extract security checks from sendmail into a specific handler

use `http.Handler` wrappers to split the sendmail handler into 2
distinct handlers: one for validating the query (currently, checking the allowed
domains) and the other one for the sendmail functionality.

main.go

@@ -27,10 +27,10 @@ import (
 )
 
 type Route struct {
-	Name        string
-	Method      string
-	Pattern     string
-	HandlerFunc http.HandlerFunc
+	Name    string
+	Method  string
+	Pattern string
+	Handler http.Handler
 }
 
 type Routes []Route
@@ -40,13 +40,13 @@ var routes = Routes{
 		"SendMail",
 		"POST",
 		"/sendmail",
-		SendMail,
+		MuxSecAllowedDomainsHandler(http.HandlerFunc(SendMail)),
 	},
 	Route{
 		"Healthz",
 		"GET",
 		"/healthz",
-		Healthz,
+		http.HandlerFunc(Healthz),
 	},
 }
 
@@ -67,9 +67,7 @@ func MuxLoggerHandler(inner http.Handler, name string) http.Handler {
 func NewRouter() *mux.Router {
 	router := mux.NewRouter().StrictSlash(true)
 	for _, route := range routes {
-		var handler http.Handler
-		handler = route.HandlerFunc
-		handler = MuxLoggerHandler(handler, route.Name)
+		handler := MuxLoggerHandler(route.Handler, route.Name)
 
 		router.
 			Methods(route.Method).

sendmail.go

@@ -148,32 +148,40 @@ func (m *SendMailRequest) ParseTemplate(templateFileName string, data interface{
 	return nil
 }
 
+// MuxSecAllowedDomainsHandler is a security middleware which controls allowed domains.
+func MuxSecAllowedDomainsHandler(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		allowedDomains := strings.Split(viper.GetString("ALLOWED_ORIGINS"), ",")
+		allowedOrigins := make(map[string]bool)
+
+		for _, domain := range allowedDomains {
+			domainTrimmed := strings.TrimSpace(domain)
+			allowedOrigins[fmt.Sprintf("http://%s", domainTrimmed)] = true
+			allowedOrigins[fmt.Sprintf("https://%s", domainTrimmed)] = true
+			allowedOrigins[fmt.Sprintf("http://www.%s", domainTrimmed)] = true
+			allowedOrigins[fmt.Sprintf("https://www.%s", domainTrimmed)] = true
+		}
+
+		if len(r.Header["Origin"]) == 0 || len(r.Header["Referer"]) == 0 {
+			rawHeader, _ := json.Marshal(r.Header)
+			log.Infoln("request with unexpected headers", string(rawHeader))
+			w.WriteHeader(http.StatusForbidden)
+			return
+		}
+
+		reqOrigin := r.Header["Origin"][0]
+		if _, domainFound := allowedOrigins[reqOrigin]; !domainFound {
+			log.Errorln("not allowed origin", reqOrigin)
+			w.WriteHeader(http.StatusForbidden)
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}
+
 // SendMail handles HTTP request to send email
 func SendMail(httpResp http.ResponseWriter, httpReq *http.Request) {
-
-	allowedDomains := strings.Split(viper.GetString("ALLOWED_ORIGINS"), ",")
-	allowedOrigins := make(map[string]bool)
-	for _, domain := range allowedDomains {
-		domainTrimmed := strings.TrimSpace(domain)
-		allowedOrigins[fmt.Sprintf("http://%s", domainTrimmed)] = true
-		allowedOrigins[fmt.Sprintf("https://%s", domainTrimmed)] = true
-		allowedOrigins[fmt.Sprintf("http://www.%s", domainTrimmed)] = true
-		allowedOrigins[fmt.Sprintf("https://www.%s", domainTrimmed)] = true
-	}
-	if len(httpReq.Header["Origin"]) == 0 || len(httpReq.Header["Referer"]) == 0 {
-		rawHeader, _ := json.Marshal(httpReq.Header)
-		log.Infoln("request with unexpected headers", string(rawHeader))
-		httpResp.WriteHeader(http.StatusForbidden)
-		return
-	}
-
-	reqOrigin := httpReq.Header["Origin"][0]
-	if _, domainFound := allowedOrigins[reqOrigin]; !domainFound {
-		log.Errorln("not allowed origin", reqOrigin)
-		httpResp.WriteHeader(http.StatusForbidden)
-		return
-	}
-
 	httpReq.ParseForm()
 
 	contactRequest := ContactRequest{