Critical TLS Hostname Verification Bypass in Bouncy Castle Java BCJSSE - CVE-2026-59638

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.

Certificate name-constraint bypass in Bouncy Castle for Java allows a crafted certificate to escape X.509 permitted/excluded subtree restrictions by appending a trailing dot to an rfc822Name (email) or URI host, or by prepending RDNs ahead of a permitted directoryName. Any relying party using BC's PKIXNameConstraintValidator (via CertPathValidator/CertPathBuilder) to enforce name constraints can be tricked into trusting an identity a constrained CA was never authorized to issue, enabling impersonation and effective authentication bypass. There is no public exploit identified at time of analysis, but the vendor rates it 9.3 (Critical) and a fix is available.

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.

Memory exhaustion in Bouncy Castle for Java's OpenPGP parser enables unauthenticated remote denial of service via a crafted User Attribute subpacket with an oversized length header. All Java applications importing untrusted OpenPGP certificates using bc-java before 1.85, bc-lts-java before 2.73.12, or BC-FJA (FIPS) in affected series are vulnerable to pre-authentication JVM heap exhaustion. No KEV listing or public exploit has been confirmed at time of analysis, but the zero-complexity unauthenticated network attack vector and CVSS 4.0 score of 8.7 signal high operational priority for services that accept OpenPGP certificates from external sources.

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.

Bouncy Castle for Java's OpenPGP session-key decryption path re-exposes the Mister-Zuccherato adaptive-chosen-ciphertext oracle by failing to suppress the legacy CFB 'quick check' on the PKESK/session-key route, enabling an attacker with decryption oracle access to recover plaintext from SEIPD v1 encrypted messages. Affected are bc-java before 1.85, bc-lts-java before 2.73.12, and BC-FJA (bcpg-fips) across the 1.0.X, 2.0.X, and 2.1.X series - three distinct product lines used broadly in enterprise and open-source Java applications. No public exploit or CISA KEV listing has been identified at time of analysis; however, the attack technique is well-established in cryptographic literature dating to the Mister-Zuccherato paper.

CPU exhaustion in Bouncy Castle for Java's MLS (Messaging Layer Security) implementation allows a network attacker to trigger unbounded HKDF key-derivation iterations by supplying a crafted 32-bit generation counter in an MLS message. The hash-ratchet in GroupKeySet.get(int generation) advances one HKDF step per skipped generation without any upper bound, so a maximally crafted counter forces up to ~4.29 billion derivation steps on the receiving process. Scored CVSS 4.0 at 8.7 with High availability impact and no authentication prerequisites per vendor scoring; no public exploit code or KEV listing has been identified at time of analysis.

Unbounded recursion in Bouncy Castle's OER (Octet Encoding Rules) parser enables remote unauthenticated denial-of-service against any Java application processing IEEE 1609.2 V2X messages. Because the IEEE 1609.2 schema is cyclic - an Ieee1609Dot2Data structure can nest inside its own SignedData payload - a few kilobytes of crafted input drive the parser into unbounded recursive descent, terminating in a JVM StackOverflowError. Critically, parsing occurs before signature verification, so no authentication or trust relationship is needed to trigger the crash. No public exploit has been identified at time of analysis, but the patch commit exposes two additional heap-exhaustion vectors alongside the primary recursion flaw, all reachable from the same pre-verification code path.