From 1714bf5ed6b8a32aa8a260e648051f2b0c8a579d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Thorben=20G=C3=BCnther?= <admin@xenrox.net>
Date: Thu, 13 Oct 2022 13:08:54 +0200
Subject: [PATCH] basicAuthMiddleware: Protect against timing attacks

Compare strings of equal length (hashed with SHA-512) with
ConstantTimeCompare.

Closes: https://todo.xenrox.net/~xenrox/ntfy-alertmanager/1
---
 main.go | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/main.go b/main.go
index 9d58cf2..9d46c1d 100644
--- a/main.go
+++ b/main.go
@@ -1,6 +1,8 @@
 package main
 
 import (
+	"crypto/sha512"
+	"crypto/subtle"
 	"encoding/base64"
 	"encoding/json"
 	"flag"
@@ -169,7 +171,15 @@ func (rcv *receiver) basicAuthMiddleware(handler http.HandlerFunc) http.HandlerF
 			return
 		}
 
-		if user != rcv.cfg.User || pass != rcv.cfg.Password {
+		inputUserHash := sha512.Sum512([]byte(user))
+		inputPassHash := sha512.Sum512([]byte(pass))
+		configUserHash := sha512.Sum512([]byte(rcv.cfg.User))
+		configPassHash := sha512.Sum512([]byte(rcv.cfg.Password))
+
+		validUser := subtle.ConstantTimeCompare(inputUserHash[:], configUserHash[:])
+		validPass := subtle.ConstantTimeCompare(inputPassHash[:], configPassHash[:])
+
+		if validUser != 1 || validPass != 1 {
 			http.Error(w, "Unauthorized", http.StatusUnauthorized)
 			rcv.logger.Debug("basic auth: wrong user or password")
 			return