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

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Nov 27, 2023
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Fixed crash when loading a PKCS#7 bundle with no certificates #9926

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5 changes: 4 additions & 1 deletion src/cryptography/hazmat/backends/openssl/backend.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1111,9 +1111,12 @@ def _load_pkcs7_certificates(self, p7) -> list[x509.Certificate]:
_Reasons.UNSUPPORTED_SERIALIZATION,
)

certs: list[x509.Certificate] = []
if p7.d.sign == self._ffi.NULL:
return certs
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@davidben davidben Nov 29, 2023 •
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I don't think this fix is quite right. I believe this should actually raise an exception here, not return the empty list, because it's analogous to the block above, where you reject content types other than signed data.

Specifically, this is NULL if you've got a content-less ContentInfo:
https://datatracker.ietf.org/doc/html/rfc2315#section-7

The spec says:

    o    content is the content. The field is optional, and
         if the field is not present, its intended value must be
         supplied by other means. Its type is defined along with the
         object identifier for contentType.

It honestly doesn't make any sense for the signed-data ContentInfos that you're parsing here. It's more for the data ContentInfo inside the signed-data ContentInfo. As the spec says later on here:

    3.   The optional omission of the content field makes
         it possible to construct "external signatures," for
         example, without modification to or replication of the
         content to which the signatures apply. In the case of
         external signatures, the content being signed would be
         omitted from the "inner" encapsulated ContentInfo value
         included in the signed-data content type.

That is, an omitted one means "this is a signed data, but I am omitting the signature, certificates, etc., with the expectation that you have it out of band somewhere".

Setting aside whether there are any use cases for that, it certainly doesn't make sense in the context of your function. By returning the empty list, you are telling the caller "yup, this contained zero certificates!". But this actually may have contained a ton of certificates, you just didn't have them available. So this is actually a PKCS#7 blob that isn't of the type you expect.

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I should add that, as far as compatibility goes, nothing can possibly be relying on you returning the empty list here because it used to crash. :-)


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)
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6 changes: 6 additions & 0 deletions tests/hazmat/primitives/test_pkcs7.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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