Go: Improve QHelp for go/unvalidated-url-redirection.

The example showed a different (and better) fix from what the help claimed, but the suggestion also had a subtle bug that I fixed at the same time.

go/ql/src/Security/CWE-601/OpenUrlRedirect.qhelp

@@ -16,6 +16,10 @@ To guard against untrusted URL redirection, it is advisable to avoid putting use
 a redirect URL. Instead, maintain a list of authorized redirects on the server; then choose from
 that list based on the user input provided.
 </p>
+<p>
+If this is not possible, then the user input should be validated in some other way,
+for example, by verifying that the target URL is local and does not redirect to a different host.
+</p>
 </recommendation>
 
 <example>
@@ -27,11 +31,17 @@ validating the input, which facilitates phishing attacks:
 <sample src="OpenUrlRedirect.go"/>
 
 <p>
-One way to remedy the problem is to validate the user input against a known fixed string
-before doing the redirection:
+One way to remedy the problem is to parse the target URL and check that its hostname is empty,
+which means that it is a relative URL:
 </p>
 
 <sample src="OpenUrlRedirectGood.go"/>
+
+<p>
+Note that some browsers treat backslashes in URLs as forward slashes. To account for this,
+we replace all backslashes with forward slashes before parsing the URL and checking its hostname.
+</p>
+
 </example>
 
 <references>

go/ql/src/Security/CWE-601/OpenUrlRedirectGood.go

@@ -3,21 +3,26 @@ package main
 import (
 	"net/http"
 	"net/url"
+	"strings"
 )
 
-func serve() {
+func serve1() {
 	http.HandleFunc("/redir", func(w http.ResponseWriter, r *http.Request) {
 		r.ParseForm()
-		target, err := url.Parse(r.Form.Get("target"))
+		targetUrl := r.Form.Get("target")
+		// replace all backslashes with forward slashes before parsing the URL
+		targetUrl = strings.ReplaceAll(targetUrl, "\\", "/")
+
+		target, err := url.Parse(targetUrl)
 		if err != nil {
 			// ...
 		}
 
-		if target.Hostname() == "semmle.com" {
-			// GOOD: checking hostname
+		if target.Hostname() == "" {
+			// GOOD: check that it is a local redirect
 			http.Redirect(w, r, target.String(), 302)
 		} else {
-			http.WriteHeader(400)
+			w.WriteHeader(400)
 		}
 	})
 }

go/ql/test/query-tests/Security/CWE-601/OpenUrlRedirect/OpenUrlRedirectGood.go

@@ -3,17 +3,23 @@ package main
 import (
 	"net/http"
 	"net/url"
+	"strings"
 )
 
 func serve1() {
 	http.HandleFunc("/redir", func(w http.ResponseWriter, r *http.Request) {
 		r.ParseForm()
-		target, err := url.Parse(r.Form.Get("target"))
+		targetUrl := r.Form.Get("target")
+		// replace all backslashes with forward slashes before parsing the URL
+		targetUrl = strings.ReplaceAll(targetUrl, "\\", "/")
+
+		target, err := url.Parse(targetUrl)
 		if err != nil {
 			// ...
 		}
-		if target.Hostname() == "semmle.com" {
-			// GOOD: checking hostname
+
+		if target.Hostname() == "" {
+			// GOOD: check that it is a local redirect
 			http.Redirect(w, r, target.String(), 302)
 		} else {
 			w.WriteHeader(400)