fix: convert non-UTF-8 email body charset to UTF-8

go.mod

@@ -34,6 +34,7 @@ require (
 	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
 	go.yaml.in/yaml/v2 v2.4.2 // indirect
 	golang.org/x/sys v0.42.0 // indirect
+	golang.org/x/text v0.35.0 // indirect
 	google.golang.org/protobuf v1.36.8 // indirect
 	modernc.org/libc v1.70.0 // indirect
 	modernc.org/mathutil v1.7.1 // indirect

go.sum

@@ -71,9 +71,12 @@ golang.org/x/oauth2 v0.35.0 h1:Mv2mzuHuZuY2+bkyWXIHMfhNdJAdwW3FuWeCPYN5GVQ=
 golang.org/x/oauth2 v0.35.0/go.mod h1:lzm5WQJQwKZ3nwavOZ3IS5Aulzxi68dUSgRHujetwEA=
 golang.org/x/sync v0.19.0 h1:vV+1eWNmZ5geRlYjzm2adRgW2/mcpevXNg50YZtPCE4=
 golang.org/x/sync v0.19.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=
+golang.org/x/sync v0.20.0 h1:e0PTpb7pjO8GAtTs2dQ6jYa5BWYlMuX047Dco/pItO4=
 golang.org/x/sys v0.6.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.42.0 h1:omrd2nAlyT5ESRdCLYdm3+fMfNFE/+Rf4bDIQImRJeo=
 golang.org/x/sys v0.42.0/go.mod h1:4GL1E5IUh+htKOUEOaiffhrAeqysfVGipDYzABqnCmw=
+golang.org/x/text v0.35.0 h1:JOVx6vVDFokkpaq1AEptVzLTpDe9KGpj5tR4/X+ybL8=
+golang.org/x/text v0.35.0/go.mod h1:khi/HExzZJ2pGnjenulevKNX1W67CUy0AsXcNubPGCA=
 golang.org/x/tools v0.42.0 h1:uNgphsn75Tdz5Ji2q36v/nsFSfR/9BRFvqhGBaJGd5k=
 golang.org/x/tools v0.42.0/go.mod h1:Ma6lCIwGZvHK6XtgbswSoWroEkhugApmsXyrUmBhfr0=
 google.golang.org/protobuf v1.36.8 h1:xHScyCOEuuwZEc6UtSOvPbAT4zRh0xcNRYekJwfqyMc=

modules/smtp/handler.go

@@ -19,6 +19,7 @@ import (
 	"github.com/buggregator/go-buggregator/internal/event"
 	"github.com/buggregator/go-buggregator/internal/storage"
 	gosmtp "github.com/emersion/go-smtp"
+	"golang.org/x/text/encoding/ianaindex"
 )
 
 // smtpServer wraps go-smtp and implements tcp.Starter.
@@ -226,6 +227,7 @@ func parseEmail(raw []byte, recipients []string) (*ParsedEmail, []parsedAttachme
 	if err != nil || !strings.HasPrefix(mediaType, "multipart/") {
 		body, _ := io.ReadAll(msg.Body)
 		decoded := decodeContent(body, msg.Header.Get("Content-Transfer-Encoding"))
+		decoded = convertToUTF8(decoded, params["charset"])
 		if strings.HasPrefix(mediaType, "text/html") {
 			parsed.HTML = string(decoded)
 		} else {
@@ -307,6 +309,7 @@ func processPart(part *multipart.Part, parsed *ParsedEmail, atts *[]parsedAttach
 	// Body content.
 	body, _ := io.ReadAll(part)
 	decoded := decodeContent(body, part.Header.Get("Content-Transfer-Encoding"))
+	decoded = convertToUTF8(decoded, params["charset"])
 
 	if strings.HasPrefix(mediaType, "text/html") {
 		parsed.HTML += string(decoded)
@@ -345,3 +348,21 @@ func decodeContent(data []byte, encoding string) []byte {
 		return data
 	}
 }
+
+// convertToUTF8 converts data from the given charset to UTF-8.
+// If charset is empty, "utf-8", or "us-ascii", the data is returned as-is.
+func convertToUTF8(data []byte, charset string) []byte {
+	charset = strings.ToLower(strings.TrimSpace(charset))
+	if charset == "" || charset == "utf-8" || charset == "us-ascii" {
+		return data
+	}
+	enc, err := ianaindex.MIME.Encoding(charset)
+	if err != nil || enc == nil {
+		return data
+	}
+	decoded, err := enc.NewDecoder().Bytes(data)
+	if err != nil {
+		return data
+	}
+	return decoded
+}

modules/smtp/handler_test.go

@@ -128,6 +128,99 @@ func TestDecodeContent(t *testing.T) {
 	})
 }
 
+func TestConvertToUTF8(t *testing.T) {
+	t.Run("utf-8 passthrough", func(t *testing.T) {
+		input := []byte("Привет мир")
+		got := convertToUTF8(input, "utf-8")
+		if string(got) != "Привет мир" {
+			t.Errorf("got %q", got)
+		}
+	})
+
+	t.Run("empty charset passthrough", func(t *testing.T) {
+		input := []byte("hello")
+		got := convertToUTF8(input, "")
+		if string(got) != "hello" {
+			t.Errorf("got %q", got)
+		}
+	})
+
+	t.Run("windows-1250 czech", func(t *testing.T) {
+		// "týmu se vám ozve" in windows-1250
+		win1250 := []byte{0x74, 0xFD, 0x6D, 0x75, 0x20, 0x73, 0x65, 0x20, 0x76, 0xE1, 0x6D, 0x20, 0x6F, 0x7A, 0x76, 0x65}
+		got := convertToUTF8(win1250, "windows-1250")
+		want := "týmu se vám ozve"
+		if string(got) != want {
+			t.Errorf("got %q, want %q", got, want)
+		}
+	})
+
+	t.Run("iso-8859-1 latin", func(t *testing.T) {
+		// "café" in iso-8859-1: 0x63 0x61 0x66 0xe9
+		latin1 := []byte{0x63, 0x61, 0x66, 0xe9}
+		got := convertToUTF8(latin1, "iso-8859-1")
+		if string(got) != "café" {
+			t.Errorf("got %q, want %q", got, "café")
+		}
+	})
+
+	t.Run("unknown charset passthrough", func(t *testing.T) {
+		input := []byte("data")
+		got := convertToUTF8(input, "x-nonexistent-999")
+		if string(got) != "data" {
+			t.Errorf("got %q", got)
+		}
+	})
+}
+
+func TestParseEmail_CharsetConversion(t *testing.T) {
+	t.Run("single part windows-1250", func(t *testing.T) {
+		// "café" in windows-1250: 0x63 0x61 0x66 0xe9
+		body := string([]byte{0x63, 0x61, 0x66, 0xe9})
+		raw := []byte("From: sender@example.com\r\nTo: recipient@example.com\r\nSubject: Test\r\nContent-Type: text/plain; charset=windows-1250\r\n\r\n" + body)
+
+		parsed, _, err := parseEmail(raw, []string{"recipient@example.com"})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if parsed.Text != "café" {
+			t.Errorf("Text = %q, want %q", parsed.Text, "café")
+		}
+	})
+
+	t.Run("single part html iso-8859-2", func(t *testing.T) {
+		// "Vaše zpráva" in iso-8859-2: V=56 a=61 š=B9 e=65 sp=20 z=7A p=70 r=72 á=E1 v=76 a=61
+		body := string([]byte{0x56, 0x61, 0xB9, 0x65, 0x20, 0x7A, 0x70, 0x72, 0xE1, 0x76, 0x61})
+		raw := []byte("From: sender@example.com\r\nTo: recipient@example.com\r\nSubject: Test\r\nContent-Type: text/html; charset=iso-8859-2\r\n\r\n" + body)
+
+		parsed, _, err := parseEmail(raw, []string{"recipient@example.com"})
+		if err != nil {
+			t.Fatal(err)
+		}
+		want := "Vaše zpráva"
+		if parsed.HTML != want {
+			t.Errorf("HTML = %q, want %q", parsed.HTML, want)
+		}
+	})
+
+	t.Run("multipart with charset", func(t *testing.T) {
+		// "café" in iso-8859-1
+		body := string([]byte{0x63, 0x61, 0x66, 0xe9})
+		raw := []byte("From: sender@example.com\r\nTo: recipient@example.com\r\nSubject: Test\r\nContent-Type: multipart/alternative; boundary=\"bnd\"\r\n\r\n--bnd\r\nContent-Type: text/plain; charset=iso-8859-1\r\n\r\n" + body + "\r\n--bnd\r\nContent-Type: text/html; charset=iso-8859-1\r\n\r\n<p>" + body + "</p>\r\n--bnd--")
+
+		parsed, _, err := parseEmail(raw, []string{"recipient@example.com"})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if !strings.Contains(parsed.Text, "café") {
+			t.Errorf("Text = %q, want to contain %q", parsed.Text, "café")
+		}
+		if !strings.Contains(parsed.HTML, "café") {
+			t.Errorf("HTML = %q, want to contain %q", parsed.HTML, "café")
+		}
+	})
+}
+
 func TestParseAddresses(t *testing.T) {
 	t.Run("name and email", func(t *testing.T) {
 		header := make(map[string][]string)