fix(ctap2): preserve raw authenticatorData bytes

The authenticator signs over the exact authData bytes, including the
extensions block. Rebuilding authData from the parsed struct could drop
unmodeled extensions or reorder keys, breaking the relying party's
signature verification. Keep the device bytes and return them unchanged.
Platform-synthesized U2F authData still rebuilds from the fields.

Author: AlfioEmanueleFresta

libwebauthn/src/fido.rs

@@ -100,13 +100,24 @@ pub struct AuthenticatorData<T> {
     pub signature_count: u32,
     pub attested_credential: Option<AttestedCredentialData>,
     pub extensions: Option<T>,
+    /// Raw authData bytes as received from the device, preserved verbatim so
+    /// the RP's signature over authData stays valid. `None` for authData the
+    /// platform synthesizes (e.g. the U2F upgrade path), which is rebuilt from
+    /// the fields above.
+    pub raw: Option<Vec<u8>>,
 }
 
 impl<T> AuthenticatorData<T>
 where
     T: Clone + Serialize,
 {
     pub fn to_response_bytes(&self) -> Result<Vec<u8>, Error> {
+        // Return the device's authData verbatim. Re-encoding from the parsed
+        // fields would reorder or drop unmodeled extensions, invalidating the
+        // authenticator's signature over these exact bytes.
+        if let Some(raw) = &self.raw {
+            return Ok(raw.clone());
+        }
         // Name                    | Length
         // -----------------------------------
         // rpIdHash                | 32
@@ -284,6 +295,7 @@ impl<'de, T: DeserializeOwned> Deserialize<'de> for AuthenticatorData<T> {
                     signature_count,
                     attested_credential,
                     extensions,
+                    raw: Some(data.to_vec()),
                 })
             }
         }
@@ -349,6 +361,7 @@ mod tests {
             signature_count,
             attested_credential: Some(attested_credential.clone()),
             extensions: Some(extensions.clone()),
+            raw: None,
         };
         let webauthn_auth_data = auth_data.to_response_bytes().unwrap();
         assert_eq!(rp_id_hash, &webauthn_auth_data[..32]);
@@ -432,6 +445,7 @@ mod tests {
             signature_count: 1,
             attested_credential: Some(attested_credential),
             extensions: None,
+            raw: None,
         };
 
         let bytes = auth_data.to_response_bytes().unwrap();
@@ -445,4 +459,37 @@ mod tests {
             crate::proto::ctap2::Ctap2COSEAlgorithmIdentifier::RS256
         );
     }
+
+    #[test]
+    fn to_response_bytes_preserves_extensions_verbatim() {
+        // The authenticator signs over the exact authData bytes, including the
+        // extensions block. Re-encoding from the typed struct drops keys it does
+        // not model (and may reorder the rest), which would break relying-party
+        // signature verification. The bytes must round-trip unchanged.
+        use crate::proto::ctap2::Ctap2MakeCredentialsResponseExtensions;
+
+        let flags = AuthenticatorDataFlags::USER_PRESENT | AuthenticatorDataFlags::EXTENSION_DATA;
+        let mut input = [0x11u8; 32].to_vec();
+        input.push(flags.bits());
+        input.extend_from_slice(&[0x00, 0x00, 0x00, 0x07]); // signCount
+
+        // CBOR: { "credBlob": true, "thirdPartyPayment": true }. The second key
+        // is unmodeled, so the typed struct drops it on re-encode.
+        input.extend_from_slice(&[
+            0xA2, // map(2)
+            0x68, b'c', b'r', b'e', b'd', b'B', b'l', b'o', b'b', 0xF5, // "credBlob": true
+            0x71, b't', b'h', b'i', b'r', b'd', b'P', b'a', b'r', b't', b'y', b'P', b'a', b'y',
+            b'm', b'e', b'n', b't', 0xF5, // "thirdPartyPayment": true
+        ]);
+
+        let wrapped = cbor::to_vec(&ByteBuf::from(input.clone())).unwrap();
+        let parsed: AuthenticatorData<Ctap2MakeCredentialsResponseExtensions> =
+            cbor::from_slice(&wrapped).unwrap();
+
+        assert_eq!(
+            parsed.to_response_bytes().unwrap(),
+            input,
+            "authenticatorData must be preserved byte-for-byte"
+        );
+    }
 }

libwebauthn/src/ops/u2f.rs

@@ -143,6 +143,7 @@ impl UpgradableResponse<MakeCredentialResponse, MakeCredentialRequest> for Regis
             signature_count,
             attested_credential: Some(attested_cred_data),
             extensions: None,
+            raw: None,
         };
 
         // Let attestationStatement be a CBOR map (see "attStmtTemplate" in Generating an Attestation Object [WebAuthn])
@@ -201,6 +202,7 @@ impl UpgradableResponse<GetAssertionResponse, SignRequest> for SignResponse {
             signature_count,
             attested_credential: None,
             extensions: None,
+            raw: None,
         };
 
         // Let authenticatorGetAssertionResponse be a CBOR map with the following keys whose values are as follows: [..]

libwebauthn/src/ops/webauthn/get_assertion.rs

@@ -1215,6 +1215,7 @@ mod tests {
             signature_count: 1,
             attested_credential: None,
             extensions: None,
+            raw: None,
         };
 
         Assertion {

libwebauthn/src/ops/webauthn/make_credential.rs

@@ -1382,6 +1382,7 @@ mod tests {
             signature_count: 0,
             attested_credential: Some(attested_credential),
             extensions: None,
+            raw: None,
         };
 
         MakeCredentialResponse {

libwebauthn/src/proto/ctap2/model/get_assertion.rs

@@ -569,6 +569,7 @@ mod tests {
                 signature_count: 0,
                 attested_credential: None,
                 extensions: None,
+                raw: None,
             },
             signature: ByteBuf::from(vec![0u8; 32]),
             user: None,