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// CVE · PUBLISHED 2026-10-02

CVE-2026-63569

Legion of the Bouncy Castle Inc. / bc-csharp

9.1 · CVSS 4.0Critical

Description

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.

References

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This product uses the NVD API but is not endorsed or certified by the NVD. CVSS scores do not establish exploitation.