Improper Certificate Validation in Bouncy Castle’s bc-csharp Product
CVE-2026-63577

8.2HIGH

Key Information:

Vendor

Legion Of The Bouncy Castle Inc.

Status
Bc-csharp
Vendor
CVE Published:
2 October 2026

What is CVE-2026-63577?

The bc-csharp library from Legion of the Bouncy Castle Inc. contains an improper certificate validation flaw. Specifically, the name-constraint check, 'PkixNameConstraintValidator.WithinDNSubtree', does not enforce correct validation protocols as stipulated in RFC 5280. This vulnerability allows attackers, who control or can have certificates issued by a name-constrained intermediate CA, to have their certificates erroneously accepted by the PKIX path validation process. They can exploit this flaw by using a subject distinguished name or directoryName subjectAltName that falls outside the permitted subtrees, facilitated by arranging RDNs in a manner that undermines the intended constraints.

Affected Version(s)

bc-csharp 0 < 2.7.0

References

https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63577
vendor-advisory
https://github.com/bcgit/bc-csharp/commit/606e9153b97a265...
patch
https://github.com/bcgit/bc-csharp/commit/75c3c5766028861...
patch

CVSS V4

Score:
8.2
Severity:
HIGH
Confidentiality:
None
Integrity:
High
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
Physical
Privileges Required:
Undefined
User Interaction:
None

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Discovered by Claude, Anthropic's AI assistant, and triaged by the Anthropic security team in collaboration with Anthropic Research.
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