CVE-2026-71889: PKIXCertPathReviewer does not apply X.509 name constraints to the target certificate
In Bouncy Castle for Java before 1.86, neither copy of PKIXCertPathReviewer - org.bouncycastle.pkix.jcajce.PKIXCertPathReviewer nor the legacy org.bouncycastle.x509.PKIXCertPathReviewer - applied X.509 name constraints to the end-entity certificate. checkNameConstraints walked the path with a loop bound of index greater than zero, which is the bound the CA-only steps require, but index zero is the target certificate under the standard CertPath ordering, so the permitted and excluded subtree checks of RFC 5280 sec. 6.1.3 (b) and (c) never ran against the leaf's subject DN or its subjectAltName. A chain whose leaf violated a NameConstraints extension imposed by its own issuing CA therefore reported isValidCertPath() true with an empty error list, while CertPathValidator.getInstance("PKIX", "BC"), which shares no code with the reviewer, rejected the identical chain against the identical trust anchor. An application using the reviewer to make the trust decision rather than for diagnostics alongside a real validation accepted a certificate the constrained CA was never authorised to issue. Both copies now check every certificate in the path including the target, waive the sec. 4.2.1.10 self-issued exemption for the final certificate as sec. 6.1.3 requires, and skip the sec. 6.1.4 (g) constraint-accumulation step for the target. This issue also affects Bouncy Castle for Java LTS before 2.73.13, which carries only the org.bouncycastle.pkix.jcajce copy of the reviewer. It also affects Bouncy Castle for Java FIPS (BC-FJA) before bcpkix-fips 1.0.13 (1.0.X series), 2.0.13 (2.0.X series) and 2.1.13 (2.1.X series).
Affected Software
Event History
Frequently Asked Questions
Which applications are exposed to this issue?
Applications are exposed if they use either org.bouncycastle.pkix.jcajce.PKIXCertPathReviewer or org.bouncycastle.x509.PKIXCertPathReviewer to make a certificate trust decision. Applications using CertPathValidator.getInstance("PKIX", "BC") for validation are not affected by this reviewer-specific behavior.
What must an attacker provide to exploit the issue?
The attacker needs to present a certificate chain whose target certificate violates permitted or excluded X.509 NameConstraints imposed by its issuing CA. The application must then rely on PKIXCertPathReviewer reporting the path as valid rather than performing real PKIX validation.
How can I determine whether an application is affected?
Review code for use of either PKIXCertPathReviewer class and determine whether isValidCertPath() or its error list is used to accept or reject a chain. Affected behavior is indicated when such a reviewer accepts a chain whose leaf violates the issuing CA's NameConstraints, while the BC PKIX CertPathValidator rejects the same chain and trust anchor.
What can be done before upgrading?
Do not use PKIXCertPathReviewer as the trust-decision mechanism. Validate certificate paths with CertPathValidator.getInstance("PKIX", "BC") instead, and retain the reviewer only for diagnostics if needed.