Prevent redirect to Host (2) (#19175)

Unhelpfully Locations starting with `/\` will be converted by the
browser to `//` because ... well I do not fully understand. Certainly
the RFCs and MDN do not indicate that this would be expected. Providing
"compatibility" with the (mis)behaviour of a certain proprietary OS is
my suspicion. However, we clearly have to protect against this.

Therefore we should reject redirection locations that match the regular
expression: `^/[\\\\/]+`

Reference #9678

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/context/context.go

@@ -181,6 +181,12 @@ func (ctx *Context) RedirectToFirst(location ...string) {
 			continue
 		}
 
+		// Unfortunately browsers consider a redirect Location with preceding "//" and "/\" as meaning redirect to "http(s)://REST_OF_PATH"
+		// Therefore we should ignore these redirect locations to prevent open redirects
+		if len(loc) > 1 && loc[0] == '/' && (loc[1] == '/' || loc[1] == '\\') {
+			continue
+		}
+
 		u, err := url.Parse(loc)
 		if err != nil || ((u.Scheme != "" || u.Host != "") && !strings.HasPrefix(strings.ToLower(loc), strings.ToLower(setting.AppURL))) {
 			continue