CVE-2026-42770
LowCVSS 3.7Exploitation Probability (EPSS)
Low risk18th percentile - higher than 18% of all known CVEs
Summary
The vulnerability in OpenSSL affects the EVP_PKEY_derive_set_peer() function when using a DHX (X9.42) peer key. A malicious peer can exploit the lack of proper subgroup membership check, leading to the recovery of the victim's private key after a few key exchange attempts.
Risk Assessment
The risk for the organization is the potential recovery of the private key by an attacker in scenarios with long-lived DHX keys, especially in CMP environments or government applications. This could compromise communication confidentiality.
Recommendation
It is recommended to immediately update OpenSSL to a patched version that includes a fix for CVE-2026-42770. If an update is not possible, limit the use of DHX X9.42 keys in interactive protocols.
Original NVD description (English source)
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 priv mod r. Repeating for each small-prime factor of the cofactor and combining via CRT recovers the full private key (Lim–Lee / small-subgroup-confinement attack). The realistic attack surface is narrow: principally CMP deployments with long-lived RA/CA DHX keys and bespoke enterprise or government applications using X9.42 DHX static keys with interactive protocols and therefore this issue was assigned Low severity. The FIPS modules in 4.0, 3.6, 3.5, 3.4, 3.1.2 and 3.0 are affected by this issue.