fix(kubernetes): sanitize non-UTF-8 host in metrics (#7998)

plugin/kubernetes/metrics.go

@@ -3,7 +3,9 @@ package kubernetes
 import (
 	"context"
 	"net/url"
+	"strings"
 	"time"
+	"unicode/utf8"
 
 	"github.com/coredns/coredns/plugin"
 
@@ -64,7 +66,7 @@ type latencyAdapter struct {
 }
 
 func (l *latencyAdapter) Observe(_ context.Context, verb string, u url.URL, latency time.Duration) {
-	l.m.WithLabelValues(verb, u.Host).Observe(latency.Seconds())
+	l.m.WithLabelValues(verb, sanitizeLabelValue(u.Host)).Observe(latency.Seconds())
 }
 
 type resultAdapter struct {
@@ -72,5 +74,14 @@ type resultAdapter struct {
 }
 
 func (r *resultAdapter) Increment(_ context.Context, code, method, host string) {
-	r.m.WithLabelValues(code, method, host).Inc()
+	r.m.WithLabelValues(code, method, sanitizeLabelValue(host)).Inc()
+}
+
+// sanitizeLabelValue ensures the string is valid UTF-8, which is required by
+// Prometheus for label values. Invalid bytes are replaced with U+FFFD.
+func sanitizeLabelValue(s string) string {
+	if utf8.ValidString(s) {
+		return s
+	}
+	return strings.ToValidUTF8(s, "\uFFFD")
 }

plugin/kubernetes/metrics_test.go

@@ -0,0 +1,27 @@
+package kubernetes
+
+import "testing"
+
+func TestSanitizeLabelValue(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{"valid ASCII", "example.com", "example.com"},
+		{"valid UTF-8", "例え.jp", "例え.jp"},
+		{"empty string", "", ""},
+		{"invalid single byte", "host\xff:443", "host\uFFFD:443"},
+		{"consecutive invalid bytes", "\xff\xfe\xfd", "\uFFFD"},
+		{"mixed valid and invalid", "ok\xffok\xfeok", "ok\uFFFDok\uFFFDok"},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := sanitizeLabelValue(tc.input)
+			if got != tc.expected {
+				t.Errorf("sanitizeLabelValue(%q) = %q, want %q", tc.input, got, tc.expected)
+			}
+		})
+	}
+}