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cryptography: PKCS#7 EnvelopedData decryption exposes a Bleichenbacher oracle through distinguishable errors and timingGHSA-g6cj-pr64-35w5

Fix: pyca/cryptography#15369

GHSA-g6cj-pr64-35w5 is a CWE-208 vulnerability in cryptography. A fix is available for cryptography — see the affected versions and patch details below.

Also known asCVE-2026-69247PYSEC-2026-3552
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-g6cj-pr64-35w5.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs20th percentile — riskier than 20% of all scored CVEsHighest risk
0.00%0.26%0.52%0.77%0.2%0.3%0.3%0.3%Sep 26Nov 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected
🐍cryptography

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIcryptography≥ 44.0.0&&< 50.0.050.0.0pip install --upgrade 'cryptography==50.0.0'

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for cryptography, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update cryptography to 50.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g6cj-pr64-35w5 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Red Hat rates this Moderate (CVSS 3.1 5.9). Exploitation requires an application that decrypts attacker-supplied PKCS#7 EnvelopedData via pkcs7_decrypt_* and leaks errors or timing. GitHub CNA CVSS 4.0 8.2 assumes that oracle is readily available.

Workaround published by Red Hat
The vulnerable APIs were introduced in cryptography 44.0.0 and fixed in 50.0.0.
Source: Red Hat security advisory for GHSA-g6cj-pr64-35w5 (CC BY 4.0)
ProductFixed inAdvisory
Red Hat AI Inference Server 3.4rhaii/vllm-cpu-rhel9:1790075793RHSA-2026:70965
Red Hat AI Inference Server 3.4rhaii/vllm-spyre-rhel9:1790076141RHSA-2026:70969
Red Hat AI Inference Server 3.4rhaii/vllm-cuda-rhel9:1790090131RHSA-2026:70979
Red Hat AI Inference Server 3.4rhaii/vllm-rocm-rhel9:1790109620RHSA-2026:70995
Red Hat Hardened Imagespython-cryptography-main-50.0.0-1.hum1RHSA-2026:55543
Ubuntu
USN-8776-1Ubuntu

Frequently Asked Questions

### 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
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