Critical Certificate Name-Constraint Bypass in Bouncy Castle for Java - CVE-2026-8763

Certificate revocation-check bypass in the Bouncy Castle for Java (bc-java) PKIX provider allows a validly-signed OCSP response issued for one certificate to be accepted as proof-of-status for a different certificate, because the stapled response is not bound to the CertID of the certificate actually being checked. This means a revoked end-entity certificate can pass PKIXRevocationChecker validation when a 'good' OCSP response for another serial (e.g. the CA's) is supplied, defeating revocation enforcement in TLS/PKI clients that rely on OCSP stapling. Affected releases are bc-java before 1.85, bc-lts-java before 2.73.12, and BC-FJA (FIPS) before 2.0.2 and 2.1.3; no public exploit identified at time of analysis and it is not listed in CISA KEV.

TLS hostname-verification bypass in the Bouncy Castle for Java JSSE provider (BCJSSE) ships the legacy certificate-CN fallback enabled by default, even though the vendor documentation states it is opt-in and gated behind the org.bouncycastle.jsse.client.checkCnFallback property. Affected are BC-Java before 1.85, BC-LTS before 2.73.12, and BC-FJA (FIPS) TLS module bctls-fips before 1.0.24, 2.0.24 and 2.1.24; a MITM-positioned attacker can present a SAN-less certificate whose Subject CN names the victim host and have it silently accepted, defeating name-constraint protections and enabling interception of TLS traffic (CWE-297). This carries a CVSS 4.0 base of 9.3, but there is no public exploit identified at time of analysis and it is not in CISA KEV.

Private key recovery in Bouncy Castle for Java (bc-java before 1.85, bc-lts-java before 2.73.12) affects the DH agreement code path, where a peer-supplied public value is exponentiated with the local private key without full range/subgroup validation. A malicious protocol peer can submit small-order or out-of-range Diffie-Hellman elements to mount a small-subgroup confinement attack and, when the local private key is static/reused, gradually recover it via CRT - enabling decryption and impersonation. No public exploit identified at time of analysis; the fix is a vendor-released patch and the vulnerability is not listed in CISA KEV.

CCMBlockCipher in Bouncy Castle for Java decrypts ciphertext directly into the caller-supplied output buffer before verifying the authentication tag, leaving unverified CTR-decrypted plaintext in memory when tag verification fails and violating NIST SP 800-38C §6.2. Every edition - bc-java before 1.85, bc-lts-java before 2.73.12, and BC-FJA across three active release lines (1.0.x, 2.0.x, 2.1.x) - is affected. Applications that reuse pooled byte buffers, log failure-path output, or otherwise expose the buffer on an authentication failure inadvertently become CTR decryption oracles, allowing attackers to submit forged ciphertexts and recover plaintext without possessing a valid authentication tag. No public exploit and no CISA KEV listing exist at time of analysis.

Uncontrolled memory allocation in Bouncy Castle for Java's HSS public-key parser allows remote unauthenticated attackers to exhaust JVM heap memory by supplying a crafted public key with an arbitrarily large HSS level count (L value), causing denial of service during signature verification. All three product lines are affected: bc-java before 1.85, bc-lts-java before 2.73.12, and BC-FIPS (bc-fja) before 2.0.2/2.1.3 depending on series. No public exploit has been identified at time of analysis and no CISA KEV listing exists, but the AV:N/AC:L/PR:N attack profile and Bouncy Castle's ubiquity as a Java crypto dependency make this a credible availability risk for any service accepting external HSS/LMS key material.

CMS AuthenticatedData integrity verification in Bouncy Castle for Java is bypassable when authenticated attributes (authAttrs) are present, allowing an attacker to substitute arbitrary content while the recipient MAC check passes without error. Affected are bc-java before 1.85, bc-lts-java before 2.73.12, and BC-FJA across the 1.0.x, 2.0.x, and 2.1.x series - all widely deployed in Java enterprise and FIPS environments. The CVSS 4.0 vector (AV:N/AC:L/PR:N/UI:N/VI:H) reflects unauthenticated remote exploitation with high integrity impact; no public exploit or CISA KEV listing is confirmed at time of analysis.

CPU exhaustion in Bouncy Castle for Java allows remote attackers to cause denial of service by supplying X.500 distinguished name attribute values densely packed with RFC 4514 special characters or leading/trailing spaces. The root cause is a quadratic-time O(n²) escaping algorithm in `IETFUtils.valueToString()` that inserts backslashes into the buffer being actively scanned - triggerable through `X500Name.toString()`, `.equals()`, or `.hashCode()`, methods commonly invoked during TLS certificate and LDAP DN processing. No public exploit has been identified beyond the vendor's own regression test, and this vulnerability is not listed in CISA KEV, though the CVSS 4.0 score of 8.7 with AV:N/PR:N signals meaningful exposure for Internet-facing Java services.

OpenPGP inline-signature policy enforcement in Bouncy Castle for Java silently discards rejection exceptions, causing signatures that use prohibited weak hash algorithms (MD5, SHA-1, RIPEMD-160) or otherwise violate the configured OpenPGPPolicy to be reported as cryptographically valid. Affected are bc-java before 1.85 and bcpg-fips (bc-fja) before 2.0.13; the detached-signature and prefixed-signature verification paths are not affected - only the inline one-pass path. A vendor patch is available; no public exploit code has been identified at time of analysis, though the CVSS 4.0 score of 8.7 reflects a high integrity impact driven by the unauthenticated network vector.