fix: require Destination on all SAML responses

Previously, unsigned responses with an empty Destination were accepted
without any Destination validation. This allowed an attacker to replay
a captured response to a different SP, since the Destination was the
only non-cryptographic binding to the intended recipient.

Now Destination is always required and validated, regardless of whether
the response is signed. The SAML spec says Destination MUST be present
on signed responses and SHOULD be present otherwise — we upgrade the
SHOULD to a MUST for defense in depth.

Fixes #12

Author: aashh

service_provider.go

@@ -986,12 +986,8 @@ const (
 // and properties are met.
 func (sp *ServiceProvider) parseResponse(responseEl *etree.Element, possibleRequestIDs []string, now time.Time, signatureRequirement signatureRequirement, currentURL url.URL) (*Assertion, error) {
 	var responseSignatureErr error
-	var responseHasSignature bool
 	if signatureRequirement == signatureRequired {
 		responseSignatureErr = sp.validateSignature(responseEl)
-		if responseSignatureErr != errSignatureElementNotPresent {
-			responseHasSignature = true
-		}
 
 		// Note: we're deferring taking action on the signature validation until after we've
 		// processed the request attributes, because certain test cases seem to require this mis-feature.
@@ -1005,14 +1001,18 @@ func (sp *ServiceProvider) parseResponse(responseEl *etree.Element, possibleRequ
 			return nil, fmt.Errorf("cannot unmarshal response: %v", err)
 		}
 
-		// If the response is *not* signed, the Destination may be omitted.
-		if responseHasSignature || response.Destination != "" {
-			// Per section 3.4.5.2 of the SAML spec, Destination must match the location at which the response was received, i.e. currentURL.
-			// Historically, we checked against the SP's ACS URL instead of currentURL, which is usually the same but may differ in query params.
-			// To mitigate the risk of switching to comparing against currentURL, we still allow it if the ACS URL matches, even if the current URL doesn't.
-			if response.Destination != currentURL.String() && response.Destination != sp.AcsURL.String() {
-				return nil, fmt.Errorf("`Destination` does not match requested URL or AcsURL (destination %q, requested %q, acs %q)", response.Destination, currentURL.String(), sp.AcsURL.String())
-			}
+		// Per section 3.4.5.2 of the SAML spec, Destination MUST be present
+		// when a signature is present, and SHOULD be present otherwise. We
+		// require it always: for unsigned responses the Destination is the
+		// only non-cryptographic binding to this SP, and omitting it would
+		// let an attacker replay a response across service providers.
+		if response.Destination == "" {
+			return nil, fmt.Errorf("`Destination` is required but missing")
+		}
+		// Historically, we checked against the SP's ACS URL instead of currentURL, which is usually the same but may differ in query params.
+		// To mitigate the risk of switching to comparing against currentURL, we still allow it if the ACS URL matches, even if the current URL doesn't.
+		if response.Destination != currentURL.String() && response.Destination != sp.AcsURL.String() {
+			return nil, fmt.Errorf("`Destination` does not match requested URL or AcsURL (destination %q, requested %q, acs %q)", response.Destination, currentURL.String(), sp.AcsURL.String())
 		}
 
 		if err := sp.validateRequestID(response, possibleRequestIDs); err != nil {

service_provider_test.go

@@ -946,7 +946,7 @@ func (test *ServiceProviderTest) replaceDestination(newDestination string) {
 		[]byte(`Destination="https://15661444.ngrok.io/saml2/acs"`), []byte(newStr), 1)
 }
 
-func TestSPCanProcessResponseWithoutDestination(t *testing.T) {
+func TestSPRejectsResponseWithoutDestination(t *testing.T) {
 	test := NewServiceProviderTest(t)
 	s := ServiceProvider{
 		Key:         test.Key,
@@ -962,7 +962,41 @@ func TestSPCanProcessResponseWithoutDestination(t *testing.T) {
 	test.replaceDestination("")
 	req.PostForm.Set("SAMLResponse", base64.StdEncoding.EncodeToString(test.SamlResponse))
 	_, err = s.ParseResponse(&req, []string{"id-9e61753d64e928af5a7a341a97f420c9"})
+	assert.Check(t, is.Error(err.(*InvalidResponseError).PrivateErr,
+		"`Destination` is required but missing"))
+}
+
+func TestSPRejectsUnsignedResponseWithoutDestination(t *testing.T) {
+	// Regression test for issue #12: On main, unsigned responses without
+	// Destination were silently accepted. The fix requires Destination always,
+	// because for unsigned responses it is the only binding to this SP.
+	test := NewServiceProviderTest(t)
+	s := ServiceProvider{
+		Key:         test.Key,
+		Certificate: test.Certificate,
+		MetadataURL: mustParseURL("https://15661444.ngrok.io/saml2/metadata"),
+		AcsURL:      mustParseURL("https://15661444.ngrok.io/saml2/acs"),
+		IDPMetadata: &EntityDescriptor{},
+	}
+	err := xml.Unmarshal(test.IDPMetadata, &s.IDPMetadata)
 	assert.Check(t, err)
+
+	// Build a minimal unsigned SAML response with no Destination attribute.
+	// This simulates an IdP sending a response without any audience binding.
+	unsignedResp := `<saml2p:Response xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol"` +
+		` ID="_test123" InResponseTo="id-9e61753d64e928af5a7a341a97f420c9"` +
+		` IssueInstant="2015-12-01T01:56:21.375Z" Version="2.0">` +
+		`<saml2:Issuer xmlns:saml2="urn:oasis:names:tc:SAML:2.0:assertion"` +
+		` Format="urn:oasis:names:tc:SAML:2.0:nameid-format:entity">` +
+		`https://idp.testshib.org/idp/shibboleth</saml2:Issuer>` +
+		`<saml2p:Status><saml2p:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/></saml2p:Status>` +
+		`</saml2p:Response>`
+
+	req := http.Request{PostForm: url.Values{}, URL: &s.AcsURL}
+	req.PostForm.Set("SAMLResponse", base64.StdEncoding.EncodeToString([]byte(unsignedResp)))
+	_, err = s.ParseResponse(&req, []string{"id-9e61753d64e928af5a7a341a97f420c9"})
+	assert.Check(t, is.Error(err.(*InvalidResponseError).PrivateErr,
+		"`Destination` is required but missing"))
 }
 
 func (test *ServiceProviderTest) responseDom(t *testing.T) (doc *etree.Document) {
@@ -1079,7 +1113,7 @@ func TestServiceProviderMissingDestinationWithSignaturePresent(t *testing.T) {
 	req.PostForm.Set("SAMLResponse", base64.StdEncoding.EncodeToString(bytes))
 	_, err = s.ParseResponse(&req, []string{"id-9e61753d64e928af5a7a341a97f420c9"})
 	assert.Check(t, is.Error(err.(*InvalidResponseError).PrivateErr,
-		"`Destination` does not match requested URL or AcsURL (destination \"\", requested \"https://15661444.ngrok.io/saml2/acs\", acs \"https://15661444.ngrok.io/saml2/acs\")"))
+		"`Destination` is required but missing"))
 }
 
 func TestSPMismatchedDestinationsWithSignaturePresent(t *testing.T) {
@@ -1142,7 +1176,7 @@ func TestSPMissingDestinationWithSignaturePresent(t *testing.T) {
 	req.PostForm.Set("SAMLResponse", base64.StdEncoding.EncodeToString(bytes))
 	_, err = s.ParseResponse(&req, []string{"id-9e61753d64e928af5a7a341a97f420c9"})
 	assert.Check(t, is.Error(err.(*InvalidResponseError).PrivateErr,
-		"`Destination` does not match requested URL or AcsURL (destination \"\", requested \"https://15661444.ngrok.io/saml2/acs\", acs \"https://15661444.ngrok.io/saml2/acs\")"))
+		"`Destination` is required but missing"))
 }
 
 func TestSPInvalidAssertions(t *testing.T) {