CVE-2026-42770

/// Technical description

Issue summary: When EVP_PKEY_derive_set_peer() is called with a DHX (X9.42)

peer key, the peer key is not properly checked for the subgroup membership.

Impact summary: A malicious peer which presents an X9.42 key carrying the

victim's p and g parameters, a forged q = r (a small prime factor of the

cofactor (p−1)/q_local), and a public value Y of order r can recover the

victim's private key after a small number of key exchange attempts.

When EVP_PKEY_derive_set_peer() is called with a DHX (X9.42) peer key, the

subgroup membership check Y^q ≡ 1 (mod p) is performed using the peer's

own q parameter, not the local key's q. The peer's domain parameters are

then matched against the domain parameters of the private key, but the value

of q is not compared.

A malicious peer who presents an X9.42 key carrying the victim's p, g,

a forged q = r (a small prime factor of the cofactor), and a public

value Y of order r passes all checks. The shared secret then takes only

r distinct values, leaking

/// 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