Bouncy Castle bc-csharp: Name constraints bypass via trailing dot in PKIX validation
Summary
Improper certificate validation in PkixNameConstraintValidator of bc-csharp allows certificates to be accepted for names inside an excluded subtree of a name-constrained intermediate CA. The bypass works when a rfc822Name, dNSName or URI host ends with a dot, because the trailing dot is not removed before comparison. An attacker must control or obtain certificates from such a name-constrained CA, so the precondition is narrow.
Affected
- bc-csharp (BouncyCastle.Cryptography NuGet package) before 2.7.0
- Vendor wiki lists BC C# .NET 2.6.2 and earlier, and the 2.7.0-beta.98 pre-release
- Applies to PkixCertPathValidator and PkixCertPathBuilder on all CLRs
Fixed in
- BC C# .NET 2.7.0 and later (fix released before the CVE was published on 2 Oct 2026; release date sources differ, 30 July or 7 Aug 2026)
Workaround
- No vendor workaround documented; upgrading is the remediation.
- Application-level alternative (untested): reject any email address, DNS name or URI host ending with a dot, or strip the dot and compare against excluded subtrees of the CA certificates in the path.
Exploitation
- CISA SSVC: exploitation none, automatable no, technical impact partial.
- No known exploitation in the wild and no public proof of concept found in the reviewed sources; CISA KEV status still to be verified directly.
Our recommendation
- Inventory .NET applications and NuGet references to BouncyCastle.Cryptography.
- Upgrade to bc-csharp 2.7.0 or later; do not use the 2.7.0-beta.98 pre-release.
- Re-test certificate validation for applications that rely on excluded subtrees, especially those using third-party or sub-CAs.
- Check any Java-based services for the related BC Java issue CVE-2026-8763, fixed in BC Java 1.85.
- Confirm the CISA KEV status before closing the ticket.
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- CVE-2026-103602 vendor wiki (Bouncy Castle) github.com
- Fix commit: removes trailing dot before comparison github.com
- Fix commit: rejects hosts with empty labels github.com
- Fix commit: regression tests github.com
- OSV entry CVE-2026-103602 osv.dev
- Bouncy Castle C# 2.7.0 release news www.bouncycastle.org
- BC Java 1.85 CVE summary github.com
- Bouncy Castle C# download page www.bouncycastle.org
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 1 Nov 2026, 17:32.
Original description (NVD)
Improper certificate validation in PkixNameConstraintValidator in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows an attacker who controls, or can obtain certificates from, a name-constrained intermediate CA to have certificates accepted during PKIX certification path validation for email addresses, DNS names or URI hosts that lie within excluded subtrees applying to that CA, via an rfc822Name, dNSName or uniformResourceIdentifier name whose host ends with a dot, because names and constraints were compared without first removing the RFC 1034 root-label trailing dot, so a fully qualified host name did not match an excluded subtree for the same host written without the dot.
NVD references (4)
- https://github.com/bcgit/bc-csharp/commit/75c3c576602886180ed92a63c89419e3bd63b392
- https://github.com/bcgit/bc-csharp/commit/be276e057775a6a674a62e9d9a2583b229978f73
- https://github.com/bcgit/bc-csharp/commit/f196a22bbf7765cc19b0c4f4645c74ea7ad35e09
- https://github.com/bcgit/bc-csharp/wiki/CVE-2026-103602
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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