attach: fix content-transfer-encoding for rfc822 attachments

Fix the content-transfer-encoding for attachments with message/rfc822
mime types.

It's not allowed by RFC2046 to set any other content-transfer-encoding
than 7bit, 8bit, or binary for message/rfc822 mime types (see RFC2046,
section 5.2.1).

We can enforce this by setting a content-transfer-encoding header for
attachments to 'binary' explicitly. 'binary' is more lenient than '8bit'
with respect to line length and CRLF semantics and thus seems more
suitable.

Link: https://datatracker.ietf.org/doc/html/rfc2046#section-5.2.1
Link: https://lists.sr.ht/~rjarry/aerc-devel/%3CD48A6YOQOXRG.3KKB6UTGMT8LY@maslowski.xyz%3E
Reported-by: Piotr Masłowski <piotr@maslowski.xyz>
Reported-by: inwit <inwit@sindominio.net>
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/attachment.go b/lib/attachment.go
index 4dd3f41d..63b8f161 100644
--- a/lib/attachment.go
+++ b/lib/attachment.go
@@ -90,6 +90,8 @@ func (fa *FileAttachment) WriteTo(w *mail.Writer) error {
 	// setting the filename auto sets the content disposition
 	ah.SetFilename(filename)
 
+	fixContentTransferEncoding(mimeType, &ah)
+
 	aw, err := w.CreateAttachment(ah)
 	if err != nil {
 		return errors.Wrap(err, "CreateAttachment")
@@ -127,6 +129,8 @@ func (pa *PartAttachment) WriteTo(w *mail.Writer) error {
 	// setting the filename auto sets the content disposition
 	ah.SetFilename(pa.Name())
 
+	fixContentTransferEncoding(pa.part.MimeType, &ah)
+
 	aw, err := w.CreateAttachment(ah)
 	if err != nil {
 		return errors.Wrap(err, "CreateAttachment")
@@ -174,3 +178,15 @@ func FindMimeType(filename string, reader *bufio.Reader) (string, map[string]str
 	// so we need to break them apart before passing them to the headers
 	return mime.ParseMediaType(mimeString)
 }
+
+// fixContentTransferEncoding checks the mime type of the attachment and
+// corrects the content-transfer-encoding if necessary.
+//
+// It's expressly forbidden by RFC2046 to set any other
+// content-transfer-encoding than 7bit, 8bit, or binary for
+// message/rfc822 mime types (see RFC2046, section 5.2.1)
+func fixContentTransferEncoding(mimeType string, header *mail.AttachmentHeader) {
+	if strings.ToLower(mimeType) == "message/rfc822" {
+		header.Add("Content-Transfer-Encoding", "binary")
+	}
+}