Fixed crash when loading a PKCS#7 bundle with no certificates (pyca#9926)

Author: alex

src/cryptography/hazmat/backends/openssl/backend.py

@@ -1890,9 +1890,12 @@ def _load_pkcs7_certificates(self, p7) -> typing.List[x509.Certificate]:
                 _Reasons.UNSUPPORTED_SERIALIZATION,
             )
 
+        certs: list[x509.Certificate] = []
+        if p7.d.sign == self._ffi.NULL:
+            return certs
+
         sk_x509 = p7.d.sign.cert
         num = self._lib.sk_X509_num(sk_x509)
-        certs = []
         for i in range(num):
             x509 = self._lib.sk_X509_value(sk_x509, i)
             self.openssl_assert(x509 != self._ffi.NULL)

tests/hazmat/primitives/test_pkcs7.py

@@ -89,6 +89,12 @@ def test_load_pkcs7_unsupported_type(self, backend):
                 mode="rb",
             )
 
+    def test_load_pkcs7_empty_certificates(self):
+        der = b"\x30\x0B\x06\x09\x2A\x86\x48\x86\xF7\x0D\x01\x07\x02"
+
+        certificates = pkcs7.load_der_pkcs7_certificates(der)
+        assert certificates == []
+
 
 # We have no public verification API and won't be adding one until we get
 # some requirements from users so this function exists to give us basic