Ensure user IDs match the spec

event.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"fmt"
 	"reflect"
+	"regexp"
 	"strings"
 	"time"
 
@@ -709,11 +710,27 @@ func (e *Event) CheckFields() error { // nolint: gocyclo
 	return nil
 }
 
-func checkID(id, kind string, sigil byte) (domain string, err error) {
-	domain, err = domainFromID(id)
-	if err != nil {
+// https://matrix.org/docs/spec/appendices#id12
+var validUserIDRegex = regexp.MustCompile(`^[0-9a-z\._=\-/]+$`)
+
+func checkID(id, kind string, sigil byte) (domain ServerName, err error) {
+	var user string
+	if user, domain, err = SplitID(sigil, id); err != nil {
+		err = fmt.Errorf("gomatrixserverlib: invalid ID %q, invalid format: %w", id, err)
 		return
 	}
+	// Check that the characters in the ID are valid for the type
+	switch sigil {
+	case '@':
+		if len(id) > 255 {
+			err = fmt.Errorf("gomatrixserverlib: invalid ID %q, too long", id)
+			return
+		}
+		if !validUserIDRegex.MatchString(user) {
+			err = fmt.Errorf("gomatrixserverlib: invalid ID %q, user part %q contains invalid characters", id, user)
+			return
+		}
+	}
 	if id[0] != sigil {
 		err = fmt.Errorf(
 			"gomatrixserverlib: invalid %s ID, wanted first byte to be '%c' got '%c'",

event_test.go

@@ -180,3 +180,28 @@ func TestHeaderedEventToNewEventFromUntrustedJSON(t *testing.T) {
 		t.Fatal("expected an UnexpectedHeaderedEvent error but got:", err)
 	}
 }
+
+func TestValidUserID(t *testing.T) {
+	userIDsShouldPass := []string{
+		"@foo:bar.com",
+		"@foo-baz:bar.com",
+		"@foo_qux/baz:bar.com",
+		"@foo.baz:bar.com",
+	}
+	userIDsShouldFail := []string{
+		"@Foo:bar.com",
+		"@foo%:bar.com",
+		"@fooé:bar.com",
+		"@℉oo:bar.com",
+	}
+	for _, id := range userIDsShouldPass {
+		if _, err := checkID(id, "user", '@'); err != nil {
+			t.Fatalf("%q should have passed but didn't: %s", id, err)
+		}
+	}
+	for _, id := range userIDsShouldFail {
+		if _, err := checkID(id, "user", '@'); err == nil {
+			t.Fatalf("%q should have failed but didn't", id)
+		}
+	}
+}