Add support for HTML parts

go.sum

@@ -37,6 +37,8 @@ github.com/mattn/go-isatty v0.0.9 h1:d5US/mDsogSGW37IV293h//ZFaeajb69h+EHFsv2xGg
 github.com/mattn/go-isatty v0.0.9/go.mod h1:YNRxwqDuOph6SZLI9vUUz6OYw3QyUt7WiY2yME+cCiQ=
 github.com/mattn/go-isatty v0.0.11 h1:FxPOTFNqGkuDUGi3H/qkUbQO4ZiBa2brKq5r0l8TGeM=
 github.com/mattn/go-isatty v0.0.11/go.mod h1:PhnuNfih5lzO57/f3n+odYbM4JtupLOxQOAqxQCu2WE=
+github.com/microcosm-cc/bluemonday v1.0.2 h1:5lPfLTTAvAbtS0VqT+94yOtFnGfUWYyx0+iToC3Os3s=
+github.com/microcosm-cc/bluemonday v1.0.2/go.mod h1:iVP4YcDBq+n/5fb23BhYFvIMq/leAFZyRl6bYmGDlGc=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=

plugins/base/public/assets/style.css

@@ -1 +1,5 @@
-/* TODO */
+iframe {
+	width: 100%;
+	height: 400px;
+	border: 0;
+}

plugins/base/public/message.html

@@ -112,7 +112,12 @@
 
 {{if .Body}}
 	<p><a href="{{.Message.Uid}}/reply?part={{.PartPath}}">Reply</a></p>
-	<pre>{{.Body}}</pre>
+	{{if .IsHTML}}
+		<!-- TODO: add a src fallback -->
+		<iframe srcdoc="{{.Body}}" sandbox></iframe>
+	{{else}}
+		<pre>{{.Body}}</pre>
+	{{end}}
 {{else}}
 	<p>Can't preview this message part.</p>
 	<a href="{{.Message.Uid}}/raw?part={{.PartPath}}">Download</a>

plugins/base/routes.go

@@ -16,6 +16,7 @@ import (
 	"github.com/emersion/go-message"
 	"github.com/emersion/go-smtp"
 	"github.com/labstack/echo/v4"
+	"github.com/microcosm-cc/bluemonday"
 )
 
 func registerRoutes(p *koushin.GoPlugin) {
@@ -162,6 +163,7 @@ type MessageRenderData struct {
 	Mailbox     *imap.MailboxStatus
 	Message     *IMAPMessage
 	Body        string
+	IsHTML      bool
 	PartPath    string
 	MailboxPage int
 	Flags       map[string]bool
@@ -216,6 +218,7 @@ func handleGetPart(ctx *koushin.Context, raw bool) error {
 
 		// TODO: set Content-Length if possible
 
+		// Be careful not to serve types like text/html as inline
 		if !strings.EqualFold(mimeType, "text/plain") || strings.EqualFold(disp, "attachment") {
 			dispParams := make(map[string]string)
 			if filename != "" {
@@ -241,6 +244,13 @@ func handleGetPart(ctx *koushin.Context, raw bool) error {
 		body = string(b)
 	}
 
+	isHTML := false
+	if strings.EqualFold(mimeType, "text/html") {
+		p := bluemonday.UGCPolicy()
+		body = p.Sanitize(body)
+		isHTML = true
+	}
+
 	flags := make(map[string]bool)
 	for _, f := range mbox.PermanentFlags {
 		f = imap.CanonicalFlag(f)
@@ -256,6 +266,7 @@ func handleGetPart(ctx *koushin.Context, raw bool) error {
 		Mailbox:        mbox,
 		Message:        msg,
 		Body:           body,
+		IsHTML:         isHTML,
 		PartPath:       partPathString,
 		MailboxPage:    int(mbox.Messages-msg.SeqNum) / messagesPerPage,
 		Flags:          flags,

server.go

@@ -302,7 +302,9 @@ func New(e *echo.Echo, options *Options) (*Server, error) {
 
 	e.Use(func(next echo.HandlerFunc) echo.HandlerFunc {
 		return func(ectx echo.Context) error {
-			ectx.Response().Header().Set("Content-Security-Policy", "default-src 'self'")
+			// `style-src 'unsafe-inline'` is required for e-mails with
+			// embedded stylesheets
+			ectx.Response().Header().Set("Content-Security-Policy", "default-src 'self'; style-src 'self' 'unsafe-inline'")
 			return next(ectx)
 		}
 	})