GMS-2023-1583: ginuerzh/gost vulnerable to Timing Attack

Timing attacks occur when an attacker can guess a secret by observing a difference in processing time for valid and invalid inputs. Sensitive secrets such as passwords, token and API keys should be compared only using a constant-time comparision function. More information on this attack type can be found in this blog post.

Root Cause Analysis

In this case, the vulnerability occurs due to the following code.

https://github.com/ginuerzh/gost/blob/1c62376e0880e4094bd3731e06bd4f7842638f6a/auth.go#L46-L46

Here, a untrusted input, sourced from a HTTP header, is compared directly with a secret. Since, this comparision is not secure, an attacker can mount a side-channel timing attack to guess the password.

Remediation

This can be easily fixed using a constant time comparing function such as crypto/subtle’s ConstantTimeCompare. An example fix can be found in https://github.com/runatlantis/atlantis/commit/48870911974adddaa4c99c8089e79b7d787fa820 Alternatively, one can apply the patch below

From d18cff85e1a565f688f717fd8f2cacea62ff9dbf Mon Sep 17 00:00:00 2001
From: Porcupiney Hairs <porcupiney.hairs@protonmail.com>
Date: Sun, 7 May 2023 01:03:33 +0530
Subject: [PATCH] Fix : Timing attack

---
 auth.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/auth.go b/auth.go
index 1be96e9..be13f23 100644
--- a/auth.go
+++ b/auth.go
@@ -2,6 +2,7 @@ package gost
 
 import (
 	"bufio"
+	"crypto/subtle"
 	"io"
 	"strings"
 	"sync"
@@ -43,7 +44,8 @@ func (au *LocalAuthenticator) Authenticate(user, password string) bool {
 	}
 
 	v, ok := au.kvs[user]
-	return ok && (v == "" || password == v)
+	passOk := subtle.ConstantTimeCompare([]byte(password), []byte(v)) == 0
+	return ok && (v == "" || passOk)
 }
 
 // Add adds a key-value pair to the Authenticator.
-- 
2.25.1