CVE-2026-63577 Name Constraints bypass: directoryName constraint matched at any position in the DN instead of as a prefix
Attackers can bypass certificate name constraints in Bouncy Castle C# by placing permitted RDNs after other RDNs, allowing unauthorized certificates to pass validation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-63577?
CVE-2026-63577 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. bc-csharp, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-295
Which versions are affected by CVE-2026-63577?
It affects 1 product.
Products include Bouncycastle Bc-Csharp.
How do I fix CVE-2026-63577?
No fix has been published for CVE-2026-63577 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-63577 being exploited?
There is no known exploitation of CVE-2026-63577 in the wild and no public exploit code.
Other vulnerabilities in Legion of the Bouncy Castle Inc. bc-csharp
- CVE-2026-63569 MTI/A0 DHAgreement does not validate the peer's ephemeral value
- CVE-2026-103600 Unbounded ASN.1 nesting depth causes process-terminating stack overflow
- CVE-2026-103603 Unbounded HSS public key level count allows huge array allocation during signature verification
- CVE-2026-103604 Quadratic-time escaping when converting X.509 distinguished names to strings
- CVE-2026-16000 KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used
Sources
Also known as GHSA-437r-hr8h-5f45.