mime: fix panic parsing 'encoded-word', be stricter

Fixes #19416

Change-Id: I23c69ff637abaa202909f1cba6ed41b3cfe3d117
Reviewed-on: https://go-review.googlesource.com/37812Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/encodedword.go

@@ -194,22 +194,29 @@ type WordDecoder struct {
 
 // Decode decodes an RFC 2047 encoded-word.
 func (d *WordDecoder) Decode(word string) (string, error) {
-	if !strings.HasPrefix(word, "=?") || !strings.HasSuffix(word, "?=") || strings.Count(word, "?") != 4 {
+	// See https://tools.ietf.org/html/rfc2047#section-2
+	if len(word) < 9 || !strings.HasPrefix(word, "=?") || !strings.HasSuffix(word, "?=") || strings.Count(word, "?") != 4 {
 		return "", errInvalidWord
 	}
 	word = word[2 : len(word)-2]
 
 	// split delimits the first 2 fields
 	split := strings.IndexByte(word, '?')
-	// the field after split must only be one byte
-	if word[split+2] != '?' {
-		return "", errInvalidWord
-	}
 
 	// split word "UTF-8?q?ascii" into "UTF-8", 'q', and "ascii"
 	charset := word[:split]
+	if len(charset) == 0 {
+		return "", errInvalidWord
+	}
 	encoding := word[split+1]
+	// the field after split must only be one byte
+	if word[split+2] != '?' {
+		return "", errInvalidWord
+	}
 	text := word[split+3:]
+	if len(text) == 0 {
+		return "", errInvalidWord
+	}
 
 	content, err := decode(encoding, text)
 	if err != nil {

src/mime/encodedword_test.go

@@ -88,6 +88,8 @@ func TestDecodeWord(t *testing.T) {
 		{"=?UTF-8?Q?A=B?=", "", true},
 		{"=?UTF-8?Q?=A?=", "", true},
 		{"=?UTF-8?A?A?=", "", true},
+		{"=????=", "", true},
+		{"=?UTF-8?Q??=", "", true},
 	}
 
 	for _, test := range tests {