GHSA-g6cj-pr64-35w5

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GHSA-g6cj-pr64-35w5

Summary: cryptography: PKCS#7 EnvelopedData decryption exposes a Bleichenbacher oracle through distinguishable errors and timing

Details: ### Summary `pkcs7_decrypt_der`, `pkcs7_decrypt_pem`, and `pkcs7_decrypt_smime` reported the outcome of decrypting a `RecipientInfo`'s `encryptedKey` in several distinguishable ways, one of which disclosed the exact length recovered from the RSA operation. The same distinction was also observable by timing. An application that decrypts attacker-supplied `EnvelopedData` and reflects the outcome gives the attacker a Bleichenbacher oracle against the content-encryption key. Introduced in 44.0.0. Fixed in 50.0.0. ### Details Decryption ran as: RSA PKCS#1 v1.5 decrypt of `encryptedKey` → build an AES cipher from the result → AES-CBC decrypt and PKCS#7 unpad. Each stage failed differently, with no RFC 3218 mitigation: 1. invalid RSA padding → `Decryption failed` 2. valid padding, bad key length → `Invalid key size (N) for AES.`, disclosing `N` 3. correct length, wrong key → `Invalid padding bytes.` 4. the real key → plaintext Case 1 is reachable only where the linked library lacks implicit rejection: OpenSSL 3.0 and 3.1, LibreSSL, and BoringSSL. On OpenSSL 3.2+, used in our wheels, invalid padding instead returns a synthetic plaintext of pseudorandom length, so the error channel does not distinguish conforming ciphertexts. Exploitation requires a service that auto-decrypts untrusted `EnvelopedData` matching the victim certificate and answers adaptively at high volume, such as an S/MIME gateway or mail filter. ### Fix Per RFC 3218, the content-encryption algorithm is now resolved before the private key is used, so the expected key length is known in advance. If the RSA decryption fails or recovers a key of the wrong length, a random key of the expected length is substituted and decryption continues down an identical path. All failures now report identically and perform the same work. ### Not addressed by this fix `EnvelopedData` does not authenticate its content. Tampering with `encryptedContent` alone yields a CBC padding oracle that recovers plaintext at roughly 256 queries per byte, without recovering any key, on every backend. This is a property of PKCS#7 rather than of this implementation, cannot be fixed in the library, and is now documented. ### Credit Reported by @X1AOxiang.

References: https://github.com/pyca/cryptography/security/advisories/GHSA-g6cj-pr64-35w5, https://github.com/pyca/cryptography/pull/15369, https://github.com/pyca/cryptography/commit/53fccd93413a8d7f07d6d8999681f27b75cffa3f, https://github.com/pyca/cryptography