config: reject case-insensitive subdomain domains (#5474)

pkg/config/v1/validation/proxy.go

@@ -79,9 +79,11 @@ func validateDomainConfigForClient(c *v1.DomainConfig) error {
 }
 
 func validateDomainConfigForServer(c *v1.DomainConfig, s *v1.ServerConfig) error {
+	subDomainHost := strings.ToLower(s.SubDomainHost)
 	for _, domain := range c.CustomDomains {
-		if s.SubDomainHost != "" && len(strings.Split(s.SubDomainHost, ".")) < len(strings.Split(domain, ".")) {
-			if strings.HasSuffix(domain, "."+s.SubDomainHost) {
+		canonicalDomain := strings.ToLower(domain)
+		if subDomainHost != "" && len(strings.Split(subDomainHost, ".")) < len(strings.Split(canonicalDomain, ".")) {
+			if strings.HasSuffix(canonicalDomain, "."+subDomainHost) {
 				return fmt.Errorf("custom domain [%s] should not belong to subdomain host [%s]", domain, s.SubDomainHost)
 			}
 		}

pkg/config/v1/validation/proxy_test.go

@@ -0,0 +1,76 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package validation
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	v1 "github.com/fatedier/frp/pkg/config/v1"
+)
+
+func TestValidateDomainConfigForServerRejectsSubdomainHostCaseInsensitively(t *testing.T) {
+	tests := []struct {
+		name          string
+		subDomainHost string
+		customDomain  string
+		wantErr       bool
+	}{
+		{
+			name:          "lowercase subdomain",
+			subDomainHost: "frp.example.com",
+			customDomain:  "victim.frp.example.com",
+			wantErr:       true,
+		},
+		{
+			name:          "mixed case custom domain",
+			subDomainHost: "frp.example.com",
+			customDomain:  "victim.FRP.example.com",
+			wantErr:       true,
+		},
+		{
+			name:          "mixed case wildcard domain",
+			subDomainHost: "frp.example.com",
+			customDomain:  "*.FRP.example.com",
+			wantErr:       true,
+		},
+		{
+			name:          "mixed case subdomain host",
+			subDomainHost: "FRP.Example.Com",
+			customDomain:  "victim.frp.example.com",
+			wantErr:       true,
+		},
+		{
+			name:          "external domain",
+			subDomainHost: "frp.example.com",
+			customDomain:  "victim.example.net",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateDomainConfigForServer(
+				&v1.DomainConfig{CustomDomains: []string{tt.customDomain}},
+				&v1.ServerConfig{SubDomainHost: tt.subDomainHost},
+			)
+			if tt.wantErr {
+				require.ErrorContains(t, err, "should not belong to subdomain host")
+				return
+			}
+			require.NoError(t, err)
+		})
+	}
+}