Enforce name limits server-side and reject hidden runes in model names

Security review follow-ups. The 200-character registry-name cap the UI
forms promise now holds in domain.Validate for categories, tags,
merchants and instruments, so a non-browser client cannot persist an
unbounded name that every subsequent state response would carry. And a
model-supplied merchant or taxonomy name containing control or format
code points — bidi overrides, zero-width characters — is dropped like
an identifier-shaped one: React escaping already prevented injection,
but such names could visually spoof or reorder the review UI the
operator approves from.

internal/classification/client.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 	"sync/atomic"
 	"time"
+	"unicode"
 	"unicode/utf8"
 
 	"finance-duck/internal/domain"
@@ -251,6 +252,13 @@ func (c *Client) Classify(ctx context.Context, facts domain.Facts, data domain.D
 	return result, nil
 }
 
+// hasHiddenRunes reports control or format code points — bidi overrides,
+// zero-width characters — that would let model-supplied text spoof or
+// reorder review UI. Legitimate payee names never need them.
+func hasHiddenRunes(s string) bool {
+	return strings.ContainsFunc(s, func(r rune) bool { return unicode.IsControl(r) || unicode.Is(unicode.Cf, r) })
+}
+
 // resolveAnswer maps one schema-valid provider answer onto enrichment,
 // revalidating every id against the local registry. proposed collects newly
 // minted merchants by normalized name so several rows resolved against the
@@ -280,11 +288,12 @@ func resolveAnswer(answer answer, facts domain.Facts, data domain.Dataset, candi
 	}
 	if answer.NewMerchant != nil {
 		name := strings.Join(strings.Fields(*answer.NewMerchant), " ")
-		// An identifier-shaped or oversized name is dropped, never stored, but
+		// An identifier-shaped, oversized or hidden-rune name is dropped,
-		// the row keeps its independently enum-validated category and tags: a
+		// never stored, but the row keeps its independently enum-validated
-		// legitimate payee whose spelling trips the redactor (observed in the
+		// category and tags: a legitimate payee whose spelling trips the
-		// field) must not lose its whole classification.
+		// redactor (observed in the field) must not lose its whole
-		if !utf8.ValidString(name) || utf8.RuneCountInString(name) > 100 || normalize(name) == "" || normalize(clean(name)) != normalize(name) {
+		// classification.
+		if !utf8.ValidString(name) || utf8.RuneCountInString(name) > 100 || normalize(name) == "" || normalize(clean(name)) != normalize(name) || hasHiddenRunes(name) {
 			// no merchant
 		} else if existing := duplicateMerchant(name, data.Merchants); existing != nil {
 			e.MerchantID = existing.ID

internal/classification/client_test.go

@@ -343,7 +343,7 @@ func TestTransactionAmountAndCounterpartyAreSent(t *testing.T) {
 }
 
 func TestUnsafeMerchantProposalDroppedWithoutLosingClassification(t *testing.T) {
-	for _, name := range []string{"Alice Privateperson", "DE89370400440532013000", "Bank 123456789", "reference secretpayment", strings.Repeat("x", 101)} {
+	for _, name := range []string{"Alice Privateperson", "DE89370400440532013000", "Bank 123456789", "reference secretpayment", strings.Repeat("x", 101), "Rent \u202Edeifirev \u2713", "zero\u200Bwidth"} {
 		t.Run(name, func(t *testing.T) {
 			f, d := fixture()
 			f.Counterparty = "Alice Privateperson"

internal/classification/propose.go

@@ -83,7 +83,7 @@ func normalizedProposalName(value string, max int) (string, error) {
 	if !utf8.ValidString(value) || value == "" || utf8.RuneCountInString(value) > max {
 		return "", errors.New("proposal name is blank, invalid UTF-8 or too long")
 	}
-	if strings.ContainsAny(value, "{}[]()<>/\\") || strings.Contains(value, "___") {
+	if strings.ContainsAny(value, "{}[]()<>/\\") || strings.Contains(value, "___") || hasHiddenRunes(value) {
 		return "", errors.New("proposal name is identifier-shaped")
 	}
 	return value, nil

internal/domain/domain.go

@@ -250,6 +250,11 @@ func validHint(s string) bool {
 	return utf8.ValidString(s) && utf8.RuneCountInString(s) <= 200
 }
 
+// validName bounds registry display names at the 200 runes every UI form
+// already enforces, so no client can persist an unbounded name that every
+// later state response would carry.
+func validName(s string) bool { return nonblank(s) && utf8.RuneCountInString(s) <= 200 }
+
 // ValidISIN reports a syntactically valid ISIN: two country letters, nine
 // alphanumerics and a check digit.
 func ValidISIN(s string) bool { return isinPattern.MatchString(s) }
@@ -292,7 +297,7 @@ func Validate(d Dataset) error {
 		if err := register(c.ID, "category"); err != nil {
 			return err
 		}
-		if !nonblank(c.Name) || !validHint(c.Hint) || (c.Kind != "expense" && c.Kind != "income") {
+		if !validName(c.Name) || !validHint(c.Hint) || (c.Kind != "expense" && c.Kind != "income") {
 			return fmt.Errorf("category %q: invalid name, hint or kind", c.ID)
 		}
 		categories[c.ID] = c
@@ -330,7 +335,7 @@ func Validate(d Dataset) error {
 		if err := register(t.ID, "tag"); err != nil {
 			return err
 		}
-		if !nonblank(t.Name) || !validHint(t.Hint) {
+		if !validName(t.Name) || !validHint(t.Hint) {
 			return fmt.Errorf("tag %q: name or hint invalid", t.ID)
 		}
 		tags[t.ID] = true
@@ -339,8 +344,8 @@ func Validate(d Dataset) error {
 		if err := register(m.ID, "merchant"); err != nil {
 			return err
 		}
-		if !nonblank(m.Name) {
+		if !validName(m.Name) {
-			return fmt.Errorf("merchant %q: name required", m.ID)
+			return fmt.Errorf("merchant %q: valid name of at most 200 characters required", m.ID)
 		}
 		if m.DefaultCategoryID != "" {
 			if _, ok := categories[m.DefaultCategoryID]; !ok || children[m.DefaultCategoryID] {
@@ -375,7 +380,7 @@ func Validate(d Dataset) error {
 		if other, ok := isins[v.ISIN]; ok {
 			return fmt.Errorf("instrument %q: ISIN %s already held by %q", v.ID, v.ISIN, other)
 		}
-		if !nonblank(v.Name) || !currencyPattern.MatchString(v.Currency) || !validText(v.Symbol) {
+		if !validName(v.Name) || !currencyPattern.MatchString(v.Currency) || !validText(v.Symbol) {
 			return fmt.Errorf("instrument %q: valid UTF-8 name and symbol and three-letter uppercase currency required", v.ID)
 		}
 		// A quote without its day cannot be judged stale, and a day without a

internal/domain/domain_test.go

@@ -75,6 +75,8 @@ func TestDomainRejectsBrokenReferencesAndTaxonomy(t *testing.T) {
 		{"duplicate identity", func(d *Dataset) { d.Tags[0].ID = "acc_main" }},
 		{"invalid provenance date", func(d *Dataset) { d.Transactions[0].Enrichment.Classification.Timestamp = "yesterday" }},
 		{"nonleaf merchant default", func(d *Dataset) { d.Merchants[0].DefaultCategoryID = "cat_food" }},
+		{"oversized tag name", func(d *Dataset) { d.Tags[0].Name = strings.Repeat("x", 201) }},
+		{"oversized category name", func(d *Dataset) { d.Categories[2].Name = strings.Repeat("x", 201) }},
 	}
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {