feat(webauthn): scrub attestation for attestation=none conveyance

WebAuthn L3 §5.4.1 requires the platform to anonymise the attestation
object when the relying party requests attestation conveyance "none":
replace fmt with "none", attStmt with an empty map, and zero the AAGUID
in the authenticator data.

libwebauthn parsed the conveyance preference but discarded it, forwarding
the authenticator's real format, attStmt, and AAGUID verbatim. This honours
the preference at response assembly time. The WebAuthn IDL default of "none"
is applied at the IDL boundary, so JSON requests that omit attestation are
scrubbed too. Requests built directly with attestation None pass through.

Author: AlfioEmanueleFresta

libwebauthn-tests/tests/basic_ctap2.rs

@@ -46,6 +46,7 @@ async fn test_webauthn_basic_ctap2() {
         resident_key: Some(ResidentKeyRequirement::Discouraged),
         user_verification: UserVerificationRequirement::Preferred,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions: None,
         timeout: TIMEOUT,

libwebauthn-tests/tests/large_blob.rs

@@ -53,6 +53,7 @@ async fn test_webauthn_large_blob_read_returns_planted_blob() {
         resident_key: Some(ResidentKeyRequirement::Required),
         user_verification: UserVerificationRequirement::Discouraged,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions: Some(MakeCredentialsRequestExtensions {
             large_blob: Some(MakeCredentialLargeBlobExtensionInput {
@@ -245,6 +246,7 @@ async fn register_with_large_blob(
         resident_key: Some(ResidentKeyRequirement::Required),
         user_verification: UserVerificationRequirement::Discouraged,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions: Some(MakeCredentialsRequestExtensions {
             large_blob: Some(MakeCredentialLargeBlobExtensionInput {

libwebauthn-tests/tests/preflight.rs

@@ -55,6 +55,7 @@ async fn make_credential_call_with_rp(
         resident_key: Some(ResidentKeyRequirement::Discouraged),
         user_verification: UserVerificationRequirement::Preferred,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: exclude_list,
         extensions: None,
         timeout: TIMEOUT,

libwebauthn-tests/tests/prf.rs

@@ -105,6 +105,7 @@ async fn test_webauthn_prf_eval_at_create_degrades_when_unsupported() {
         resident_key: Some(ResidentKeyRequirement::Discouraged),
         user_verification: UserVerificationRequirement::Discouraged,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions: Some(extensions),
         timeout: TIMEOUT,
@@ -179,6 +180,7 @@ async fn run_test_battery(channel: &mut HidChannel<'_>, using_pin: bool) {
         resident_key: Some(ResidentKeyRequirement::Discouraged),
         user_verification: UserVerificationRequirement::Preferred,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions: Some(extensions),
         timeout: TIMEOUT,
@@ -672,6 +674,7 @@ async fn test_webauthn_prf_variable_length_input() {
         resident_key: Some(ResidentKeyRequirement::Discouraged),
         user_verification: UserVerificationRequirement::Preferred,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions: Some(MakeCredentialsRequestExtensions {
             prf: Some(MakeCredentialPrfInput { eval: None }),
@@ -797,6 +800,7 @@ fn basic_make_credential_request(
         resident_key: Some(ResidentKeyRequirement::Discouraged),
         user_verification,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions,
         timeout: TIMEOUT,

libwebauthn-tests/tests/signature_roundtrip.rs

@@ -75,6 +75,7 @@ async fn test_ctap2_signature_roundtrip() {
         resident_key: Some(ResidentKeyRequirement::Discouraged),
         user_verification: UserVerificationRequirement::Preferred,
         algorithms: vec![Ctap2CredentialType::default()],
+        attestation: None,
         exclude: None,
         extensions: Some(MakeCredentialsRequestExtensions {
             hmac_create_secret: Some(true),

libwebauthn/examples/features/webauthn_extensions_hid.rs

@@ -65,6 +65,7 @@ pub async fn main() -> Result<(), Box<dyn Error>> {
             algorithms: vec![Ctap2CredentialType::default()],
             exclude: None,
             extensions: Some(extensions.clone()),
+            attestation: None,
             timeout: TIMEOUT,
         };
 

libwebauthn/examples/features/webauthn_preflight_hid.rs

@@ -120,6 +120,7 @@ async fn make_credential_call(
         algorithms: vec![Ctap2CredentialType::default()],
         exclude: exclude_list,
         extensions: None,
+        attestation: None,
         timeout: TIMEOUT,
     };
 

libwebauthn/examples/features/webauthn_prf_cable.rs

@@ -118,6 +118,7 @@ async fn create() -> Result<(), Box<dyn Error>> {
         algorithms: vec![Ctap2CredentialType::default()],
         exclude: None,
         extensions: Some(extensions),
+        attestation: None,
         timeout: TIMEOUT,
     };
 

libwebauthn/examples/features/webauthn_prf_hid.rs

@@ -58,6 +58,7 @@ pub async fn main() -> Result<(), Box<dyn Error>> {
             algorithms: vec![Ctap2CredentialType::default()],
             exclude: None,
             extensions: Some(extensions.clone()),
+            attestation: None,
             timeout: TIMEOUT,
         };
 

libwebauthn/src/ops/webauthn/make_credential.rs

@@ -80,6 +80,35 @@ fn registration_transports(transport: Option<Transport>) -> Vec<String> {
     tokens
 }
 
+fn scrub_aaguid(authenticator_data: &mut [u8]) -> Result<(), ResponseSerializationError> {
+    const AAGUID_OFFSET: usize = 37;
+    const AAGUID_LEN: usize = 16;
+    authenticator_data
+        .get_mut(AAGUID_OFFSET..AAGUID_OFFSET + AAGUID_LEN)
+        .ok_or_else(|| {
+            ResponseSerializationError::AuthenticatorDataError(
+                "authenticator data too short to scrub AAGUID".into(),
+            )
+        })?
+        .fill(0);
+    Ok(())
+}
+
+fn build_attestation_object(
+    format: &str,
+    attestation_statement: &Ctap2AttestationStatement,
+    authenticator_data_bytes: &[u8],
+) -> Result<Vec<u8>, ResponseSerializationError> {
+    let attestation_object = AttestationObject {
+        format,
+        auth_data: authenticator_data_bytes,
+        attestation_statement,
+    };
+
+    cbor::to_vec(&attestation_object)
+        .map_err(|e| ResponseSerializationError::AttestationObjectError(e.to_string()))
+}
+
 impl WebAuthnIDLResponse for MakeCredentialResponse {
     type IdlModel = RegistrationResponseJSON;
     type Context = MakeCredentialRequest;
@@ -102,11 +131,16 @@ impl WebAuthnIDLResponse for MakeCredentialResponse {
         let id = base64_url::encode(&attested.credential_id);
         let raw_id = Base64UrlString::from(attested.credential_id.clone());
 
-        let authenticator_data_bytes = self
+        let mut authenticator_data_bytes = self
             .authenticator_data
             .to_response_bytes()
             .map_err(|e| ResponseSerializationError::AuthenticatorDataError(e.to_string()))?;
 
+        let scrub_attestation = request.attestation.as_deref() == Some("none");
+        if scrub_attestation {
+            scrub_aaguid(&mut authenticator_data_bytes)?;
+        }
+
         let public_key_algorithm = i64::from(
             cose::read_alg(&attested.credential_public_key)
                 .map_err(|e| ResponseSerializationError::PublicKeyError(e.to_string()))?,
@@ -120,7 +154,14 @@ impl WebAuthnIDLResponse for MakeCredentialResponse {
             .map(Base64UrlString::from);
 
         // Build attestation object (CBOR map with authData, fmt, attStmt)
-        let attestation_object_bytes = self.build_attestation_object(&authenticator_data_bytes)?;
+        let none_statement = Ctap2AttestationStatement::None(BTreeMap::new());
+        let (format, attestation_statement) = if scrub_attestation {
+            ("none", &none_statement)
+        } else {
+            (self.format.as_str(), &self.attestation_statement)
+        };
+        let attestation_object_bytes =
+            build_attestation_object(format, attestation_statement, &authenticator_data_bytes)?;
 
         // WebAuthn getTransports(): the authenticator's getInfo 0x09 transports
         // folded with the ceremony transport, unique tokens lexicographically sorted.
@@ -151,20 +192,6 @@ impl WebAuthnIDLResponse for MakeCredentialResponse {
 }
 
 impl MakeCredentialResponse {
-    fn build_attestation_object(
-        &self,
-        authenticator_data_bytes: &[u8],
-    ) -> Result<Vec<u8>, ResponseSerializationError> {
-        let attestation_object = AttestationObject {
-            format: &self.format,
-            auth_data: authenticator_data_bytes,
-            attestation_statement: &self.attestation_statement,
-        };
-
-        cbor::to_vec(&attestation_object)
-            .map_err(|e| ResponseSerializationError::AttestationObjectError(e.to_string()))
-    }
-
     fn build_client_extension_results(&self) -> AuthenticationExtensionsClientOutputsJSON {
         let mut results = AuthenticationExtensionsClientOutputsJSON::default();
         let unsigned_ext = &self.unsigned_extensions_output;
@@ -374,6 +401,8 @@ pub struct MakeCredentialRequest {
     pub exclude: Option<Vec<Ctap2PublicKeyCredentialDescriptor>>,
     /// extensions
     pub extensions: Option<MakeCredentialsRequestExtensions>,
+    /// Attestation conveyance preference. `Some("none")` scrubs attestation.
+    pub attestation: Option<String>,
     pub timeout: Duration,
 }
 
@@ -468,6 +497,8 @@ impl FromIdlModel<PublicKeyCredentialCreationOptionsJSON> for MakeCredentialRequ
                 )
             },
             extensions: inner.extensions,
+            // WebAuthn IDL defaults attestation conveyance to "none".
+            attestation: inner.attestation.or_else(|| Some("none".to_string())),
             timeout,
         })
     }
@@ -631,6 +662,7 @@ impl MakeCredentialRequest {
             algorithms: vec![Ctap2CredentialType::default()],
             exclude: None,
             extensions: None,
+            attestation: None,
             resident_key: None,
             user_verification: UserVerificationRequirement::Discouraged,
             timeout: Duration::from_secs(10),
@@ -847,6 +879,7 @@ mod tests {
             algorithms: vec![Ctap2CredentialType::default()],
             exclude: None,
             extensions: None,
+            attestation: Some("none".to_string()),
             timeout: Duration::from_secs(30),
         }
     }
@@ -1504,6 +1537,7 @@ mod tests {
             algorithms: vec![Ctap2CredentialType::default()],
             exclude: None,
             extensions: None,
+            attestation: None,
             timeout: Duration::from_secs(30),
         }
     }
@@ -1731,6 +1765,114 @@ mod tests {
         }
     }
 
+    fn create_attested_response(aaguid: [u8; 16]) -> MakeCredentialResponse {
+        use crate::fido::{AttestedCredentialData, AuthenticatorData, AuthenticatorDataFlags};
+        use crate::proto::ctap2::FidoU2fAttestationStmt;
+        use cosey::Bytes;
+        use serde_bytes::ByteBuf;
+
+        let cose_public_key = cosey::PublicKey::P256Key(cosey::P256PublicKey {
+            x: Bytes::from_slice(&[0u8; 32]).unwrap(),
+            y: Bytes::from_slice(&[0u8; 32]).unwrap(),
+        });
+        let credential_public_key = cbor::to_vec(&cose_public_key).unwrap();
+
+        let authenticator_data = AuthenticatorData {
+            rp_id_hash: [0u8; 32],
+            flags: AuthenticatorDataFlags::USER_PRESENT
+                | AuthenticatorDataFlags::ATTESTED_CREDENTIALS,
+            signature_count: 0,
+            attested_credential: Some(AttestedCredentialData {
+                aaguid,
+                credential_id: vec![0x01, 0x02, 0x03, 0x04],
+                credential_public_key,
+            }),
+            extensions: None,
+            raw: None,
+        };
+
+        MakeCredentialResponse {
+            format: "fido-u2f".to_string(),
+            authenticator_data,
+            attestation_statement: Ctap2AttestationStatement::FidoU2F(FidoU2fAttestationStmt {
+                signature: ByteBuf::from(vec![0xAA; 16]),
+                certificates: vec![ByteBuf::from(vec![0xBB; 8])],
+            }),
+            enterprise_attestation: None,
+            large_blob_key: None,
+            unsigned_extensions_output: MakeCredentialsResponseUnsignedExtensions::default(),
+            transport: None,
+            authenticator_transports: None,
+        }
+    }
+
+    #[test]
+    fn attestation_none_conveyance_scrubs_fmt_attstmt_and_aaguid() {
+        let response = create_attested_response([0x11u8; 16]);
+        let mut request = create_test_request();
+        request.attestation = Some("none".to_string());
+
+        let model = response.to_idl_model(&request).unwrap();
+
+        let auth_data = &model.response.authenticator_data.0;
+        assert_eq!(&auth_data[37..53], &[0u8; 16], "top-level authData AAGUID");
+
+        let attestation: cbor::Value =
+            cbor::from_slice(&model.response.attestation_object.0).unwrap();
+        let cbor::Value::Map(map) = attestation else {
+            panic!("attestation object should be a CBOR map");
+        };
+        let value_for = |key: &str| {
+            map.iter()
+                .find(|(k, _)| matches!(k, cbor::Value::Text(s) if s == key))
+                .map(|(_, v)| v)
+        };
+        assert!(
+            matches!(value_for("fmt"), Some(cbor::Value::Text(s)) if s == "none"),
+            "fmt must be scrubbed to none"
+        );
+        match value_for("attStmt") {
+            Some(cbor::Value::Map(stmt)) => assert!(stmt.is_empty(), "attStmt must be empty"),
+            other => panic!("attStmt must be an empty map, got {other:?}"),
+        }
+        match value_for("authData") {
+            Some(cbor::Value::Bytes(embedded)) => {
+                assert_eq!(&embedded[37..53], &[0u8; 16], "embedded authData AAGUID");
+            }
+            other => panic!("authData must be CBOR bytes, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn attestation_direct_preserves_attestation() {
+        let response = create_attested_response([0x11u8; 16]);
+        let mut request = create_test_request();
+        request.attestation = Some("direct".to_string());
+
+        let model = response.to_idl_model(&request).unwrap();
+
+        let auth_data = &model.response.authenticator_data.0;
+        assert_eq!(
+            &auth_data[37..53],
+            &[0x11u8; 16],
+            "AAGUID must be preserved"
+        );
+
+        let attestation: cbor::Value =
+            cbor::from_slice(&model.response.attestation_object.0).unwrap();
+        let cbor::Value::Map(map) = attestation else {
+            panic!("attestation object should be a CBOR map");
+        };
+        let fmt = map
+            .iter()
+            .find(|(k, _)| matches!(k, cbor::Value::Text(s) if s == "fmt"))
+            .map(|(_, v)| v);
+        assert!(
+            matches!(fmt, Some(cbor::Value::Text(s)) if s == "fido-u2f"),
+            "fmt must be preserved"
+        );
+    }
+
     #[test]
     fn test_response_with_extensions() {
         let mut response = create_test_response();