Improve URL validation for external wiki and external issues (#4710)

* Improve URL validation for external wiki  and external issues

* Do not allow also localhost address for external URLs

modules/validation/helpers.go

@@ -0,0 +1,77 @@
+// Copyright 2018 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package validation
+
+import (
+	"net"
+	"net/url"
+	"strings"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+var loopbackIPBlocks []*net.IPNet
+
+func init() {
+	for _, cidr := range []string{
+		"127.0.0.0/8", // IPv4 loopback
+		"::1/128",     // IPv6 loopback
+	} {
+		if _, block, err := net.ParseCIDR(cidr); err == nil {
+			loopbackIPBlocks = append(loopbackIPBlocks, block)
+		}
+	}
+}
+
+func isLoopbackIP(ip string) bool {
+	pip := net.ParseIP(ip)
+	if pip == nil {
+		return false
+	}
+	for _, block := range loopbackIPBlocks {
+		if block.Contains(pip) {
+			return true
+		}
+	}
+	return false
+}
+
+// IsValidURL checks if URL is valid
+func IsValidURL(uri string) bool {
+	if u, err := url.ParseRequestURI(uri); err != nil ||
+		(u.Scheme != "http" && u.Scheme != "https") ||
+		!validPort(portOnly(u.Host)) {
+		return false
+	}
+
+	return true
+}
+
+// IsAPIURL checks if URL is current Gitea instance API URL
+func IsAPIURL(uri string) bool {
+	return strings.HasPrefix(strings.ToLower(uri), strings.ToLower(setting.AppURL+"api"))
+}
+
+// IsValidExternalURL checks if URL is valid external URL
+func IsValidExternalURL(uri string) bool {
+	if !IsValidURL(uri) || IsAPIURL(uri) {
+		return false
+	}
+
+	u, err := url.ParseRequestURI(uri)
+	if err != nil {
+		return false
+	}
+
+	// Currently check only if not loopback IP is provided to keep compatibility
+	if isLoopbackIP(u.Hostname()) || strings.ToLower(u.Hostname()) == "localhost" {
+		return false
+	}
+
+	// TODO: Later it should be added to allow local network IP addreses
+	//       only if allowed by special setting
+
+	return true
+}