CVE-2026-63577: legion of the bouncy castle inc. bc-csharp Improper Certificate Validation

Vulnerability summary

CVE-2026-63577 is a high (CVSS 8.2) Improper Certificate Validation vulnerability in legion of the bouncy castle inc. bc-csharp. EPSS estimates a 0.24% chance of exploitation in the next 30 days, higher than 13.6% of scored vulnerabilities.

Timeline

1. 02-10-2026
   PublishedVulnerability first disclosed.
2. 06-10-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-295Improper Certificate ValidationThe product does not validate, or incorrectly validates, a certificate.