From b7a118401996089246fcc313e2e79f3eeed603bd Mon Sep 17 00:00:00 2001
From: Nick Craig-Wood <nick@craig-wood.com>
Date: Mon, 27 Jul 2026 18:04:55 +0100
Subject: [PATCH] serve ftp: use constant time comparison for password check
 GHSA-mfvx-7rcj-9m5g

The builtin authentication compared the configured username and password
with ==, whose run time depends on how much of the value matches, giving a
timing side-channel that could in principle help guess the password.

serve sftp, serve s3 and the auth proxy already use subtle.ConstantTimeCompare
so bring serve ftp in line with them. An empty configured password
still accepts any password.

Addresses GHSA-mfvx-7rcj-9m5g finding 4.
---
 cmd/serve/ftp/ftp.go      |  9 ++++++++-
 cmd/serve/ftp/ftp_test.go | 30 ++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/cmd/serve/ftp/ftp.go b/cmd/serve/ftp/ftp.go
index e5f574ae1..6d15d4cfc 100644
--- a/cmd/serve/ftp/ftp.go
+++ b/cmd/serve/ftp/ftp.go
@@ -5,6 +5,7 @@ package ftp
 
 import (
 	"context"
+	"crypto/subtle"
 	"errors"
 	"fmt"
 	"io"
@@ -333,7 +334,13 @@ func (d *driver) CheckPasswd(sctx *ftp.Context, user, pass string) (ok bool, err
 		d.userPass[user] = oPass
 		d.userPassMu.Unlock()
 	} else {
-		ok = d.opt.User == user && (d.opt.Pass == "" || d.opt.Pass == pass)
+		userOK := subtle.ConstantTimeCompare([]byte(d.opt.User), []byte(user))
+		// No password configured means any password is accepted
+		passOK := 1
+		if d.opt.Pass != "" {
+			passOK = subtle.ConstantTimeCompare([]byte(d.opt.Pass), []byte(pass))
+		}
+		ok = (userOK & passOK) == 1
 		if !ok {
 			fs.Infof(nil, "login failed: bad credentials")
 			return false, nil
diff --git a/cmd/serve/ftp/ftp_test.go b/cmd/serve/ftp/ftp_test.go
index 83255fc77..df6655ac5 100644
--- a/cmd/serve/ftp/ftp_test.go
+++ b/cmd/serve/ftp/ftp_test.go
@@ -70,6 +70,36 @@ func TestFTP(t *testing.T) {
 	servetest.Run(t, "ftp", start)
 }
 
+// TestCheckPasswd checks the builtin authentication accepts only the
+// configured credentials, with an empty configured password accepting any
+// password.
+func TestCheckPasswd(t *testing.T) {
+	for _, test := range []struct {
+		name    string
+		optUser string
+		optPass string
+		user    string
+		pass    string
+		want    bool
+	}{
+		{name: "good", optUser: "user", optPass: "pass", user: "user", pass: "pass", want: true},
+		{name: "bad-pass", optUser: "user", optPass: "pass", user: "user", pass: "PASS", want: false},
+		{name: "bad-user", optUser: "user", optPass: "pass", user: "USER", pass: "pass", want: false},
+		{name: "wrong-length-pass", optUser: "user", optPass: "pass", user: "user", pass: "pass2", want: false},
+		{name: "empty-configured-pass", optUser: "user", optPass: "", user: "user", pass: "anything", want: true},
+		{name: "empty-configured-pass-bad-user", optUser: "user", optPass: "", user: "USER", pass: "anything", want: false},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			d := &driver{}
+			d.opt.User = test.optUser
+			d.opt.Pass = test.optPass
+			ok, err := d.CheckPasswd(nil, test.user, test.pass)
+			assert.NoError(t, err)
+			assert.Equal(t, test.want, ok)
+		})
+	}
+}
+
 func TestRc(t *testing.T) {
 	if israce.Enabled {
 		t.Skip("Skipping under race detector as underlying library is racy")