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

> An on-path attacker can bypass key authentication and potentially recover static private keys in MTI/A0 Diffie-Hellman implementations using this library.

- **Severity:** Critical 9.1 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 2 Oct 2026
- **Updated:** 2 Oct 2026
- **Also known as:** GHSA-m2m9-m35w-rfxg

## What is CVE-2026-63569?

CVE-2026-63569 is a critical vulnerability in Legion of the Bouncy Castle Inc. bc-csharp, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2026-63569?

It affects 1 product.


Products: Bouncycastle Bc-Csharp.

## How do I fix CVE-2026-63569?

No fix has been published for CVE-2026-63569 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-63569 being exploited?

There is no known exploitation of CVE-2026-63569 in the wild and no public exploit code.

## Other vulnerabilities in Legion of the Bouncy Castle Inc. bc-csharp

- [CVE-2026-103600](https://insights.sbomify.com/CVE-2026-103600/): Unbounded ASN.1 nesting depth causes process-terminating stack overflow (high 8.7)
- [CVE-2026-103603](https://insights.sbomify.com/CVE-2026-103603/): Unbounded HSS public key level count allows huge array allocation during signature verification (high 8.7)
- [CVE-2026-103604](https://insights.sbomify.com/CVE-2026-103604/): Quadratic-time escaping when converting X.509 distinguished names to strings (high 8.7)
- [CVE-2026-16000](https://insights.sbomify.com/CVE-2026-16000/): KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used (high 8.7)
- [CVE-2026-63566](https://insights.sbomify.com/CVE-2026-63566/): DTLS handshake reassembler allocates buffer from unchecked 24-bit length (high 8.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63569 and fix it if it is affected.

What it is: CVE-2026-63569 is a critical vulnerability in Legion of the Bouncy Castle Inc. bc-csharp, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-63569 in the wild and no public exploit code.

Affected product: Legion of the Bouncy Castle Inc. bc-csharp. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63569/ (Markdown: https://insights.sbomify.com/CVE-2026-63569/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63569
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63569
- OSV: https://osv.dev/vulnerability/CVE-2026-63569

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63569/
Summaries are generated and may contain mistakes; the official description above is the record.