{"id":"CVE-2026-63569","summary":"MTI/A0 DHAgreement does not validate the peer's ephemeral value","details":"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.","modified":"2026-10-04T02:46:44.485680945Z","published":"2026-10-02T07:00:23.526Z","database_specific":{"cna_assigner":"bcorg","cwe_ids":["CWE-20"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63569.json"},"references":[{"type":"WEB","url":"https://www.nuget.org/packages/BouncyCastle.Cryptography"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63569.json"},{"type":"ADVISORY","url":"https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63569"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63569"},{"type":"FIX","url":"https://github.com/bcgit/bc-csharp/commit/fe236ad207c5960eeae1ebb6560c783a3bb5b692"},{"type":"PACKAGE","url":"https://github.com/bcgit/bc-csharp"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/bcgit/bc-csharp","events":[{"introduced":"0"},{"fixed":"4007498b13582d90ee1eda5d9920c324428b98b3"},{"fixed":"fe236ad207c5960eeae1ebb6560c783a3bb5b692"}],"database_specific":{"source":["DESCRIPTION","REFERENCES"],"extracted_events":[{"introduced":"0"},{"fixed":"2.7.0"}]}}],"versions":["release-1.9.0","release-1.8.10","release-1.8.8","release-1.8.7","release-1.8.6","release-1.8.5","release-1.8.4","release-1.8.3","release-1.8.2","release-1.8.1","release-1.8.0","release-1.8.0-RC.3","release-1.8.0-RC.2","release-1.8.0-RC.1","release-1.8.0-beta.4","release-1.8.0-beta.3","release-1.8.0-beta.2","release-1.8.0-beta.1"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-63569.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"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"}]}