CVE-2026-42768

/// Technical description

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 (Ble

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References