🐛 Fix / Return missing_certificate error when SAML signature has no X509 certificate

lib/ex_saml/core/xml/dsig.ex

@@ -302,6 +302,7 @@ defmodule ExSaml.Core.Xml.Dsig do
              :bad_digest
              | :bad_signature
              | :cert_not_accepted
+             | :missing_certificate
              | :no_signature
              | :multiple_signatures
              | :insecure_algorithm
@@ -403,31 +404,39 @@ defmodule ExSaml.Core.Xml.Dsig do
 
     sig = :base64.decode(xmlText(sig_text, :value))
 
-    {key, cert_bin} = extract_public_key(element, ds_ns)
-
-    case :public_key.verify(data, hash_function, sig, key) do
-      true -> check_fingerprints(cert_bin, fingerprints)
-      false -> {:error, :bad_signature}
+    with {:ok, {key, cert_bin}} <- extract_public_key(element, ds_ns) do
+      case :public_key.verify(data, hash_function, sig, key) do
+        true -> check_fingerprints(cert_bin, fingerprints)
+        false -> {:error, :bad_signature}
+      end
     end
   end
 
   defp extract_public_key(element, ds_ns) do
-    [cert_text] =
-      :xmerl_xpath.string(~c"ds:Signature//ds:X509Certificate/text()", element, namespace: ds_ns)
-
-    cert_bin = :base64.decode(xmlText(cert_text, :value))
-    cert = :public_key.pkix_decode_cert(cert_bin, :plain)
-    tbs = certificate(cert, :tbsCertificate)
-    spki = tbs_certificate(tbs, :subjectPublicKeyInfo)
-
-    key_bin =
-      case subject_public_key_info(spki, :subjectPublicKey) do
-        {_, kb} -> kb
-        kb -> kb
-      end
+    case :xmerl_xpath.string(
+           ~c"ds:Signature//ds:X509Certificate/text()",
+           element,
+           namespace: ds_ns
+         ) do
+      [] ->
+        {:error, :missing_certificate}
+
+      # The first certificate is the signing one; the rest is its chain.
+      [cert_text | _] ->
+        cert_bin = :base64.decode(xmlText(cert_text, :value))
+        cert = :public_key.pkix_decode_cert(cert_bin, :plain)
+        tbs = certificate(cert, :tbsCertificate)
+        spki = tbs_certificate(tbs, :subjectPublicKeyInfo)
+
+        key_bin =
+          case subject_public_key_info(spki, :subjectPublicKey) do
+            {_, kb} -> kb
+            kb -> kb
+          end
 
-    key = :public_key.pem_entry_decode({:RSAPublicKey, key_bin, :not_encrypted})
-    {key, cert_bin}
+        key = :public_key.pem_entry_decode({:RSAPublicKey, key_bin, :not_encrypted})
+        {:ok, {key, cert_bin}}
+    end
   end
 
   defp check_fingerprints(_cert_bin, :any), do: :ok

lib/ex_saml/error_messages.ex

@@ -20,9 +20,11 @@ defmodule ExSaml.ErrorMessages do
   @status_responder {:saml_error, ~c"urn:oasis:names:tc:SAML:2.0:status:Responder", :undefined}
   @assertion_signature {:assertion, {:error, :no_signature}}
   @assertion_cert_not_accepted {:assertion, {:error, :cert_not_accepted}}
+  @assertion_missing_certificate {:assertion, {:error, :missing_certificate}}
   @envelop_signature {:envelope, {:error, :no_signature}}
+  @envelope_missing_certificate {:envelope, {:error, :missing_certificate}}
   @name_id_errors {:saml_error, ~c"urn:oasis:names:tc:SAML:2.0:status:Requester", :undefined}
-  @errors ~w(bad_digest assertion_cert_not_accepted missing_assertion_key bad_audience cert_no_accepted invalid_nameid_policy invalid_nonce missing_assertion_signature missing_envelope_signature status_responder)a
+  @errors ~w(bad_digest assertion_cert_not_accepted missing_assertion_key bad_audience cert_no_accepted invalid_nameid_policy invalid_nonce missing_assertion_signature missing_certificate missing_envelope_signature status_responder)a
 
   @doc false
   def get(error, locale \\ "en")
@@ -38,9 +40,15 @@ defmodule ExSaml.ErrorMessages do
   def get(@assertion_cert_not_accepted, locale),
     do: get(:assertion_cert_not_accepted, locale)
 
+  def get(@assertion_missing_certificate, locale),
+    do: get(:missing_certificate, locale)
+
   def get(@envelop_signature, locale),
     do: get(:missing_envelope_signature, locale)
 
+  def get(@envelope_missing_certificate, locale),
+    do: get(:missing_certificate, locale)
+
   def get(@name_id_errors, locale),
     do: get(:invalid_nameid_policy, locale)
 

priv/gettext/en/LC_MESSAGES/errors.po

@@ -14,6 +14,9 @@ msgstr "Missing signature. Please ensure that the assertions are properly signed
 msgid "assertion_cert_not_accepted"
 msgstr "The certificate used to sign the assertions has not been accepted. It is possible that the certificate has expired or is not supported."
 
+msgid "missing_certificate"
+msgstr "No X.509 certificate was found in the SAML signature. Please configure your Identity Provider to include its signing certificate in signed responses and assertions."
+
 msgid "missing_envelope_signature"
 msgstr "Missing envelope signature. Please make sure you uploaded correctly the Cryptr Certificate to your IdP"
 

priv/gettext/errors.pot

@@ -13,6 +13,9 @@ msgstr "Missing signature. Please ensure that the assertions are properly signed
 msgid "assertion_cert_not_accepted"
 msgstr "The certificate used to sign the assertions has not been accepted. It is possible that the certificate has expired or is not supported."
 
+msgid "missing_certificate"
+msgstr "No X.509 certificate was found in the SAML signature. Please configure your Identity Provider to include its signing certificate in signed responses and assertions."
+
 msgid "missing_envelope_signature"
 msgstr "Missing envelope signature. Please make sure you uploaded correctly the Cryptr Certificate to your IdP."
 

priv/gettext/fr/LC_MESSAGES/errors.po

@@ -13,6 +13,9 @@ msgstr "Signature manquante. Veuillez vous assurer que les assertions sont corre
 msgid "assertion_cert_not_accepted"
 msgstr "Le certificat utilisé pour signer les assertions n'a pas été accepté. Il est possible que le certificat ait expiré ou qu'il ne soit pas pris en charge."
 
+msgid "missing_certificate"
+msgstr "Aucun certificat X.509 n'a été trouvé dans la signature SAML. Veuillez configurer votre fournisseur d'identité pour inclure son certificat de signature dans les réponses et assertions signées."
+
 msgid "missing_envelope_signature"
 msgstr "La Signature pour les enveloppes est manquantes. Assurez-vous que vous avec correctement uploadé votre Certificat Cryptr sur votre IdP."
 

test/ex_saml/core/xml/dsig_test.exs

@@ -220,6 +220,27 @@ defmodule ExSaml.Core.Xml.DsigTest do
 
       assert :ok = Dsig.verify(signed_xml, [:crypto.hash(:sha, cert_bin)])
     end
+
+    test "signature without X509Certificate returns {:error, :missing_certificate}" do
+      doc =
+        parse_xml(~S|<x:foo id="test" xmlns:x="urn:foo:x:"><x:name>blah</x:name></x:foo>|)
+
+      {key, cert_bin} = test_sign_256_key()
+      signed_xml = Dsig.sign(doc, key, cert_bin, :rsa_sha256)
+
+      # Drop KeyInfo from the serialized document: the enveloped-signature
+      # transform excludes ds:Signature from the digest, so it stays valid.
+      certless_doc =
+        [signed_xml]
+        |> :xmerl.export(:xmerl_xml)
+        |> List.flatten()
+        |> to_string()
+        |> String.replace(~r|<ds:KeyInfo>.*</ds:KeyInfo>|s, "")
+        |> parse_xml()
+
+      assert {:error, :missing_certificate} =
+               Dsig.verify(certless_doc, [:crypto.hash(:sha, cert_bin)])
+    end
   end
 
   describe "strip/1" do