			return err
+		}
+	}
+	if _, err := tp.W.Write(data); err != nil {
+		return fmt.Errorf("DATA content: %w", err)
+	}
+	if err := tp.W.Flush(); err != nil {
+		return fmt.Errorf("DATA content: %w", err)
+	}
+	if err := expect(250, "end of data"); err != nil {
+		return err
+	}
+	_ = tp.PrintfLine("QUIT")
+	return nil
+}

test/e2e/port25_config_check.go

@@ -51,6 +51,9 @@ var port25Cases = []port25Case{
 			// The log tailer tells this listener's refusals from the
 			// submission ports' by the name they are logged under.
 			{param: "syslog_name", contains: "postfix/inbound", wantExact: true},
+			// The SMTP-smuggling defence, off by default on this Postfix
+			// (own-mta-builds, § 5); testInboundBareNewline drives it.
+			{param: "smtpd_forbid_bare_newline", contains: "normalize", wantExact: true},
 			// No antispam milter configured: the inbound journal milter
 			// alone. The submission chain (strict OpenDKIM + journal-milter)
 			// must not reach inbound mail.

test/e2e/port25_features_test.go

@@ -6,6 +6,8 @@ import (
 	"encoding/base64"
 	"fmt"
 	"net/url"
+	"os"
+	"path/filepath"
 	"regexp"
 	"strings"
 	"testing"
@@ -456,3 +458,58 @@ func testInboundRelay(t *testing.T, sc *scenario) {
 		t.Fatalf("deny_domain.map after removing %s:\n%s", spammer, got)
 	}
 }
+
+// testInboundBareNewline: port 25 ends DATA only at the standard
+// <CR><LF>.<CR><LF>. A "." line that a bare <LF> precedes or ends - the SMTP
+// smuggling sequence, with a second transaction's commands behind it - is
+// body, and the one message delivered carries all of it; nothing is read as
+// a second transaction (smtpd_forbid_bare_newline=normalize on smtp/inet,
+// build/postfix-config.sh; Postfix 3.7.11, src/smtpd/smtpd.c:3649-3652).
+// Runs after testInboundRelay, whose domain, upstream and recipient it uses.
+func testInboundBareNewline(t *testing.T, sc *scenario) {
+	token := uniqueToken("inbound-bare")
+	smuggled := uniqueToken("smuggled")
+	data := "From: someone@stranger.e2e.test\r\nTo: " + inboundRecipient +
+		"\r\nSubject: e2e " + token + "\r\nDate: " + time.Now().UTC().Format(time.RFC1123Z) +
+		"\r\n\r\nthe first part\n.\n" +
+		"MAIL FROM:<someone@stranger.e2e.test>\r\nRCPT TO:<" + inboundRecipient + ">\r\nDATA\r\n" +
+		"Subject: e2e " + smuggled + "\r\n\r\nthe part a smuggler would send\r\n.\r\n"
+	if err := deliverPort25Raw("someone@stranger.e2e.test", inboundRecipient, []byte(data)); err != nil {
+		t.Fatalf("port 25 did not take the message with a bare-newline \".\" inside: %v\n%s", err, mailLogTail(h, "postfix/"))
+	}
+	raw, err := findSinkMessage(h.stageDir, token, 30*time.Second)
+	if err != nil {
+		t.Fatalf("the message never reached the upstream: %v\n%s", err, mailLogTail(h, "postfix/"))
+	}
+	if !bytes.Contains(raw, []byte(smuggled)) {
+		t.Fatalf("the message was cut at the bare-newline \".\": what followed it is not in the delivered message\n%s", raw)
+	}
+	// A second transaction, had Postfix read one, would be delivered right
+	// behind the first: give it the time and count the files that carry the
+	// smuggled token.
+	time.Sleep(3 * time.Second)
+	if n := sinkMessagesWith(t, h.stageDir, smuggled); n != 1 {
+		t.Fatalf("%d messages at the upstream carry the smuggled part, want 1\n%s", n, mailLogTail(h, "postfix/"))
+	}
+}
+
+// sinkMessagesWith counts the sink's dumped messages that contain token.
+func sinkMessagesWith(t *testing.T, stageDir, token string) int {
+	t.Helper()
+	dir := filepath.Join(stageDir, "mail-stage")
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		t.Fatalf("read %s: %v", dir, err)
+	}
+	n := 0
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		b, err := os.ReadFile(filepath.Join(dir, e.Name()))
+		if err == nil && bytes.Contains(b, []byte(token)) {
+			n++
+		}
+	}
+	return n
+}