Reject U2F assertion responses that assert CTAP2 flags.

(This doesn't apply to U2F register responses because there's no flags
byte there.)

Bug: 896980
Change-Id: I02c63d4bc63db4a8e2e7256a428d8349c34f5443
Reviewed-on: https://chromium-review.googlesource.com/c/1355242
Commit-Queue: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#612300}

device/fido/authenticator_get_assertion_response.cc

@@ -36,9 +36,15 @@ AuthenticatorGetAssertionResponse::CreateFromU2fSignResponse(
   if (key_handle.empty())
     return base::nullopt;
 
-  auto flags = u2f_data.subspan<kFlagIndex, kFlagLength>();
+  auto flags = u2f_data.subspan<kFlagIndex, kFlagLength>()[0];
+  if (flags &
+      (static_cast<uint8_t>(AuthenticatorData::Flag::kExtensionDataIncluded) |
+       static_cast<uint8_t>(AuthenticatorData::Flag::kAttestation))) {
+    // U2F responses cannot assert CTAP2 features.
+    return base::nullopt;
+  }
   auto counter = u2f_data.subspan<kCounterIndex, kCounterLength>();
-  AuthenticatorData authenticator_data(relying_party_id_hash, flags[0], counter,
+  AuthenticatorData authenticator_data(relying_party_id_hash, flags, counter,
                                        base::nullopt);
 
   auto signature =

device/fido/ctap_response_unittest.cc

@@ -530,6 +530,21 @@ TEST(CTAPResponseTest, TestParseU2fSignWithNullResponse) {
   EXPECT_FALSE(response);
 }
 
+TEST(CTAPResponseTest, TestParseU2fSignWithCTAP2Flags) {
+  std::vector<uint8_t> sign_response = GetTestSignResponse();
+  // Set two flags that should only be set in CTAP2 responses and expect parsing
+  // to fail.
+  sign_response[0] |=
+      static_cast<uint8_t>(AuthenticatorData::Flag::kExtensionDataIncluded);
+  sign_response[0] |=
+      static_cast<uint8_t>(AuthenticatorData::Flag::kAttestation);
+
+  auto response = AuthenticatorGetAssertionResponse::CreateFromU2fSignResponse(
+      test_data::kApplicationParameter, sign_response,
+      GetTestCredentialRawIdBytes());
+  EXPECT_FALSE(response);
+}
+
 TEST(CTAPResponseTest, TestParseU2fSignWithNullCorruptedCounter) {
   // A sign response of less than 5 bytes.
   auto response = AuthenticatorGetAssertionResponse::CreateFromU2fSignResponse(