OpenSSL Covert Channel Vulnerability - CVE-2026-42768

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 victim39s vulnerable application as a way to decrypt or sign messages with the victim39s 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 victim39s public key the second with an arbitrary probe ciphertext obtains opportunity to iterate the 2nd KTRI to get a valid PKCS1 v1.5 padding if the error code of the application is available. That is a Bleichenbacher oracle (Bleichenbacher CRYPTO 3998): an adaptive-chosen-ciphertext side channel from which the attacker decrypts any RSA ciphertext to the victim39s key or forges any PKCS1 v1.5 signature under it. 2. When the decryption API (CMS_decrypt() PKCS7_decrypt()) is provided with the recipient certificate and the recipient is not found a random key is substituted. An attacker who authors a message and is able to compare both error code and the result of the decryption can mount a Bleichenbacher oracle. We are not aware of any applications that provide a remote attacker an opportunity to mount an attack described in these scenarios. We consider the existence of such application very unlikely and for this reason this CVE has been evaluated as Low severity. To avoid these attacks when RSA PKCS1 v1.5 Key Transport is in use the invoked EVP_PKEY_decrypt() will use the implicit rejection mechanism described in draft-irtf-cfrg-rsa-guidance. In previous OpenSSL releases the implicit rejection was explicitly disabled. The implicit rejection mechanism always returns a plaintext value the symmetric key. This result is deterministic for the ciphertext and the private key. The length of the decryption result can happen to match the length of the key of the symmetric cipher that was used for the content encryption. When a certificate is not provided the last RecipientInfo producing a key that looks valid will be used. It may cause getting garbage content on decryption. As a proper way to deal with this a recipient certificate has to be provided to identify the particular RecipientInfo for decryption. The FIPS modules in 4.0 3.6 3.5 and 3.4 are not affected by this issue as CMS and S/MIME processing happens outside the OpenSSL FIPS module boundary.