[release-branch.go1.20] net/mail: permit more characters in mail headers

We parse mail messages using net/textproto. For #53188, we tightened
up the bytes permitted by net/textproto to match RFC 7230.
However, this package uses RFC 5322 which is more permissive.
Restore the permisiveness we used to have, so that older code
continues to work.

For #58862
For #60332
Fixes #60874
Fixes #60875

Change-Id: I5437f5e18a756f6ca61c13c4d8ba727be73eff9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/504881
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

src/net/mail/message.go

@@ -14,6 +14,7 @@ Notable divergences:
     such as breaking addresses across lines.
   - No unicode normalization is performed.
   - The special characters ()[]:;@\, are allowed to appear unquoted in names.
+  - A leading From line is permitted, as in mbox format (RFC 4155).
 */
 package mail
 
@@ -53,7 +54,7 @@ type Message struct {
 func ReadMessage(r io.Reader) (msg *Message, err error) {
 	tp := textproto.NewReader(bufio.NewReader(r))
 
-	hdr, err := tp.ReadMIMEHeader()
+	hdr, err := readHeader(tp)
 	if err != nil {
 		return nil, err
 	}
@@ -64,6 +65,54 @@ func ReadMessage(r io.Reader) (msg *Message, err error) {
 	}, nil
 }
 
+// readHeader reads the message headers from r.
+// This is like textproto.ReadMIMEHeader, but doesn't validate.
+// The fix for issue #53188 tightened up net/textproto to enforce
+// restrictions of RFC 7230.
+// This package implements RFC 5322, which does not have those restrictions.
+// This function copies the relevant code from net/textproto,
+// simplified for RFC 5322.
+func readHeader(r *textproto.Reader) (map[string][]string, error) {
+	m := make(map[string][]string)
+
+	// The first line cannot start with a leading space.
+	if buf, err := r.R.Peek(1); err == nil && (buf[0] == ' ' || buf[0] == '\t') {
+		line, err := r.ReadLine()
+		if err != nil {
+			return m, err
+		}
+		return m, errors.New("malformed initial line: " + line)
+	}
+
+	for {
+		kv, err := r.ReadContinuedLine()
+		if kv == "" {
+			return m, err
+		}
+
+		// Key ends at first colon.
+		k, v, ok := strings.Cut(kv, ":")
+		if !ok {
+			return m, errors.New("malformed header line: " + kv)
+		}
+		key := textproto.CanonicalMIMEHeaderKey(k)
+
+		// Permit empty key, because that is what we did in the past.
+		if key == "" {
+			continue
+		}
+
+		// Skip initial spaces in value.
+		value := strings.TrimLeft(v, " \t")
+
+		m[key] = append(m[key], value)
+
+		if err != nil {
+			return m, err
+		}
+	}
+}
+
 // Layouts suitable for passing to time.Parse.
 // These are tried in order.
 var (

src/net/mail/message_test.go

@@ -39,6 +39,34 @@ So, "Hello".
 		},
 		body: "This is a message just to say hello.\nSo, \"Hello\".\n",
 	},
+	{
+		// RFC 5322 permits any printable ASCII character,
+		// except colon, in a header key. Issue #58862.
+		in: `From: [email protected]
+Custom/Header: v
+
+Body
+`,
+		header: Header{
+			"From":          []string{"[email protected]"},
+			"Custom/Header": []string{"v"},
+		},
+		body: "Body\n",
+	},
+	{
+		// RFC 4155 mbox format. We've historically permitted this,
+		// so we continue to permit it. Issue #60332.
+		in: `From [email protected] Mon Jun 19 00:00:00 2023
+From: [email protected]
+
+Hello, gophers!
+`,
+		header: Header{
+			"From":                               []string{"[email protected]"},
+			"From [email protected] Mon Jun 19 00": []string{"00:00 2023"},
+		},
+		body: "Hello, gophers!\n",
+	},
 }
 
 func TestParsing(t *testing.T) {