Fix open redirect vulnerability on login screen (#4312)

* Fix open redirect vulnerability on login screen

Signed-off-by: Jonas Franz <info@jonasfranz.software>

* Reorder imports

Signed-off-by: Jonas Franz <info@jonasfranz.software>

* Replace www. from Domain too

Signed-off-by: Jonas Franz <info@jonasfranz.software>

modules/util/util.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/setting"
 )
 
 // OptionalBool a boolean that can be "null"
@@ -78,6 +79,18 @@ func URLJoin(base string, elems ...string) string {
 	return joinedURL
 }
 
+// IsExternalURL checks if rawURL points to an external URL like http://example.com
+func IsExternalURL(rawURL string) bool {
+	parsed, err := url.Parse(rawURL)
+	if err != nil {
+		return true
+	}
+	if len(parsed.Host) != 0 && strings.Replace(parsed.Host, "www.", "", 1) != strings.Replace(setting.Domain, "www.", "", 1) {
+		return true
+	}
+	return false
+}
+
 // Min min of two ints
 func Min(a, b int) int {
 	if a > b {

modules/util/util_test.go

@@ -7,6 +7,8 @@ package util
 import (
 	"testing"
 
+	"code.gitea.io/gitea/modules/setting"
+
 	"github.com/stretchr/testify/assert"
 )
 
@@ -42,3 +44,36 @@ func TestURLJoin(t *testing.T) {
 		assert.Equal(t, test.Expected, URLJoin(test.Base, test.Elements...))
 	}
 }
+
+func TestIsExternalURL(t *testing.T) {
+	setting.Domain = "try.gitea.io"
+	type test struct {
+		Expected bool
+		RawURL   string
+	}
+	newTest := func(expected bool, rawURL string) test {
+		return test{Expected: expected, RawURL: rawURL}
+	}
+	for _, test := range []test{
+		newTest(false,
+			"https://try.gitea.io"),
+		newTest(true,
+			"https://example.com/"),
+		newTest(true,
+			"//example.com"),
+		newTest(true,
+			"http://example.com"),
+		newTest(false,
+			"a/"),
+		newTest(false,
+			"https://try.gitea.io/test?param=false"),
+		newTest(false,
+			"test?param=false"),
+		newTest(false,
+			"//try.gitea.io/test?param=false"),
+		newTest(false,
+			"/hey/hey/hey#3244"),
+	} {
+		assert.Equal(t, test.Expected, IsExternalURL(test.RawURL))
+	}
+}