From 84c99023e5894769954ef9319608935caf525fa0 Mon Sep 17 00:00:00 2001
From: Kristoffer Dalby <kristoffer@tailscale.com>
Date: Fri, 5 Jun 2026 14:59:10 +0000
Subject: [PATCH] util: check RNG error before slicing url-safe random string

A crypto/rand failure sliced empty output and panicked.
---
 hscontrol/util/string.go             | 13 +++++++++++--
 hscontrol/util/string_random_test.go | 24 ++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)
 create mode 100644 hscontrol/util/string_random_test.go

diff --git a/hscontrol/util/string.go b/hscontrol/util/string.go
index b0f0376a..2bcb515f 100644
--- a/hscontrol/util/string.go
+++ b/hscontrol/util/string.go
@@ -32,9 +32,18 @@ func GenerateRandomBytes(n int) ([]byte, error) {
 func GenerateRandomStringURLSafe(n int) (string, error) {
 	b, err := GenerateRandomBytes(n)
 
-	uenc := base64.RawURLEncoding.EncodeToString(b)
+	return encodeRandomURLSafe(b, n, err)
+}
 
-	return uenc[:n], err
+// encodeRandomURLSafe URL-safe base64-encodes b and truncates to n. It checks
+// err first: on an RNG failure b is nil, so slicing the empty encoding would
+// panic instead of returning the ("", err) the caller is promised.
+func encodeRandomURLSafe(b []byte, n int, err error) (string, error) {
+	if err != nil {
+		return "", err
+	}
+
+	return base64.RawURLEncoding.EncodeToString(b)[:n], nil
 }
 
 // GenerateRandomStringDNSSafe returns a DNS-safe
diff --git a/hscontrol/util/string_random_test.go b/hscontrol/util/string_random_test.go
new file mode 100644
index 00000000..c697841b
--- /dev/null
+++ b/hscontrol/util/string_random_test.go
@@ -0,0 +1,24 @@
+package util
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestEncodeRandomURLSafeChecksErrorFirst ensures the URL-safe random string
+// encoder returns ("", err) on an RNG failure instead of slicing the empty
+// base64 of nil bytes and panicking.
+func TestEncodeRandomURLSafeChecksErrorFirst(t *testing.T) {
+	require.NotPanics(t, func() {
+		s, err := encodeRandomURLSafe(nil, 32, assert.AnError)
+		assert.Empty(t, s)
+		require.Error(t, err)
+	})
+
+	s, err := encodeRandomURLSafe(bytes.Repeat([]byte{0x1}, 32), 32, nil)
+	require.NoError(t, err)
+	assert.Len(t, s, 32)
+}