From 6435c37ab681759aff37ba751d0f2238b3043767 Mon Sep 17 00:00:00 2001
From: Neal Patel <neal@golang.org>
Date: Tue, 19 May 2026 20:30:03 -0400
Subject: [PATCH] ssh: sanitize client disconnect messages

A malicious peer can send control characters
and escape sequences in SSH_MSG_DISCONNECT;
strip these characters and %q escape it.

Change-Id: I266f78630a7a816a79ce4dda568ce57d7c3f6fb0
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/780180
Commit-Queue: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
---
 ssh/messages.go      | 11 ++++++++++-
 ssh/messages_test.go | 30 ++++++++++++++++++++++++++++--
 2 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/ssh/messages.go b/ssh/messages.go
index ab22c3d..de86f71 100644
--- a/ssh/messages.go
+++ b/ssh/messages.go
@@ -44,7 +44,16 @@ type disconnectMsg struct {
 }
 
 func (d *disconnectMsg) Error() string {
-	return fmt.Sprintf("ssh: disconnect, reason %d: %s", d.Reason, d.Message)
+	return fmt.Sprintf("ssh: disconnect, reason %d: %q", d.Reason, sanitizeString(d.Message))
+}
+
+func sanitizeString(s string) string {
+	return strings.Map(func(r rune) rune {
+		if r == '\t' || (r >= ' ' && r < 0x7f) {
+			return r
+		}
+		return -1
+	}, s)
 }
 
 // See RFC 4253, section 7.1.
diff --git a/ssh/messages_test.go b/ssh/messages_test.go
index ed34628..90f2f4a 100644
--- a/ssh/messages_test.go
+++ b/ssh/messages_test.go
@@ -9,6 +9,7 @@ import (
 	"math/big"
 	"math/rand"
 	"reflect"
+	"strings"
 	"testing"
 	"testing/quick"
 )
@@ -262,6 +263,31 @@ func TestDecode(t *testing.T) {
 	}
 }
 
+func TestDisconnectMsgSanitizesMessage(t *testing.T) {
+	for _, tc := range []struct {
+		in   string
+		want string
+	}{
+		{"clean message", `"clean message"`},
+		{"line1\nline2\nline3", `"line1line2line3"`},
+		{"has\x00null\x00bytes", `"hasnullbytes"`},
+		{"\x1b[31mred\x1b[0m", `"[31mred[0m"`},
+		{"newline\r\nCRLF", `"newlineCRLF"`},
+		{"tab\there", `"tab\there"`},
+	} {
+		t.Run(tc.in, func(t *testing.T) {
+			d := &disconnectMsg{Reason: 11, Message: tc.in}
+			got := d.Error()
+			if !strings.Contains(got, tc.want) {
+				t.Errorf("got %q, want %q", got, tc.want)
+			}
+			if bad := "\n\r\x00\x1b"; strings.ContainsAny(got, bad) {
+				t.Errorf("got %q, want none of %q", got, bad)
+			}
+		})
+	}
+}
+
 func randomBytes(out []byte, rand *rand.Rand) {
 	for i := 0; i < len(out); i++ {
 		out[i] = byte(rand.Int31())
@@ -318,7 +344,7 @@ var (
 )
 
 func BenchmarkMarshalKexInitMsg(b *testing.B) {
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		Marshal(_kexInitMsg)
 	}
 }
@@ -331,7 +357,7 @@ func BenchmarkUnmarshalKexInitMsg(b *testing.B) {
 }
 
 func BenchmarkMarshalKexDHInitMsg(b *testing.B) {
-	for i := 0; i < b.N; i++ {
+	for b.Loop() {
 		Marshal(_kexDHInitMsg)
 	}
 }