Harden safecsv against whitespace formula tricks

Ignore leading Unicode space when detecting formula cells,
extend starter runes, and document export usage in package
doc.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/safecsv/cell.go

@@ -20,6 +20,12 @@
 
 package safecsv
 
+import (
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
 // SanitizeRecord returns a copy of record with spreadsheet-safe cell values.
 func SanitizeRecord(record []string) []string {
 	if len(record) == 0 {
@@ -35,15 +41,31 @@ func SanitizeRecord(record []string) []string {
 }
 
 // SanitizeCell prefixes values that spreadsheet tools may interpret as formulas.
+// Leading Unicode whitespace (including newlines) is ignored for detection only;
+// the written cell keeps the original text with a leading single-quote escape.
 func SanitizeCell(value string) string {
 	if value == "" {
 		return value
 	}
 
-	switch value[0] {
-	case '=', '+', '-', '@', '\t', '\r':
-		return "'" + value
-	default:
+	trimmed := strings.TrimLeftFunc(value, unicode.IsSpace)
+	if trimmed == "" {
 		return value
 	}
+
+	r, _ := utf8.DecodeRuneInString(trimmed)
+	if formulaLeadingRune(r) {
+		return "'" + value
+	}
+
+	return value
+}
+
+func formulaLeadingRune(r rune) bool {
+	switch r {
+	case '=', '+', '-', '@', '\\', '|', '%':
+		return true
+	default:
+		return false
+	}
 }

pkg/safecsv/cell_test.go

@@ -36,6 +36,10 @@ func TestSanitizeCell(t *testing.T) {
 	assert.Equal(t, "'+cmd", SanitizeCell("+cmd"))
 	assert.Equal(t, "'-2", SanitizeCell("-2"))
 	assert.Equal(t, "'@sum", SanitizeCell("@sum"))
+	assert.Equal(t, "' =1+1", SanitizeCell(" =1+1"))
+	assert.Equal(t, "'\n=1+1", SanitizeCell("\n=1+1"))
+	assert.Equal(t, "'\\evil", SanitizeCell("\\evil"))
+	assert.Equal(t, "'-5", SanitizeCell("-5"))
 }
 
 func TestWriterWrite(t *testing.T) {

pkg/safecsv/doc.go

@@ -0,0 +1,29 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+// Package safecsv wraps encoding/csv for exports that may contain user-controlled
+// text. Writer sanitizes every cell before encoding to reduce spreadsheet formula
+// injection when a file is opened in Excel, LibreOffice Calc, Google Sheets, or
+// similar tools.
+//
+// Use safecsv for all user-facing CSV downloads. Do not write export CSV with
+// encoding/csv alone when record fields can contain untrusted or attacker-influenced
+// content.
+package safecsv