CVE-2026-42768
Published: 09 June 2026
Summary
CVE-2026-42768 is a low-severity Covert Channel (CWE-514) vulnerability in Openssl Openssl. Its CVSS base score is 3.7 (Low).
Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Credential Access (T1212); ranked at the 27.0th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
Deeper analysis
The vulnerability affects the CMS_decrypt and PKCS7_decrypt functions in OpenSSL when processing CMS or S/MIME messages that use RSA PKCS#1 v1.5 Key Transport. It stems from insufficient protection against Bleichenbacher-style adaptive chosen-ciphertext attacks, allowing an attacker-supplied message to trigger observable differences in error codes or decryption output during recipient iteration or key substitution.
An attacker able to supply crafted messages and observe application behavior can leverage the victim's private RSA key as a decryption or signature oracle in two scenarios: when the API is called without a recipient certificate, causing iteration over all KeyTransRecipientInfo entries, or when a certificate is supplied but the recipient is not matched, leading to substitution of a random key. This could enable decryption of arbitrary RSA ciphertexts or forgery of PKCS#1 v1.5 signatures, though the description states that no realistic remote exploitation paths in applications are known and rates the issue Low severity with a CVSS score of 3.7.
Advisories and patches referenced in the OpenSSL security notice and associated commits enable the implicit rejection mechanism in EVP_PKEY_decrypt for RSA PKCS#1 v1.5 operations, which returns a deterministic plaintext instead of leaking padding validity. They also recommend always supplying the recipient certificate to CMS_decrypt or PKCS7_decrypt to ensure correct RecipientInfo selection and avoid fallback behavior that could produce garbage output. The FIPS modules in versions 4.0, 3.6, 3.5, and 3.4 are unaffected because CMS and S/MIME processing occurs outside the module boundary.
The current EPSS score remains at 0.0001 with no reported real-world exploitation.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-35485
Vulnerability details
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…
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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 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 PKCS#1 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.
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Vulnerability enables private RSA key to be used as decryption/signature oracle via adaptive chosen-ciphertext attack on CMS/PKCS7 processing, directly facilitating credential or data access through cryptographic exploitation.
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Affected Assets
Mitigating Controls
No mitigating controls mapped yet. The per-CVE control annotator has not reached this CVE.