dmarc: no custom report address on the server's own name (audit W1)

Security audit 2026-10-17, finding W1, stage 1. HandleDomainDMARC took any
address validate.Email takes; AllowedRecipients opened every stored address
on the hostname, not only the domain's hosted one; checkReportDomain
cross-checked the claimed domain only against a dmarc-reports+<tag> local
part. So a domain user of tenant A could store ops@<hostname> and from then
on anyone could mail a report claiming tenant B's domain to it, stored and
shown under B and counted toward B's 500; or store B's hosted address and
open B's mailbox without B choosing hosted. TestAllowedRecipients asserted
ops@<host> was allowed - a choice, now reversed.

Now: a custom address on the hostname is refused on the form ("choose
SelfPost hosted"); AllowedRecipients adds a domain's address only when it
is that domain's own hosted address; checkReportDomain requires a tag and
the claimed domain to equal it. The live registry (checked 2026-10-17)
holds two hosted addresses and no custom one on the hostname, so nothing
there changes.

Tests: TestCustomReportAddressOnTheHostnameIsRefused (typed, another
domain's hosted, the domain's own hosted typed as custom; an address
elsewhere still saves); TestAllowedRecipientsFollowTheDomainAddresses
inverted plus another domain's hosted address stored; untagged addresses
rejected in TestIngestMessageRejectsTagMismatch; the ingest tests deliver
to tagged addresses.

Docs: security.md § Accepted risks (DMARC), guide § DMARC reports;
CHANGELOG; plan.

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

internal/dmarc/ingest.go

@@ -113,13 +113,19 @@ func IngestMessage(st *store.Store, r io.Reader, recipient string, receivedAt ti
 }
 
 // checkReportDomain refuses a report whose policy domain is not one of this
-// relay's sending domains, or that contradicts the +tag of the address it was
-// delivered to.
+// relay's sending domains, or is not the +tag of the address it was delivered
+// to. Every mailbox here is a hosted one, tagged with its domain
+// (AllowedRecipients), so an address without a tag is no mailbox of a domain
+// and its report belongs to none (security audit 2026-10-17, W1).
 func checkReportDomain(st *store.Store, domain, recipient string) error {
 	if domain == "" {
 		return rejected(fmt.Errorf("no policy domain"))
 	}
-	if tagged := TaggedDomain(recipient); tagged != "" && tagged != domain {
+	tagged := TaggedDomain(recipient)
+	if tagged == "" {
+		return rejected(fmt.Errorf("the address it was sent to (%s) is tagged with no domain", recipient))
+	}
+	if tagged != domain {
 		return rejected(fmt.Errorf("domain %q does not match the address it was sent to (%s)", domain, recipient))
 	}
 	ok, err := st.DomainExists(domain)

internal/dmarc/parse_test.go

@@ -101,7 +101,7 @@ func TestIngestMessageStoresReport(t *testing.T) {
 	// append raw gzip for simple single-part test
 	msg.Write(gz.Bytes())
 
-	if err := IngestMessage(st, msg, "dmarc-reports@mail.example.com", time.Now().UTC()); err != nil {
+	if err := IngestMessage(st, msg, "dmarc-reports+example.com@mail.example.com", time.Now().UTC()); err != nil {
 		t.Fatalf("IngestMessage: %v", err)
 	}
 	list, err := st.ListDMARCReports(store.DMARCReportScope{AllDomains: true}, 10)
@@ -131,7 +131,7 @@ func TestIngestMessageRejectsUnknownDomain(t *testing.T) {
 	}
 	defer st.Close()
 
-	err = IngestMessage(st, bytes.NewBufferString(gzipMessage(t, sampleXML)), "dmarc-reports@mail.example.com", time.Now().UTC())
+	err = IngestMessage(st, bytes.NewBufferString(gzipMessage(t, sampleXML)), "dmarc-reports+example.com@mail.example.com", time.Now().UTC())
 	if !errors.Is(err, ErrRejected) {
 		t.Fatalf("report for an unconfigured domain: err = %v, want ErrRejected", err)
 	}
@@ -164,6 +164,14 @@ func TestIngestMessageRejectsTagMismatch(t *testing.T) {
 	if !errors.Is(err, ErrRejected) {
 		t.Fatalf("report at an address tagged for another domain: err = %v, want ErrRejected", err)
 	}
+	// An address with no tag is no domain's mailbox, whatever the report
+	// claims (security audit 2026-10-17, W1).
+	for _, untagged := range []string{"ops@mail.example.com", "dmarc-reports@mail.example.com"} {
+		err = IngestMessage(st, bytes.NewBufferString(gzipMessage(t, sampleXML)), untagged, time.Now().UTC())
+		if !errors.Is(err, ErrRejected) {
+			t.Errorf("report at the untagged %s: err = %v, want ErrRejected", untagged, err)
+		}
+	}
 }
 
 // What cannot be read as a report is rejected, not put off: Postfix would

internal/dmarc/service.go

@@ -63,17 +63,20 @@ func (s *Service) AllowedRecipients() ([]string, error) {
 		seen[addr] = true
 		out = append(out, addr)
 	}
-	// Only what a domain has chosen is open: its report address opens the
-	// mailbox it names, if that is on this host. There is no mailbox for the
-	// server as a whole.
+	// Only what a domain has chosen is open, and only the one mailbox that is
+	// its own: the hosted address tagged with its name. A stored address on
+	// this host that is anything else - typed before 2.14.4, when the form
+	// still took one - opens nothing: it was an untagged mailbox, or another
+	// domain's, open to the internet (security audit 2026-10-17, W1). There
+	// is no mailbox for the server as a whole.
 	domains, err := s.store.ListDomains()
 	if err != nil {
 		return nil, err
 	}
 	for _, d := range domains {
-		// add keeps only addresses on this host, so a domain reporting elsewhere
-		// (or nowhere) opens nothing here.
-		add(d.DMARCRua)
+		if hosted := HostedReportAddress(s.hostname, d.Name); strings.EqualFold(strings.TrimSpace(d.DMARCRua), hosted) {
+			add(hosted)
+		}
 	}
 	sort.Strings(out)
 	return out, nil

internal/dmarc/service_test.go

@@ -32,11 +32,21 @@ func TestAllowedRecipientsFollowTheDomainAddresses(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// Another domain's hosted address, stored for this one: no mailbox
+	// either (W1's second shape).
+	stolen, _ := st.AddDomain("stolen.example", "sel")
+	if err := st.SetDomainDMARCAddress(stolen.ID, HostedReportAddress("mail.example.com", "victim.example")); err != nil {
+		t.Fatal(err)
+	}
+
 	got, err := NewService(st, nil, "mail.example.com", true).AllowedRecipients()
 	if err != nil {
 		t.Fatalf("AllowedRecipients: %v", err)
 	}
-	want := []string{"dmarc-reports+hosted.example@mail.example.com", "ops@mail.example.com"}
+	// Only the hosted address of the domain that chose it: an address typed
+	// on the server's name (ops@) opens nothing (security audit 2026-10-17,
+	// W1; this test asserted the opposite until then).
+	want := []string{"dmarc-reports+hosted.example@mail.example.com"}
 	if !reflect.DeepEqual(got, want) {
 		t.Errorf("allowed recipients = %v, want %v", got, want)
 	}