CVE-2026-63569MTI/A0 DHAgreement does not validate the peer's ephemeral value

Improper input validation in DHAgreement.CalculateAgreement (MTI/A0 two-pass Diffie-Hellman) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows an on-path attacker to make the local party compute an agreed value the attacker already knows, defeating the key authentication MTI/A0 is meant to provide. It also allows a malicious peer to learn the local static private key modulo the small factors of p-1, and to recover it entirely in groups with many such factors. The attack uses a crafted out-of-range or small-order ephemeral value, and works because that value is raised to the static private key without the range and subgroup-membership checks applied to DH public keys. Only applications that call DHAgreement directly are affected.

CVSS

9.1 CRITICAL (v4.0, CNA)CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Weakness (CWE)

CWE-20

Known exploited

Not in CISA's KEV catalog

EPSS (exploitation in the next 30 days)

0.40% (percentile 32)

Assigned by

bcorg

NVD analysis

Undergoing Analysis

Record last updated

Oct 2, 2026

First seen by InfoSec Signals

Oct 9, 2026

The CVE record on cve.org · NVD's entry

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