raise ValueError for unrecognised TLS feature values

src/rust/src/x509/certificate.rs

@@ -830,7 +830,12 @@ pub fn parse_cert_ext<'p>(
 
             let features = pyo3::types::PyList::empty(py);
             for feature in ext.value::<asn1::SequenceOf<'_, u64>>()? {
-                let py_feature = tls_feature_type_to_enum.get_item(feature)?;
+                // A value that is not a known TLSFeatureType cannot be
+                // represented, so surface it as a parse error rather than
+                // letting the dict lookup raise a bare KeyError.
+                let py_feature = tls_feature_type_to_enum
+                    .get_item(feature)
+                    .map_err(|_| asn1::ParseError::new(asn1::ParseErrorKind::InvalidValue))?;
                 features.append(py_feature)?;
             }
             Ok(Some(types::TLS_FEATURE.get(py)?.call1((features,))?))

tests/x509/test_x509.py

@@ -1748,6 +1748,38 @@ def test_parse_tls_feature_extension(self):
             [x509.TLSFeatureType.status_request]
         )
 
+    def test_tls_feature_unknown_value(self, rsa_key_2048: rsa.RSAPrivateKey):
+        # A TLSFeature extension carrying a value that is not a known
+        # TLSFeatureType must raise a ValueError, not a bare KeyError.
+        private_key = rsa_key_2048
+        cert = (
+            x509.CertificateBuilder()
+            .subject_name(
+                x509.Name([x509.NameAttribute(NameOID.COMMON_NAME, "x")])
+            )
+            .issuer_name(
+                x509.Name([x509.NameAttribute(NameOID.COMMON_NAME, "x")])
+            )
+            .public_key(private_key.public_key())
+            .serial_number(1)
+            .not_valid_before(datetime.datetime(2020, 1, 1))
+            .not_valid_after(datetime.datetime(2030, 1, 1))
+            .add_extension(
+                # SEQUENCE { INTEGER 99 }; 99 is not a valid TLSFeatureType.
+                x509.UnrecognizedExtension(
+                    x509.ObjectIdentifier("1.3.6.1.5.5.7.1.24"),
+                    b"\x30\x03\x02\x01\x63",
+                ),
+                critical=False,
+            )
+            .sign(private_key, hashes.SHA256())
+        )
+        cert = x509.load_der_x509_certificate(
+            cert.public_bytes(serialization.Encoding.DER)
+        )
+        with pytest.raises(ValueError):
+            cert.extensions
+
     def test_verify_directly_issued_by_rsa_pss(
         self, rsa_key_2048: rsa.RSAPrivateKey
     ):