Check MX record hostname validity

CHANGELOG.md

@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+### Fixed
+- Invalid hostname specified in MX record bypasses e-mail address revalidation
+
 ## [0.14.2] - 2023-10-25
 ### Fixed
 - IRMA session gets stuck in communicating status when user is requested to confirm PIN in `irmaclient`

server/keyshare/email.go

@@ -150,21 +150,33 @@ func VerifyMXRecord(email string) error {
 
 	host := email[strings.LastIndex(email, "@")+1:]
 
-	if records, err := net.LookupMX(host); err != nil || len(records) == 0 {
-		if err != nil {
-			if derr, ok := err.(*net.DNSError); ok && (derr.IsTemporary || derr.IsTimeout) {
-				// When DNS is not resolving or there is no active network connection
-				server.Logger.WithField("error", err).Error("No active network connection")
-				return ErrNoNetwork
-			}
+	records, err := net.LookupMX(host)
+
+	if err != nil || len(records) == 0 {
+		if derr, ok := err.(*net.DNSError); ok && (derr.IsTemporary || derr.IsTimeout) {
+			// When DNS is not resolving or there is no active network connection
+			server.Logger.WithField("error", err).Error("No active network connection")
+			return ErrNoNetwork
 		}
 
 		// Check if there is a valid A or AAAA record which is used as fallback by mailservers
 		// when there are no MX records present
 		if records, err := net.LookupIP(host); err != nil || len(records) == 0 {
 			return ErrInvalidEmailDomain
 		}
-		return nil
 	}
+
+	invalidHosts := 0
+	for _, h := range records {
+		// Check if host specified at MX record valid
+		if addr, err := net.LookupHost(h.Host); err != nil || len(addr) == 0 {
+			invalidHosts++
+		}
+	}
+
+	if invalidHosts >= len(records) {
+		return ErrInvalidEmailDomain
+	}
+
 	return nil
 }