CVE-2026-69247
ADVISORY - githubSummary
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:
- invalid RSA padding →
Decryption failed - valid padding, bad key length →
Invalid key size (N) for AES., disclosingN - correct length, wrong key →
Invalid padding bytes. - 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.
GitHub
CVSS SCORE
8.2high| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| cryptography | pypi | - | - | >=44.0.0,<50.0.0 | 50.0.0 |
CVSS:4 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
A successful attack depends on conditions beyond the attacker's control, requiring investing a measurable amount of effort in research, preparation, or execution against the vulnerable component before a successful attack.
The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: A race condition must be won to successfully exploit the vulnerability. The successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker. The attack may need to be launched multiple times against a single target before being successful. Network injection. The attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any human user, other than the attacker. Examples include: a remote attacker is able to send packets to a target system a locally authenticated attacker executes code to elevate privileges.
There is a total loss of confidentiality, resulting in all information within the Vulnerable System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
There is no loss of integrity within the Vulnerable System.
There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
There is no impact to availability within the Vulnerable System.
There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.