reject empty subjectAltName and issuerAltName sequences

CHANGELOG.rst

@@ -8,6 +8,11 @@ Changelog
 
 .. note:: This version is not yet released and is under active development.
 
+* Parsing a ``subjectAltName`` or ``issuerAltName`` extension now rejects an
+  empty ``GeneralNames`` sequence, matching the ``SIZE (1..MAX)`` constraint
+  RFC 5280 4.2.1.6/4.2.1.7 places on the field and the strictness already
+  applied to ``extendedKeyUsage``.
+
 .. _v50-0-0:
 
 50.0.0 - 2026-07-31

docs/development/test-vectors.rst

@@ -681,6 +681,10 @@ Custom X.509 Vectors
   This is an invalid certificate per CA/B 7.1.2.7.6.
 * ``empty-eku.pem`` - A leaf certificate containing an empty EKU extension.
   This is an invalid certificate per :rfc:`5280` 4.2.1.12.
+* ``empty-san.pem`` - A leaf certificate containing an empty subjectAltName
+  extension. This is an invalid certificate per :rfc:`5280` 4.2.1.6.
+* ``empty-ian.pem`` - A leaf certificate containing an empty issuerAltName
+  extension. This is an invalid certificate per :rfc:`5280` 4.2.1.7.
 * ``malformed-san.pem`` - A certificate with a malformed SAN.
 * ``malformed-ian.pem`` - A certificate with a malformed IAN.
 * ``admissions_extension_optional_data_not_provided.pem`` -

src/rust/cryptography-x509-verification/src/lib.rs

@@ -144,7 +144,7 @@ impl Budget {
 
 struct NameChain<'a, 'chain> {
     child: Option<&'a NameChain<'a, 'chain>>,
-    sans: SubjectAlternativeName<'chain>,
+    sans: Option<SubjectAlternativeName<'chain>>,
 }
 
 impl<'a, 'chain> NameChain<'a, 'chain> {
@@ -157,10 +157,8 @@ impl<'a, 'chain> NameChain<'a, 'chain> {
             self_issued_intermediate,
             extensions.get_extension(&SUBJECT_ALTERNATIVE_NAME_OID),
         ) {
-            (false, Some(sans)) => sans.value::<SubjectAlternativeName<'chain>>()?,
-            // TODO: there really ought to be a better way to express an empty
-            // `asn1::SequenceOf`.
-            _ => asn1::parse_single(b"\x30\x00")?,
+            (false, Some(sans)) => Some(sans.value::<SubjectAlternativeName<'chain>>()?),
+            _ => None,
         };
 
         Ok(Self { child, sans })
@@ -253,7 +251,7 @@ impl<'a, 'chain> NameChain<'a, 'chain> {
             child.evaluate_constraints(constraints, budget)?;
         }
 
-        for san in self.sans.clone() {
+        for san in self.sans.iter().flat_map(|sans| sans.clone()) {
             // If there are no applicable constraints, the SAN is considered valid so the default is true.
             let mut permit = true;
             if let Some(permitted_subtrees) = &constraints.permitted_subtrees {

src/rust/cryptography-x509/src/extensions.rs

@@ -209,8 +209,8 @@ pub struct BasicConstraints {
     pub path_length: Option<u64>,
 }
 
-pub type SubjectAlternativeName<'a> = asn1::SequenceOf<'a, name::GeneralName<'a>>;
-pub type IssuerAlternativeName<'a> = asn1::SequenceOf<'a, name::GeneralName<'a>>;
+pub type SubjectAlternativeName<'a> = asn1::SequenceOf<'a, name::GeneralName<'a>, 1>;
+pub type IssuerAlternativeName<'a> = asn1::SequenceOf<'a, name::GeneralName<'a>, 1>;
 pub type ExtendedKeyUsage<'a> = asn1::SequenceOf<'a, asn1::ObjectIdentifier, 1>;
 
 pub struct KeyUsage<'a>(asn1::BitString<'a>);

src/rust/src/x509/common.rs

@@ -336,9 +336,9 @@ pub(crate) fn parse_general_name<'p>(
     Ok(py_gn)
 }
 
-pub(crate) fn parse_general_names<'a>(
+pub(crate) fn parse_general_names<'a, const MINIMUM_LEN: usize>(
     py: pyo3::Python<'a>,
-    gn_seq: &asn1::SequenceOf<'a, GeneralName<'a>>,
+    gn_seq: &asn1::SequenceOf<'a, GeneralName<'a>, MINIMUM_LEN>,
 ) -> CryptographyResult<pyo3::Bound<'a, pyo3::PyAny>> {
     let gns = pyo3::types::PyList::empty(py);
     for gn in gn_seq.clone() {

tests/x509/test_x509.py

@@ -6419,6 +6419,26 @@ def test_invalid_empty_eku(self):
         with pytest.raises(ValueError, match="InvalidSize"):
             cert.extensions.get_extension_for_class(ExtendedKeyUsage)
 
+    def test_invalid_empty_subject_alternative_name(self):
+        cert = _load_cert(
+            os.path.join("x509", "custom", "empty-san.pem"),
+            x509.load_pem_x509_certificate,
+        )
+
+        with pytest.raises(ValueError, match="InvalidSize"):
+            cert.extensions.get_extension_for_class(
+                x509.SubjectAlternativeName
+            )
+
+    def test_invalid_empty_issuer_alternative_name(self):
+        cert = _load_cert(
+            os.path.join("x509", "custom", "empty-ian.pem"),
+            x509.load_pem_x509_certificate,
+        )
+
+        with pytest.raises(ValueError, match="InvalidSize"):
+            cert.extensions.get_extension_for_class(x509.IssuerAlternativeName)
+
 
 class TestNameAttribute:
     EXPECTED_TYPES: typing.ClassVar[

vectors/cryptography_vectors/x509/custom/empty-ian.pem

@@ -0,0 +1,9 @@
+-----BEGIN CERTIFICATE-----
+MIIBMjCB2aADAgECAgQHW80VMAoGCCqGSM49BAMCMBoxGDAWBgNVBAMMD2NyeXB0
+b2dyYXBoeS5pbzAeFw0yNTAxMDEwMDAwMDBaFw0zNTAxMDEwMDAwMDBaMBoxGDAW
+BgNVBAMMD2NyeXB0b2dyYXBoeS5pbzBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IA
+BBB0GLaeloeWg3Gco3XAypqHZq48UdTm60oI6KfKH6+dtY/7EKFEJaYNj2tQfMS8
++EwNn3y2nUacAFdPweu0O92jDTALMAkGA1UdEgQCMAAwCgYIKoZIzj0EAwIDSAAw
+RQIhAIv3h7z+YD321gf/jnz2FhqcHZ+MZg7+Lhss7ym6Pt+0AiAYYE5CjExH/QTd
+muKQ8B78WqZE4glTIkKqN6050Ltokg==
+-----END CERTIFICATE-----

vectors/cryptography_vectors/x509/custom/empty-san.pem

@@ -0,0 +1,9 @@
+-----BEGIN CERTIFICATE-----
+MIIBMjCB2aADAgECAgQHW80VMAoGCCqGSM49BAMCMBoxGDAWBgNVBAMMD2NyeXB0
+b2dyYXBoeS5pbzAeFw0yNTAxMDEwMDAwMDBaFw0zNTAxMDEwMDAwMDBaMBoxGDAW
+BgNVBAMMD2NyeXB0b2dyYXBoeS5pbzBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IA
+BBB0GLaeloeWg3Gco3XAypqHZq48UdTm60oI6KfKH6+dtY/7EKFEJaYNj2tQfMS8
++EwNn3y2nUacAFdPweu0O92jDTALMAkGA1UdEQQCMAAwCgYIKoZIzj0EAwIDSAAw
+RQIgFtbOPouE/CuyKQ+TuGil2iOk2yq8ISLKjegKpMlRTV4CIQDQ5zSTl1p5nyoc
+B7o2lMSPf+6L8FVow8ASh1sb9rS18g==
+-----END CERTIFICATE-----