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Bouncy Castle CVE-2026-59638

| EUVDEUVD-2026-52016 CRITICAL
Improper Validation of Certificate with Host Mismatch (CWE-297)
2026-08-03 bcorg GHSA-8rxj-3p7p-rq39
Java Information Disclosure Bc Java Bc Lts Java Bc Fja
Critical
Disputed · 9.3 Vendor: bcorg
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Severity by source

Sources disagree (Medium–Critical)
Vendor (bcorg) PRIMARY
9.3 CRITICAL
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber
vuln.today AI
7.4 HIGH

Hostname-verification bypass is inherently MITM-gated and needs a crafted SAN-less CN-matching cert, so AC:H; no auth or user interaction (PR:N/UI:N); interception yields high confidentiality and integrity, no availability impact.

3.1 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
4.0 AV:N/AC:H/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
SUSE
MEDIUM
qualitative

vuln.today treats the vendor’s rating as authoritative. A higher third-party CVSS (e.g. CISA-ADP) is shown for transparency but does not drive the headline severity.

CVSS VectorVendor: bcorg

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber
Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
X

Lifecycle Timeline

3
Source Code Evidence Fetched
Aug 03, 2026 - 01:38 vuln.today
Analysis Generated
Aug 03, 2026 - 01:38 vuln.today
CVE Published
Aug 03, 2026 - 00:57 cve.org
CRITICAL 9.3

DescriptionCVE.org

In Bouncy Castle for Java before 1.85, JSSE hostname verifier CN-fallback enabled by default despite documented opt-in. This issue also affects Bouncy Castle for Java LTS before 2.73.12, and Bouncy Castle for Java FIPS (BC-FJA) before bctls-fips 1.0.24 (1.0.X series), 2.0.24 (2.0.X series) and 2.1.24 (2.1.X series).

Articles & Coverage 1

News Critical TLS Hostname Verification Bypass in Bouncy Castle Java BCJSSE - CVE-2026-59638 Aug 03, 2026

AnalysisAI

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). …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

Access
Gain man-in-the-middle position on client-server path
Delivery
Obtain SAN-less cert with CN naming target host
Exploit
Present cert during TLS handshake to BCJSSE client
Execution
CN-fallback default-on accepts host mismatch
Persist
Establish intercepted TLS session
Impact
Read and modify confidential traffic
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Vulnerability AssessmentAI

Exploitation Exploitation requires (1) the target application to use the Bouncy Castle JSSE provider (BCJSSE) for TLS with hostname/endpoint identification enabled - not the stock JDK JSSE; (2) an active man-in-the-middle / traffic-interception position between client and legitimate server; and (3) a certificate that validates up the trust chain but contains no matching SAN dNSName, whose Subject CN equals the victim host - practically achievable where a dNSName-name-constrained subordinate CA can mint a SAN-less leaf, since X.509 name constraints only constrain SAN entries of the constrained type. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The signals are mixed and warrant nuance. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario An attacker on the network path between a Java client (using BCJSSE) and its intended TLS server - for example on a shared Wi-Fi, a compromised router, or via BGP/DNS redirection - presents a certificate that chains to a trusted (possibly name-constrained) CA but carries no SAN, with the Subject CN set to the target hostname. Because CN fallback is on by default, HostnameUtil accepts the mismatch, the TLS handshake completes, and the attacker transparently intercepts and can modify the supposedly authenticated traffic. …
Remediation Vendor-released patch: upgrade to Bouncy Castle for Java 1.85 or later; for the LTS line upgrade to 2.73.12 or later; for BC-FJA (FIPS) upgrade the TLS module bctls-fips to 1.0.24 (1.0.X), 2.0.24 (2.0.X) or 2.1.24 (2.1.X) as appropriate to your series. … Detailed patch versions, workarounds, and compensating controls in full report.
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Recommended ActionAI

Within 24 hours, identify all applications, services, and dependencies using Bouncy Castle versions earlier than BC-Java 1.85, BC-LTS 2.73.12, or BC-FJA (FIPS) 1.0.24, 2.0.24, and 2.1.24 respectively; this includes internal applications, third-party products, and build pipelines. …

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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Affected
SUSE Linux Enterprise Module for Development Tools 15 SP7 Affected
SUSE Linux Enterprise Server 15 SP7 Affected
SUSE Linux Enterprise Server 16.0 Affected

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CVE-2026-59638 vulnerability details – vuln.today
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