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21Track

Multi-RecipientInfo Bleichenbacher Oracle in CMS_decrypt() and PKCS7_decrypt()

OpenSSL · CVE-2026-42768
0 news outlets0 CERTs and agencies0 researchers

Assessment: Track

SSVC decision points: exploitation none, automatable no, technical impact partial, deployment high.

What is it?

Issue summary: The CMS_decrypt and PKCS7_decrypt functions are vulnerable to Bleichenbacher-style attack when an attacker is able to provide the CMS or S/MIME messages and observe the error code and/or decryption output. Impact summary: The Bleichenbacher-style attack allows an attacker to use the victim's vulnerable application as a way to decrypt or sign messages with the victim's private RSA key. The attack is possible in 2 variants. 1. The decryption API (CMS_decrypt(), PKCS7_decrypt()) is used without providing the recipient certificate. In this case OpenSSL iterates over every KeyTransRecipientInfo (KTRI) without stopping at the first success. An attacker who authors a message with two KTRI entries — the first one wrapping a real CEK under the victim's public key, the second with an arbitrary probe ciphertext — obtains opportunity to iterate the 2nd KTRI to get a valid PKCS#1 v1.5 padding if the error code of the application is available. That is a Bleichenbacher oracle (Bleichenbacher, CRYPTO '98): an adaptive-chosen-ciphertext side channel from which the attacker decrypts any RSA ciphertext to the victim's key or forges any PKCS#1 v1.5 signature under it. 2. When the

Evidence

No coverage recorded yet.

Exploitation

  • Public proof of concept
  • Exploited in the wild
  • Listed as known exploited
CVSS
3.7
EPSS
0.8%
First seen
2 d ago
Status
corroborated

All OpenSSL vulnerabilities