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CVE-2026-42770 is an improper subgroup validation vulnerability in OpenSSL's EVP_PKEY_derive_set_peer() when processing DHX (X9.42) peer keys. The subgroup membership test Y^q ≡ 1 (mod p) uses the peer-supplied q rather than the local key's q, and subsequent domain-parameter comparison does not compare q. A malicious peer can retain the victim's p and g while supplying a small prime q = r dividing the local cofactor and a public value of order r. This confines the shared secret to r possible values, leaking the private exponent modulo r. Repeated exchanges with suitable subgroup orders and Chinese remainder theorem reconstruction can recover the private key through a Lim–Lee small-subgroup-confinement attack. Exposure principally involves long-lived static DHX keys in CMP registration-authority or certificate-authority deployments and bespoke interactive applications. Affected FIPS modules include the 4.0, 3.6, 3.5, 3.4, 3.1.2, and 3.0 versions.
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OpenSSL validates DHX subgroup membership using the peer-supplied q parameter without comparing it with the local key's q. Malicious peers can exploit small subgroups to recover a victim's private key through repeated exchanges. The described exposure is narrow, principally involving long-lived CMP RA/CA DHX keys and bespoke static-DHX protocols. OpenSSL FIPS modules 4.0, 3.6, 3.5, 3.4, and 3.0 are also affected.
OpenSSL validates DHX subgroup membership using an attacker-supplied q parameter without matching it to the local key's q. A malicious peer can conduct small-subgroup-confinement attacks to recover a victim's private key through repeated exchanges. The described attack surface primarily involves long-lived static DHX keys in CMP and bespoke applications. Unlike most other listed flaws, this issue affects the listed OpenSSL FIPS modules.
A vulnerability addressed by the referenced Alma Linux 9.2 security update; no technical flaw details are provided.
An OpenSSL-related vulnerability addressed by the EulerOS_SA-2026-3812 security advisory.
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Every observed campaign linking this CVE to a named adversary.
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Cross-references every affected SKU, including bundled OEM variants.
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