Legion of the Bouncy Castle bc-csharp up to 2.6.x DirectoryName Name-Constraint Check PkixNameConstraintValidator.WithinDNSubtree certificate validation

Summaryinfo

A vulnerability identified as problematic has been detected in Legion of the Bouncy Castle bc-csharp up to 2.6.x. This vulnerability affects the function PkixNameConstraintValidator.WithinDNSubtree of the component DirectoryName Name-Constraint Check. The manipulation leads to certificate validation. This vulnerability is traded as CVE-2026-63577. It is possible to initiate the attack remotely. There is no exploit available. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in Legion of the Bouncy Castle bc-csharp up to 2.6.x. It has been rated as problematic. This issue affects the function PkixNameConstraintValidator.WithinDNSubtree of the component DirectoryName Name-Constraint Check. The manipulation with an unknown input leads to a certificate validation vulnerability. Using CWE to declare the problem leads to CWE-295. The product does not validate, or incorrectly validates, a certificate. Impacted is integrity. The summary by CVE is:

Improper certificate validation in the directoryName name-constraint check (PkixNameConstraintValidator.WithinDNSubtree) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows an attacker who controls, or can have certificates issued by, a name-constrained intermediate CA to get certificates accepted by PKIX path validation whose subject distinguished name, or a directoryName subjectAltName, lies outside the CA's permitted subtrees, via a name that places other RDNs ahead of a copy of the permitted RDN sequence, because the check looks for the constraint's first RDN anywhere in the name and compares the remaining RDNs from that position, instead of requiring the constraint to be an initial prefix of the name as RFC 5280 sections 4.2.1.10 and 7.1 require.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-63577 since 07/17/2026. The exploitation is known to be difficult. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1587.003 for this issue.

Upgrading to version 2.7.0 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at CERT Bund (WID-SEC-2026-3713). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Affected

• Open Source Bouncy Castle

Productinfo

Vendor

• Legion of the Bouncy Castle

Name

• bc-csharp

Version

• 2.0
• 2.1
• 2.2
• 2.3
• 2.4
• 2.5
• 2.6

CPE 2.3info

• 🔒
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CPE 2.2info

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Entryinfo

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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