CVE-2026-63569Network attack vectorHinoki check available

Key compromise via unchecked peer ephemeral value

Published Oct 2, 2026 · Updated Oct 2, 2026

Improper input validation in Bouncy Castle C# .NET before 2.7.0 allows attackers to compromise MTI/A0 key agreement via a crafted peer value. DHAgreement.CalculateAgreement raises the peer-supplied ephemeral value to the local static private key without checking its range or subgroup membership. Only applications that call DHAgreement directly are exposed: an on-path attacker can force a known agreed value, while a malicious peer can recover the static key in groups with enough small factors.

CVSS severityUnavailable
Unscored
EPSS probability0.40%
Next 30 days · Oct 19, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
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Summary

What happened

Improper input validation in Bouncy Castle C# .NET before 2.7.0 allows attackers to compromise MTI/A0 key agreement via a crafted peer value. DHAgreement.CalculateAgreement raises the peer-supplied ephemeral value to the local static private key without checking its range or subgroup membership. Only applications that call DHAgreement directly are exposed: an on-path attacker can force a known agreed value, while a malicious peer can recover the static key in groups with enough small factors.

The record

CVE
CVE-2026-63569
Published
Oct 2, 2026
Updated
Oct 2, 2026
Vendor
Legion of the Bouncy Castle Inc.
Product
Bouncy Castle C# .NET
Classifications
T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 2, 2026CVE publishedPublication date reported by the CVE source.
  2. Oct 2, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <2.7.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Legion of the Bouncy Castle Inc. · Bouncy Castle C# .NETversion=0 <2.7.0

Published CVSS scores

No CVSS assessment is available in this record.

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Activity data is delayed. The latest report is Sep 28, 2026.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 22, 2026Sep 28, 2026
Latest reporting daySep 28, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

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Technologies

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