force <style></style> removal in html part of incoming emails

utils/email.go

@@ -6,6 +6,7 @@ import (
 	"encoding/pem"
 	"fmt"
 	"net/mail"
+	"regexp"
 	"strings"
 	"time"
 
@@ -16,6 +17,8 @@ import (
 	"maunium.net/go/mautrix/id"
 )
 
+var styleRegex = regexp.MustCompile("<style((.|\n|\r)*?)<\\/style>")
+
 // IncomingFilteringOptions for incoming mail
 type IncomingFilteringOptions interface {
 	SpamcheckSMTP() bool
@@ -82,11 +85,8 @@ func NewEmail(messageID, inReplyTo, references, subject, from, to, text, html st
 	}
 
 	if html != "" {
-		var err error
-		html, err = StripHTMLTag(html, "style")
-		if err == nil {
-			email.HTML = html
-		}
+		html = styleRegex.ReplaceAllString(html, "")
+		email.HTML = html
 	}
 
 	return email

utils/html.go

@@ -1,44 +0,0 @@
-package utils
-
-import (
-	"bytes"
-	"strings"
-
-	"golang.org/x/net/html"
-)
-
-// StripHTMLTag from text
-//
-// Source: https://siongui.github.io/2018/01/16/go-remove-html-inline-style/
-func StripHTMLTag(text, tag string) (string, error) {
-	doc, err := html.Parse(strings.NewReader(text))
-	if err != nil {
-		return "", err
-	}
-	stripHTMLTag(doc, tag)
-
-	var out bytes.Buffer
-	err = html.Render(&out, doc)
-	if err != nil {
-		return "", err
-	}
-
-	return out.String(), nil
-}
-
-func stripHTMLTag(node *html.Node, tag string) {
-	i := -1
-	for index, attr := range node.Attr {
-		if attr.Key == tag {
-			i = index
-			break
-		}
-	}
-	if i != -1 {
-		node.Attr = append(node.Attr[:i], node.Attr[i+1:]...)
-	}
-
-	for child := node.FirstChild; child != nil; child = child.NextSibling {
-		stripHTMLTag(child, tag)
-	}
-}