Potential Subgroup Membership Issue in OpenSSL's DHX Key Derivation

CVE-2026-42770

What is CVE-2026-42770?

A vulnerability exists in OpenSSL when using the EVP_PKEY_derive_set_peer() function with DHX (X9.42) peer keys. The system fails to properly verify subgroup membership, which can be exploited by a malicious actor presenting a forged key. This leads to the potential exposure of a victim's private key after a limited series of key exchange attempts. The attack hinges on using domain parameters that match those of the victim, thus bypassing essential checks that would normally prevent unauthorized key recovery. The risk is primarily associated with CMP deployments and specialized applications employing long-lived DHX keys.

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