feat: add config option to set Access-Control-Allow-Origin header on responses

config/config.go

@@ -7,8 +7,9 @@ type Config struct {
 }
 
 type ServerConfig struct {
-	Port int       `mapstructure:"port"`
-	SSL  SSLConfig `mapstructure:"ssl"`
+	Port                     int       `mapstructure:"port"`
+	SSL                      SSLConfig `mapstructure:"ssl"`
+	AccessControlAllowOrigin string    `mapstructure:"accessControlAllowOrigin"`
 }
 
 type ProxyConfig struct {

config/config_test.yaml

@@ -4,6 +4,7 @@ server:
     enable: false
     certFilePath: "/etc/ssl/certs/cert.crt"
     keyFilePath: "/etc/ssl/private/private.key"
+  accessControlAllowOrigin: "*"
 
 proxy:
   upstreamTarget: "https://api.form3.tech/v1"

config/loader_test.go

@@ -129,6 +129,7 @@ func TestLoadConfig(t *testing.T) {
 				CertFilePath: "/etc/ssl/certs/cert.crt",
 				KeyFilePath:  "/etc/ssl/private/private.key",
 			},
+			AccessControlAllowOrigin: "*",
 		},
 		Log: LogConfig{
 			Level:  "debug",

example/config_example.yaml

@@ -11,6 +11,8 @@ server:
     certFilePath: "/etc/ssl/certs/cert.crt"
     # Location of the proxy's private key, if SSL is enabled
     keyFilePath: "/etc/ssl/private/private.key"
+  # Value to be used in the Access-Control-Allow-Origin response header
+  accessControlAllowOrigin: "*"
 
 # Request forward proxy config
 proxy:

proxy/handler_test.go

@@ -84,6 +84,69 @@ func TestHandler(t *testing.T) {
 	}
 }
 
+func TestHandlerCORS(t *testing.T) {
+	tests := []struct {
+		name                     string
+		accessControlAllowOrigin string
+	}{
+		{
+			"no value",
+			"",
+		},
+		{
+			"*",
+			"*",
+		},
+		{
+			"single domain",
+			"https://test",
+		},
+	}
+
+	expectedRespBody := "OK"
+	mockURL := "mock"
+	mockCtrl := gomock.NewController(t)
+	defer mockCtrl.Finish()
+
+	// Mock dependencies
+	mockReqSigner := mockReqSigner(mockCtrl)
+	mockMetricPublisher := mockMetricPublisher(mockCtrl, mockURL)
+
+	// Test upstream target that returns 200 OK
+	targetSrv := testTargetServer(expectedRespBody)
+
+	// Reverse proxy pointing to test target
+	rs, err := NewReverseProxy(targetSrv.URL)
+	require.NoError(t, err)
+
+	// Test handler
+	var w *test.TestResponseRecorder
+
+	for _, tt := range tests {
+		h := NewHandler(rs, mockReqSigner, mockMetricPublisher)
+		_, e := gin.CreateTestContext(w)
+		e.NoRoute(
+			RecoverMiddleware(mockMetricPublisher),
+			LogAndMetricsMiddleware(mockMetricPublisher),
+			CORSMiddleware(tt.accessControlAllowOrigin),
+			h.ForwardRequest,
+		)
+
+		t.Run(tt.name, func(t *testing.T) {
+			w = test.NewTestResponseRecorder()
+
+			// Test request
+			req, err := http.NewRequest(http.MethodGet, mockURL, nil)
+			require.NoError(t, err)
+
+			e.ServeHTTP(w, req)
+
+			require.Equal(t, http.StatusOK, w.Code)
+			require.Equal(t, w.Header().Get(AccessControlAllowOriginHeader), tt.accessControlAllowOrigin)
+		})
+	}
+}
+
 func mockReqSigner(mockCtrl *gomock.Controller) *MockRequestSigner {
 	mockReqSigner := NewMockRequestSigner(mockCtrl)
 	mockReqSigner.EXPECT().SignRequest(gomock.Any()).DoAndReturn(func(r *http.Request) (*http.Request, error) {

proxy/middleware.go

@@ -13,6 +13,10 @@ import (
 	log "github.com/sirupsen/logrus"
 )
 
+const (
+	AccessControlAllowOriginHeader string = "Access-Control-Allow-Origin"
+)
+
 func RecoverMiddleware(metricPublisher MetricPublisher) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		defer func() {
@@ -71,3 +75,12 @@ func LogAndMetricsMiddleware(metricPublisher MetricPublisher) gin.HandlerFunc {
 		}).Info("request summary")
 	}
 }
+
+func CORSMiddleware(accessControlAllowOrigin string) gin.HandlerFunc {
+	if accessControlAllowOrigin != "" {
+		return func(c *gin.Context) {
+			c.Writer.Header().Set(AccessControlAllowOriginHeader, accessControlAllowOrigin)
+		}
+	}
+	return func(_ *gin.Context) {}
+}

proxy/server.go

@@ -33,6 +33,7 @@ func NewServer(cfg config.ServerConfig, handler Handler, metric MetricPublisher)
 	router.NoRoute(
 		RecoverMiddleware(metric),
 		LogAndMetricsMiddleware(metric),
+		CORSMiddleware(cfg.AccessControlAllowOrigin),
 		handler.ForwardRequest,
 	)
 

test/e2e_test.go

@@ -15,7 +15,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/form3tech-oss/go-http-message-signatures"
+	httpsignatures "github.com/form3tech-oss/go-http-message-signatures"
 	"github.com/form3tech-oss/http-message-signing-proxy/cmd"
 	"github.com/stretchr/testify/suite"
 )
@@ -37,6 +37,7 @@ const (
 
 type e2eTestSuite struct {
 	suite.Suite
+	accessControlAllowOrigin string
 }
 
 func (s *e2eTestSuite) msgVerifier() *httpsignatures.MessageVerifier {
@@ -86,10 +87,11 @@ func (s *e2eTestSuite) runProxy(upstreamTarget string) {
 	rootCmd.SetArgs(append(
 		[]string{"--config", cfgFile},
 		genSetFlags(map[string]string{
-			"server.ssl.certFilePath":  sslCertFile,
-			"server.ssl.keyFilePath":   sslKeyFile,
-			"proxy.signer.keyFilePath": privateKeyFile,
-			"proxy.upstreamTarget":     upstreamTarget,
+			"server.ssl.certFilePath":         sslCertFile,
+			"server.ssl.keyFilePath":          sslKeyFile,
+			"proxy.signer.keyFilePath":        privateKeyFile,
+			"proxy.upstreamTarget":            upstreamTarget,
+			"server.accessControlAllowOrigin": s.accessControlAllowOrigin,
 		})...,
 	))
 	go func() {
@@ -167,6 +169,7 @@ func (s *e2eTestSuite) TestProxy() {
 			r, err := http.DefaultClient.Do(req)
 			s.NoError(err)
 			s.Equal(test.expectedStatus, r.StatusCode)
+			s.Equal(s.accessControlAllowOrigin, r.Header.Get("Access-Control-Allow-Origin"))
 
 			if test.expectedStatus == http.StatusOK {
 				resp, err := readHttpResp[successResp](r)
@@ -186,7 +189,7 @@ func (s *e2eTestSuite) TestProxy() {
 }
 
 func TestE2ETestSuite(t *testing.T) {
-	suite.Run(t, new(e2eTestSuite))
+	suite.Run(t, &e2eTestSuite{accessControlAllowOrigin: "*"})
 }
 
 func genSetFlags(m map[string]string) []string {