Sign in
Deutsch English Español Polski
High

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

  • Disclosed: 03.08.2026
  • Updated: 11.10.2026 15:17

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 software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) cryptography >= 44.0.0, < 50.0.0 50.0.0

References

CVSS score -
CVSS vector
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
Weakness (CWE)
CWE-208, CWE-209
Platform
Python (PyPI)
Package
cryptography
Disclosed
03.08.2026
Updated
11.10.2026 15:17

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2026-3552 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-g6cj-pr64-35w5 - CC BY 4.0

Vulnerability history: cryptography

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (24) in the archive
Total entries
24
First → latest
03.2017 08.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • High 12
  • Medium 7
  • Low 5

Disclosures per year

Severity:
  1. python-cryptography: Duplicate self-signed intermediates can cause exponential path-building
    High CVE-2026-69249 >= 42.0.0, < 49.0.0 49.0.0
  2. cryptography: PKCS#7 EnvelopedData decryption exposes a Bleichenbacher oracle through distinguishable errors and timing this entry
    High CVE-2026-69247 >= 44.0.0, < 50.0.0 50.0.0
  3. Vulnerable OpenSSL included in cryptography wheels
    High · 7.5 ghsa-537c-gmf6-5ccf >= 0.5.0, < 48.0.1 48.0.1
  4. cryptography Vulnerable to a Subgroup Attack Due to Missing Subgroup Validation for SECT Curves
    High CVE-2026-26007 < 46.0.5 46.0.5
  5. cryptography NULL pointer dereference with pkcs12.serialize_key_and_certificates when called with a non-matching certificate and private key and an hmac_hash override
    High · 7.5 CVE-2024-26130 >= 38.0.0, < 42.0.4 42.0.4
  6. Python Cryptography package vulnerable to Bleichenbacher timing oracle attack
    High · 7.5 CVE-2023-50782 < 42.0.0 42.0.0
  7. cryptography mishandles SSH certificates
    High · 7.5 CVE-2023-38325 >= 40.0.0, < 41.0.2 41.0.2
  8. Vulnerable OpenSSL included in cryptography wheels
    High · 7.4 CVE-2023-0286 >= 0.8.1, < 39.0.1 39.0.1
  9. PyCA Cryptography symmetrically encrypting large values can lead to integer overflow
    High · 9.1 CVE-2020-36242 >= 3.1, < 3.3.2 3.3.2
  10. RSA decryption vulnerable to Bleichenbacher timing vulnerability
    High · 5.9 CVE-2020-25659 < 3.2 3.2
  11. PyCA Cryptography vulnerable to GCM tag forgery
    High · 7.5 CVE-2018-10903 >= 1.9.0, < 2.3 2.3
  12. Improper input validation in cryptography
    High · 7.5 CVE-2016-9243 < 1.5.3 1.5.3

Know which vulnerabilities affect your application

With the DockRay integration and software inventory enabled, DockRay checks reported component versions against known vulnerabilities and notifies you about confirmed matches. Coverage depends on the supported technology and available advisory data.

Connect your application
DockRay newsletter

Application security in your inbox

Important vulnerabilities, actionable guidance and practical advice for your applications. Choose your technologies and follow what matters to your project.

  • A daily digest of new vulnerabilities, critical ones included.
  • Notifications about important updates.
  • Practical advice on application security and monitoring.

Leave empty to receive updates for every technology.

Privacy policy

Free to subscribe. Unsubscribe with one click in any email.