milter: the sender check keeps two From: and two Sender: values (audit M5)

Security audit 2026-10-17, finding M5 (low), stage 0. senderSession.Header
appended every From:/Sender: value; Postfix bounds one header at 60 000
bytes but not their number, and the signer's 1 MiB cap runs after the
sender check, so an application could make it hold megabytes per
connection (the reader's test: a hundred lines, 6 MB). judge needs only
none / one / more, so two of each are kept; the refusal text joins what is
kept, as before.

Tests: "a hundred From: lines" in the judgement table (refused, more than
one From: line); TestSenderKeepsTwoFromAndSenderLines. go test
./internal/milter.

Docs: security.md (the sentence "a few headers each"); CHANGELOG; plan.

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

CHANGELOG.md

@@ -18,6 +18,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   removed, so `RCPT TO:<"; from=<x> to=<y> …"@domain>` spelled a second
   sender and recipient there, and the parser took the first. It now takes
   Postfix's own tail at the end of the line.
+- **The sender check keeps two `From:` and two `Sender:` lines** (audit M5),
+  not every one an application sends: the judgement needs no more, and a
+  hundred long lines were megabytes held per connection.
 - **The entrypoint follows no link at the DMARC maps** (audit I2): the
   three `dmarc_*` maps root creates at the start were missing from the list
   of paths where a symbolic link is removed first.

docs/security-audit-2026-10-17.md

@@ -704,8 +704,8 @@ the argument.
   tail may hold none of the neutered characters and each SASL field is one
   word (a `.*` SASL tail would have let a quoted sender spell the whole
   tail); TestInboundRecipientCannotWriteTheRow.
-- [ ] **M5.** Keep at most two `From:`/`Sender:` values; security.md:810
-  reworded. — **Opus**, 0.5 h.
+- [x] **M5.** Keep at most two `From:`/`Sender:` values; security.md:810
+  reworded. — **Opus**, 0.5 h. *Done 2026-10-17:* TestSenderKeepsTwoFromAndSenderLines.
 - [ ] **A4.** Duplicate-cookie line through `loggedPath`. — **Sonnet**, 0.25 h.
 - [ ] **S6.** `smtpd_etrn_restrictions = reject`, `smtpd_discard_ehlo_keywords
   = etrn` in postfix-config.sh; architecture.md § Postfix one line. Done when

docs/security.md

@@ -824,7 +824,10 @@ deferred item from the roadmap.
   `headerLimit`; `2.8.0`): it keeps no more than that and drops its list
   once the block it signs is built. No mail program writes such a section,
   and the refusal is permanent, so a sender is told rather than retried.
-  The other milters keep a few headers each (From, Sender, Subject).
+  The other milters keep a few headers each: the sender check two `From:`
+  and two `Sender:` values at most - it needs only none, one or more - and
+  the first Subject (until `2.14.4` it kept every `From:`/`Sender:` line,
+  which Postfix bounds in length but not in number; audit 2026-10-17, M5).
   Until `2.8.0` the milter library itself added every header of a message
   to a map of its own in each milter - up to the port's message size limit,
   from strangers on port 25 - which none of the milters read; the panel

internal/milter/sender.go

@@ -309,11 +309,18 @@ func (s *senderSession) RcptTo(rcpt string, m *milter.Modifier) (milter.Response
 
 func (s *senderSession) Header(name, value string, m *milter.Modifier) (milter.Response, error) {
 	// The name is trimmed: the obsolete syntax allows blanks before the colon.
+	// Two of each are kept: the judgement needs only none, one or more, and
+	// Postfix bounds a header's length but not their number (audit
+	// 2026-10-17, M5: a hundred 60 000-byte From: lines were 6 MB held).
 	switch textproto.CanonicalMIMEHeaderKey(strings.TrimSpace(name)) {
 	case "From":
+		if len(s.froms) < 2 {
 			s.froms = append(s.froms, value)
+		}
 	case "Sender":
+		if len(s.senders) < 2 {
 			s.senders = append(s.senders, value)
+		}
 	case "Subject":
 		if s.subject == "" {
 			s.subject = mailhdr.DecodeSubject(value)

internal/milter/sender_test.go

@@ -132,6 +132,7 @@ func TestSenderCheck(t *testing.T) {
 		{"a subdomain of its own", "shop", [][2]string{from("a@sub.shop.example.org")}, false, "not an address this application sends as"},
 		{"no From: line", "shop", [][2]string{{"Subject", "hello"}}, false, "no From: line"},
 		{"two From: lines", "shop", [][2]string{from("a@shop.example.org"), from("director@example.org")}, false, "more than one From: line"},
+		{"a hundred From: lines", "shop", manyFroms(100), false, "more than one From: line"},
 		{"two addresses in one line", "shop", [][2]string{from("a@shop.example.org, director@example.org")}, false, "one address"},
 		{"not an address", "shop", [][2]string{from("the shop")}, false, "as a mail program reads it"},
 		{"an address before a list comma, then another", "shop", [][2]string{from("director@example.org, <a@shop.example.org>")}, false, "one address"},
@@ -460,3 +461,24 @@ func TestSenderLineHeldToTheSameRule(t *testing.T) {
 		})
 	}
 }
+
+// manyFroms is n From: lines; the sender check keeps two of them (audit
+// 2026-10-17, M5).
+func manyFroms(n int) [][2]string {
+	out := make([][2]string, n)
+	for i := range out {
+		out[i] = [2]string{"From", "a@shop.example.org"}
+	}
+	return out
+}
+
+func TestSenderKeepsTwoFromAndSenderLines(t *testing.T) {
+	s := &senderSession{}
+	for i := 0; i < 100; i++ {
+		s.Header("From", "a@shop.example.org", nil)
+		s.Header("Sender", "b@shop.example.org", nil)
+	}
+	if len(s.froms) != 2 || len(s.senders) != 2 {
+		t.Errorf("kept %d From: and %d Sender: values, want 2 and 2", len(s.froms), len(s.senders))
+	}
+}