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.
Metrics
Affected Vendors & Products
References
History
Sun, 04 Oct 2026 22:15:00 +0000
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Legion Of The Bouncy Castle Inc.
Legion Of The Bouncy Castle Inc. bc-csharp |
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| Vendors & Products |
Legion Of The Bouncy Castle Inc.
Legion Of The Bouncy Castle Inc. bc-csharp |
Fri, 02 Oct 2026 20:30:00 +0000
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ssvc
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Fri, 02 Oct 2026 07:15:00 +0000
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| 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. | |
| Title | MTI/A0 DHAgreement does not validate the peer's ephemeral value | |
| Weaknesses | CWE-20 | |
| References |
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| Metrics |
cvssV4_0
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Status: PUBLISHED
Assigner: bcorg
Published: 2026-10-02T07:00:23.526Z
Updated: 2026-10-02T19:07:59.005Z
Reserved: 2026-07-16T23:48:46.076Z
Link: CVE-2026-63569
Updated: 2026-10-02T19:07:22.966Z
Status : Awaiting Analysis
Published: 2026-10-02T07:16:37.430
Modified: 2026-10-02T19:16:41.913
Link: CVE-2026-63569
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