CVE-2026-63576: CWE-295 Improper Certificate Validation in Legion of the Bouncy Castle Inc. bc-csharp
Improper certificate validation in PkixNameConstraintValidator (ExtractHostFromURL) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a name-constrained subordinate CA, or anyone able to obtain certificates with chosen subjectAltName URIs from such a CA, to bypass permitted or excluded uniformResourceIdentifier name constraints during certification path validation via a URI whose path, query, fragment or userinfo contains characters such as '@' or ':', because the host was extracted by string slicing without first isolating the RFC 3986 authority component, so the host compared against the constraints could differ from the URI's actual host.
AI Analysis
Technical Summary
The vulnerability CVE-2026-63576 affects the bc-csharp library from Legion of the Bouncy Castle Inc. in versions before 2.7.0. It arises from improper certificate validation in the PkixNameConstraintValidator, where the ExtractHostFromURL function extracts the host from a URI by string slicing without isolating the authority component as defined in RFC 3986. This allows a name-constrained subordinate CA or anyone able to obtain certificates with chosen subjectAltName URIs from such a CA to bypass permitted or excluded uniformResourceIdentifier name constraints during certification path validation. The bypass is possible when the URI's path, query, fragment, or userinfo contains characters such as '@' or ':', which can cause the extracted host to differ from the actual host, undermining the validation process.
Potential Impact
An attacker who can obtain certificates with crafted subjectAltName URIs from a name-constrained subordinate CA can bypass URI name constraints during certificate path validation. This could allow unauthorized certificates to be accepted as valid, potentially undermining trust in the certificate validation process and enabling man-in-the-middle or impersonation attacks in systems relying on bc-csharp for certificate validation.
Mitigation Recommendations
A fix is available in bc-csharp version 2.7.0. Users should upgrade to version 2.7.0 or later to remediate this vulnerability. No other mitigation guidance is provided or necessary as the issue is resolved by the official patch.
CVE-2026-63576: CWE-295 Improper Certificate Validation in Legion of the Bouncy Castle Inc. bc-csharp
Description
Improper certificate validation in PkixNameConstraintValidator (ExtractHostFromURL) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a name-constrained subordinate CA, or anyone able to obtain certificates with chosen subjectAltName URIs from such a CA, to bypass permitted or excluded uniformResourceIdentifier name constraints during certification path validation via a URI whose path, query, fragment or userinfo contains characters such as '@' or ':', because the host was extracted by string slicing without first isolating the RFC 3986 authority component, so the host compared against the constraints could differ from the URI's actual host.
CVSS v4.0
Score 8.2high
Affected software
Legion of the Bouncy Castle Inc.
bc-csharp
pkg:nuget/BouncyCastle.CryptographyRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The vulnerability CVE-2026-63576 affects the bc-csharp library from Legion of the Bouncy Castle Inc. in versions before 2.7.0. It arises from improper certificate validation in the PkixNameConstraintValidator, where the ExtractHostFromURL function extracts the host from a URI by string slicing without isolating the authority component as defined in RFC 3986. This allows a name-constrained subordinate CA or anyone able to obtain certificates with chosen subjectAltName URIs from such a CA to bypass permitted or excluded uniformResourceIdentifier name constraints during certification path validation. The bypass is possible when the URI's path, query, fragment, or userinfo contains characters such as '@' or ':', which can cause the extracted host to differ from the actual host, undermining the validation process.
Potential Impact
An attacker who can obtain certificates with crafted subjectAltName URIs from a name-constrained subordinate CA can bypass URI name constraints during certificate path validation. This could allow unauthorized certificates to be accepted as valid, potentially undermining trust in the certificate validation process and enabling man-in-the-middle or impersonation attacks in systems relying on bc-csharp for certificate validation.
Mitigation Recommendations
A fix is available in bc-csharp version 2.7.0. Users should upgrade to version 2.7.0 or later to remediate this vulnerability. No other mitigation guidance is provided or necessary as the issue is resolved by the official patch.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- bcorg
- Date Reserved
- 2026-07-16T23:51:51.306Z
- Cvss Version
- 4.0
- State
- PUBLISHED
Threat ID: 6abf5de7a43b0b3b8988eab0
Added to database: 10/02/2026, 07:31:51 UTC
Last enriched: 10/02/2026, 07:46:12 UTC
Last updated: 10/02/2026, 21:45:56 UTC
Views: 14
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