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CVE-2026-63569

6.5 · MEDIUM

Improper input validation in DHAgreement.CalculateAgreement (MTI/A0 two-pass Diffie-Hellman) in Legion of the Bouncy Castle Inc.

Published Oct 2, 2026bouncycastleCWE-20EPSS 0.33%

Overview

CVE-2026-63569 is a medium-severity vulnerability affecting bouncycastle bc-csharp. It was published on October 2, 2026 and has a CVSS 3.1 base score of 6.5 (MEDIUM).

This vulnerability has a CVSS 3.1 base score of 6.5, rated MEDIUM. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories and patches from bouncycastle. Update bc-csharp to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

Affected products

VendorProductVersionsStatus
bouncycastlebc-csharp>= 0, < 2.7.0Affected

References

GitHubgithub.com/bcgit/bc-csharp/commit/fe236ad207c5960eeae1ebb6560c783a3bb5b692GitHubgithub.com/bcgit/bc-csharp/wiki/CVE-2026-63569

How CyberStrike helps

Frequently asked questions

What is CVE-2026-63569?

CVE-2026-63569 is a medium-severity vulnerability affecting bouncycastle bc-csharp. It was published on October 2, 2026 and has a CVSS 3.1 base score of 6.5 (MEDIUM).

How severe is CVE-2026-63569?

This vulnerability has a CVSS 3.1 base score of 6.5, rated MEDIUM. It can be exploited remotely over the network. Some level of privileges is required for exploitation.

How do I fix or remediate CVE-2026-63569?

Check the references section for vendor advisories and patches from bouncycastle. Update bc-csharp to the latest patched version. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2026-63569?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2026-63569 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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