Add post payload length protection for auth pages

auth/handlers/handler.go

@@ -134,6 +134,12 @@ const (
 	TmplMfaTotp                  = "mfa-totp.html.tpl"
 	TmplMfaTotpDisable           = "mfa-totp-disable.html.tpl"
 	TmplInternalError            = "error-internal.html.tpl"
+
+	// 1k of data per POST field is all we allow
+	maxPostValueLength = 2 << 9
+
+	// general limitation on number of fields
+	maxPostFields = 10
 )
 
 func init() {
@@ -146,6 +152,7 @@ func init() {
 // handles auth request and prepares request struct with request, session and response helper
 func (h *AuthHandlers) handle(fn handlerFn) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
+
 		var (
 			req = &request.AuthReq{
 				Response:   w,
@@ -169,6 +176,11 @@ func (h *AuthHandlers) handle(fn handlerFn) http.HandlerFunc {
 				return
 			}
 
+			if !validFormPost(r) {
+				req.Status = http.StatusRequestEntityTooLarge
+				return
+			}
+
 			req.Client = request.GetOauth2Client(req.Session)
 
 			req.AuthUser = request.GetAuthUser(req.Session)
@@ -413,3 +425,29 @@ func anonyOnly(fn handlerFn) handlerFn {
 func translator(req *request.AuthReq, ns string) func(key string, rr ...string) string {
 	return req.Locale.NS(req.Context(), ns)
 }
+
+// general validation of posted data
+//
+// quite primitive for now but should be effective against out-of-bounds attacks
+//
+// in the future, more sophisticated validation might be needed
+func validFormPost(r *http.Request) bool {
+	if len(r.Form) > maxPostFields {
+		// auth does not have any large forms
+		return false
+	}
+
+	// None of the values from the post fields should be longer than max length
+	for k, _ := range r.Form {
+		if len(r.Form[k]) > 1 {
+			// assuming only one value per field!
+			return false
+		}
+
+		if len(r.Form[k][0]) > maxPostValueLength {
+			return false
+		}
+	}
+
+	return true
+}