From ea7e5efe9741e1b1787a58af16cb15b40c23be5a Mon Sep 17 00:00:00 2001
From: Benjamin Dauvergne <bdauvergne@entrouvert.com>
Date: Mon, 8 Mar 2021 11:33:26 +0100
Subject: Fix signature checking on unsigned response with multiple assertions

CVE-2021-28091 : when AuthnResponse messages are not signed (which is
permitted by the specifiation), all assertion's signatures should be
checked, but currently after the first signed assertion is checked all
following assertions are accepted without checking their signature, and
the last one is considered the main assertion.

This patch :
* check signatures from all assertions if the message is not signed,
* refuse messages with assertion from different issuers than the one on
  the message, to prevent assertion bundling event if they are signed.

diff --git a/lasso/saml-2.0/login.c b/lasso/saml-2.0/login.c
index 0d4bb1d..cf62c1c 100644
--- a/lasso/saml-2.0/login.c
+++ b/lasso/saml-2.0/login.c
@@ -1257,7 +1257,11 @@ lasso_saml20_login_check_assertion_signature(LassoLogin *login,
 	original_node = lasso_node_get_original_xmlnode(LASSO_NODE(assertion));
 	goto_cleanup_if_fail_with_rc(original_node, LASSO_PROFILE_ERROR_CANNOT_VERIFY_SIGNATURE);
 
-	rc = profile->signature_status = lasso_provider_verify_saml_signature(remote_provider, original_node, NULL);
+	/* Shouldn't set the profile->signature_status here as we're only
+	 * checking the assertion signature.
+	 * Instead, we'll set the status after all the assertions are iterated.
+	 */
+	rc = lasso_provider_verify_saml_signature(remote_provider, original_node, NULL);
 
 #define log_verify_assertion_signature_error(msg) \
 			message(G_LOG_LEVEL_WARNING, "Could not verify signature of assertion" \
@@ -1282,18 +1286,6 @@ cleanup:
 	return rc;
 }
 
-static gboolean
-_lasso_check_assertion_issuer(LassoSaml2Assertion *assertion, const gchar *provider_id)
-{
-	if (! LASSO_SAML2_ASSERTION(assertion) || ! provider_id)
-		return FALSE;
-
-	if (! assertion->Issuer || ! assertion->Issuer->content)
-		return FALSE;
-
-	return lasso_strisequal(assertion->Issuer->content,provider_id);
-}
-
 static gint
 _lasso_saml20_login_decrypt_assertion(LassoLogin *login, LassoSamlp2Response *samlp2_response)
 {
@@ -1358,11 +1350,23 @@ _lasso_saml20_login_decrypt_assertion(LassoLogin *login, LassoSamlp2Response *sa
 	return 0;
 }
 
+/* Verify that an assertion comes from a designated Issuer */
+static gboolean
+_lasso_check_assertion_issuer(LassoSaml2Assertion *assertion, const gchar *provider_id)
+{
+	if (! LASSO_SAML2_ASSERTION(assertion) || ! provider_id)
+		return FALSE;
+	if (! assertion->Issuer || ! assertion->Issuer->content)
+		return FALSE;
+	return lasso_strisequal(assertion->Issuer->content,provider_id);
+}
+
 static gint
 lasso_saml20_login_process_response_status_and_assertion(LassoLogin *login)
 {
 	LassoSamlp2StatusResponse *response;
 	LassoSamlp2Response *samlp2_response = NULL;
+	LassoSaml2Assertion *last_assertion = NULL;
 	LassoProfile *profile;
 	char *status_value;
 	lasso_error_t rc = 0;
@@ -1404,34 +1408,62 @@ lasso_saml20_login_process_response_status_and_assertion(LassoLogin *login)
 
 	/* Decrypt all EncryptedAssertions */
 	_lasso_saml20_login_decrypt_assertion(login, samlp2_response);
-	/* traverse all assertions */
-	goto_cleanup_if_fail_with_rc (samlp2_response->Assertion != NULL,
-			LASSO_PROFILE_ERROR_MISSING_ASSERTION);
 
+	/* Check there is at least one assertion */
+	goto_cleanup_if_fail_with_rc (samlp2_response->Assertion != NULL, LASSO_PROFILE_ERROR_MISSING_ASSERTION);
+
+	/* In case of verify_hint as 'FORCE', if there's no response signature,
+	 * we reject.
+	 * In case of 'MAYBE', if response signature is present and valid, or
+	 * not present, then we proceed with checking assertion signature(s).
+	 * In any case, if there's a response signature and it's not valid,
+	 * we reject.
+	*/
 	verify_hint = lasso_profile_get_signature_verify_hint(profile);
+	if (profile->signature_status == LASSO_DS_ERROR_SIGNATURE_NOT_FOUND) {
+		if (verify_hint == LASSO_PROFILE_SIGNATURE_VERIFY_HINT_FORCE) {
+			goto_cleanup_with_rc(profile->signature_status);
+		}
+	} else if (profile->signature_status != 0) {
+		goto_cleanup_with_rc(profile->signature_status);
+	}
 
 	lasso_foreach_full_begin(LassoSaml2Assertion*, assertion, it, samlp2_response->Assertion);
 		LassoSaml2Subject *subject = NULL;
 
-		lasso_assign_gobject (login->private_data->saml2_assertion, assertion);
+		/* All Assertions MUST come from the same issuer as the Response. */
+		if (! _lasso_check_assertion_issuer(assertion, profile->remote_providerID)) {
+			goto_cleanup_with_rc(LASSO_PROFILE_ERROR_INVALID_ISSUER);
+		}
 
-		/* If signature has already been verified on the message, and assertion has the same
-		 * issuer as the message, the assertion is covered. So no need to verify a second
-		 * time */
-		if (profile->signature_status != 0 
-			|| ! _lasso_check_assertion_issuer(assertion,
-				profile->remote_providerID)
-			|| verify_hint == LASSO_PROFILE_SIGNATURE_VERIFY_HINT_FORCE) {
+		if (profile->signature_status != 0) {
+			/* When response signature is not present */
+			if (verify_hint == LASSO_PROFILE_SIGNATURE_VERIFY_HINT_MAYBE) {
+				assertion_signature_status =
+					lasso_saml20_login_check_assertion_signature(login, assertion);
+				if (assertion_signature_status) {
+					goto_cleanup_with_rc(assertion_signature_status);
+				}
+			}
+		} else {
+			/* response signature is present and valid */
 			assertion_signature_status = lasso_saml20_login_check_assertion_signature(login,
-					assertion);
-			/* If signature validation fails, it is the return code for this function */
+							assertion);
 			if (assertion_signature_status) {
-				rc = LASSO_PROFILE_ERROR_CANNOT_VERIFY_SIGNATURE;
+				/* assertion signature is not valid or not present */
+				if (verify_hint == LASSO_PROFILE_SIGNATURE_VERIFY_HINT_FORCE) {
+					/* In case of FORCE, we reject right away */
+					goto_cleanup_with_rc(assertion_signature_status);
+				} else if (verify_hint == LASSO_PROFILE_SIGNATURE_VERIFY_HINT_MAYBE) {
+					/* In case of MAYBE, if assertion signature is present and invalid, then we reject */
+					if (assertion_signature_status != LASSO_DS_ERROR_SIGNATURE_NOT_FOUND) {
+						goto_cleanup_with_rc(assertion_signature_status);
+					}
+				}
 			}
 		}
-
 		lasso_extract_node_or_fail(subject, assertion->Subject, SAML2_SUBJECT,
-				LASSO_PROFILE_ERROR_MISSING_SUBJECT);
+			LASSO_PROFILE_ERROR_MISSING_SUBJECT);
 
 		/* Verify Subject->SubjectConfirmationData->InResponseTo */
 		if (login->private_data->request_id) {
@@ -1446,8 +1478,20 @@ lasso_saml20_login_process_response_status_and_assertion(LassoLogin *login)
 		/** Handle nameid */
 		lasso_check_good_rc(lasso_saml20_profile_process_name_identifier_decryption(profile,
 					&subject->NameID, &subject->EncryptedID));
+
+		last_assertion = assertion;
 	lasso_foreach_full_end();
 
+	/* set the profile signature status only after all the signatures are
+	 * verified.
+	 */
+	profile->signature_status = rc;
+
+	/* set the default assertion to the last one */
+	if (last_assertion) {
+		lasso_assign_gobject (login->private_data->saml2_assertion, last_assertion);
+	}
+
 	switch (verify_hint) {
 		case LASSO_PROFILE_SIGNATURE_VERIFY_HINT_FORCE:
 		case LASSO_PROFILE_SIGNATURE_VERIFY_HINT_MAYBE:
-- 
cgit v0.10.2