GCVE Workshop - 22 September 2026 (14:00-18:00), Luxembourg Before The Vulnopticon Conference - Registration

GHSA-G6CJ-PR64-35W5

Vulnerability from github – Published: 2026-08-03 21:17 – Updated: 2026-08-03 21:17
VLAI
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.

Severity
8.2 (High)
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
Show details on source website
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "cryptography"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "44.0.0"
            },
            {
              "fixed": "50.0.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-69247"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-208",
      "CWE-209"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-03T21:17:00Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\n`pkcs7_decrypt_der`, `pkcs7_decrypt_pem`, and `pkcs7_decrypt_smime` reported the\noutcome of decrypting a `RecipientInfo`\u0027s `encryptedKey` in several\ndistinguishable ways, one of which disclosed the exact length recovered from the\nRSA operation. The same distinction was also observable by timing. An\napplication that decrypts attacker-supplied `EnvelopedData` and reflects the\noutcome gives the attacker a Bleichenbacher oracle against the\ncontent-encryption key.\n\nIntroduced in 44.0.0. Fixed in 50.0.0.\n\n### Details\n\nDecryption ran as: RSA PKCS#1 v1.5 decrypt of `encryptedKey` \u2192 build an AES\ncipher from the result \u2192 AES-CBC decrypt and PKCS#7 unpad. Each stage failed\ndifferently, with no RFC 3218 mitigation:\n\n1. invalid RSA padding \u2192 `Decryption failed`\n2. valid padding, bad key length \u2192 `Invalid key size (N) for AES.`, disclosing `N`\n3. correct length, wrong key \u2192 `Invalid padding bytes.`\n4. the real key \u2192 plaintext\n\nCase 1 is reachable only where the linked library lacks implicit rejection:\nOpenSSL 3.0 and 3.1, LibreSSL, and BoringSSL. On OpenSSL 3.2+, used in our wheels,\ninvalid padding instead returns a synthetic plaintext of\npseudorandom length, so the error channel does not distinguish conforming\nciphertexts.\n\nExploitation requires a service that auto-decrypts untrusted `EnvelopedData`\nmatching the victim certificate and answers adaptively at high volume, such as\nan S/MIME gateway or mail filter.\n\n### Fix\n\nPer RFC 3218, the content-encryption algorithm is now resolved before the\nprivate key is used, so the expected key length is known in advance. If the RSA\ndecryption fails or recovers a key of the wrong length, a random key of the\nexpected length is substituted and decryption continues down an identical path.\nAll failures now report identically and perform the same work.\n\n### Not addressed by this fix\n\n`EnvelopedData` does not authenticate its content. Tampering with\n`encryptedContent` alone yields a CBC padding oracle that recovers plaintext at\nroughly 256 queries per byte, without recovering any key, on every backend. This\nis a property of PKCS#7 rather than of this implementation, cannot be fixed in\nthe library, and is now documented.\n\n### Credit\n\nReported by @X1AOxiang.",
  "id": "GHSA-g6cj-pr64-35w5",
  "modified": "2026-08-03T21:17:00Z",
  "published": "2026-08-03T21:17:00Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/pyca/cryptography/security/advisories/GHSA-g6cj-pr64-35w5"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pyca/cryptography/pull/15369"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pyca/cryptography/commit/53fccd93413a8d7f07d6d8999681f27b75cffa3f"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/pyca/cryptography"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "cryptography: PKCS#7 EnvelopedData decryption exposes a Bleichenbacher oracle through distinguishable errors and timing"
}



Log in or create an account to share your comment.




Tags
Taxonomy of the tags.





Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

Author Source Type Date Other

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.