# CVE-2026-63576: Bouncy Castle C# improper certificate validation in URI name constraints

Source: https://junglewise.ai/threats/cve-2026-63576-improper-certificate-validation-in-pkixnameconstraintvalidator  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-02. Updated: 2026-10-05  
License: CC BY 4.0 with a link back

CVE-2026-63576 · Severity: info · Published 2026-10-02

Bouncy Castle C# is a cryptography library used to validate X.509 digital certificates. This vulnerability allows attackers with access to a subordinate Certificate Authority to bypass name constraints by crafting URIs with special characters, potentially enabling them to impersonate legitimate domains and compromise certificate-based security checks.

Technologies: [Legion of the Bouncy Castle BouncyCastle.Cryptography](https://junglewise.ai/threats/technologies/legion-of-the-bouncy-castle-bouncycastle-cryptography). Vendors: [Legion of the Bouncy Castle](https://junglewise.ai/threats/vendors/legion-of-the-bouncy-castle).

## Quick answers

**Is CVE-2026-63576 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-63576 being exploited in the wild.

**How severe is CVE-2026-63576?** Informational severity, published 2 October 2026.

**Which products and versions are affected?** Legion of the Bouncy Castle BouncyCastle.Cryptography before 2.7.0.

**Is there a fix for CVE-2026-63576?** Yes: fixed in BouncyCastle.Cryptography 2.7.0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-63576 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-63576?** Check whether you run Legion of the Bouncy Castle BouncyCastle.Cryptography at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in BouncyCastle.Cryptography 2.7.0

## Technical details

The PkixNameConstraintValidator's ExtractHostFromURL function improperly parses URIs by using string slicing without first isolating the RFC 3986 authority component, allowing characters like '@' or ':' in the URI's path, query, fragment, or userinfo to cause the extracted hostname to differ from the actual host. This enables bypassing of uniformResourceIdentifier name constraints during certification path validation. A name-constrained subordinate CA can obtain certificates with crafted subjectAltName URIs to exploit this parsing flaw.

## Affected products

- Legion of the Bouncy Castle BouncyCastle.Cryptography before 2.7.0

## Timeline

- 2026-07-02: patched
- 2026-10-02: advisory

## References

- [https://www.nuget.org/packages/BouncyCastle.Cryptography](https://www.nuget.org/packages/BouncyCastle.Cryptography)
- [https://github.com/bcgit/bc-csharp/commit/f196a22bbf7765cc19b0c4f4645c74ea7ad35e09](https://github.com/bcgit/bc-csharp/commit/f196a22bbf7765cc19b0c4f4645c74ea7ad35e09)

## Related threats

- [Bouncy Castle Cryptography unbounded resource allocation in password-based key decryption](https://junglewise.ai/threats/cve-2026-63578-allocation-of-resources-without-limits-in-password-based-private)
- [Bouncy Castle improper certificate name-constraint validation](https://junglewise.ai/threats/cve-2026-63577-improper-certificate-validation-in-the-directoryname-name)
- [Bouncy Castle C# PKCS#12 key derivation denial of service](https://junglewise.ai/threats/cve-2026-63575-loop-with-unreachable-exit-condition-in-the-pkcs-12-key)
- [Bouncy Castle memory exhaustion in OpenPGP subpacket parsing](https://junglewise.ai/threats/cve-2026-63574-memory-allocation-with-excessive-size-value-in-the-openpgp)
- [Bouncy Castle RSA PKCS#1 v1.5 padding oracle in CMS key-transport](https://junglewise.ai/threats/cve-2026-63573-observable-discrepancy-in-the-cms-rsa-pkcs-1-v1-5-key-transport)