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Java CVE-2026-5588

| EUVDEUVD-2026-22871 MEDIUM
Use of a Broken or Risky Cryptographic Algorithm (CWE-327)
2026-04-15 bcorg GHSA-wg6q-6289-32hp
Java Information Disclosure Bc Java Legion Of The Bouncy Castle Inc
6.3
CVSS 4.0 · Vendor: bcorg
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Craft certificate with empty signature sequence
  2. Delivery
    Submit to PKIX CompositeVerifier
  3. Exploit
    Bypass signature validation
  4. Execution
    Accept forged certificate as valid
  5. Impact
    Establish unauthorized trust relationship

Vulnerability Assessment

Weakness
CWE
CWE-327
OWASP Top 10:2025
A04:2025 - Cryptographic Failures
ASVS 5.0
V11 - Cryptography Approved algorithm selection
Exploitation Conditions

Requires target application to utilize the experimental CompositeVerifier component within Bouncy Castle's bcpkix module for signature verification operations. This is NOT the default signature verification path in Bouncy Castle - applications must explicitly implement or configure composite signature scheme handling per draft IETF specifications. Standard PKIX certificate validation using production Bouncy Castle APIs (e.g., CertificateFactory, CertPathValidator) are not vulnerable. The flaw is specific to applications that have adopted the draft composite cryptography features, which are typically found in research implementations, cutting-edge PKI deployments experimenting with post-quantum cryptography, or systems explicitly configured for composite signature schemes. Once this specific API is
Impact

This represents an exceptionally severe cryptographic implementation flaw with maximum CVSS 4.0 score of 10.0 (AV:N/AC:L/AT:N/PR:N/UI:N with high impact across all confidentiality, integrity, and availability metrics for both vulnerable and subsequent systems). The attack vector is network-based with low complexity, requiring no privileges, no user interaction, and no attack prerequisites - indicating trivial exploitability once applications using the vulnerable CompositeVerifier are identified. Despite the critical CVSS score, real-world risk requires nuanced assessment: the vulnerability affects a draft implementation (CompositeVerifier) rather than the widely-used production signature verification APIs, potentially limiting exposure to applications explicitly adopting experimental composite signature schemes. No EPSS score provided in input data, and no CISA KEV listing indicates no confirmed active exploitation at time of analysis. However, the fundamental nature of the flaw (accepting empty signatures) means exploitation requires minimal sophistication - attackers need only submit null signature payloads to bypass validation. The 35-version span (1.49-1.83) suggests extensive deployment surface across legacy and current Java applications. Priority should be highest for applications using bcpkix for certificate validation in authentication flows, code signing verification, or secure communications protocols where signature forgery enables complete security bypass.
Exploit ScenarioDerived

An attacker identifies a Java-based API gateway using Bouncy Castle BC-JAVA 1.78 for client certificate authentication via the experimental CompositeVerifier. The attacker crafts an authentication request with a valid X.509 certificate but submits an empty signature sequence in the composite signature field. The vulnerable CompositeVerifier incorrectly validates the empty signature as legitimate, granting the attacker authenticated access without possessing the corresponding private key. The attacker leverages this authentication bypass to access restricted API endpoints, exfiltrate sensitive data, or inject malicious transactions under the identity of the forged certificate subject.
This Month Plan within 30 days. Why: MEDIUM severity (CVSS 4.0-6.9).
Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv1 – Apr 15, 2026

Signature verification bypass in Bouncy Castle BC-JAVA (bcpkix module versions 1.49-1.83) allows remote unauthenticated attackers to forge cryptographic signatures by submitting empty signature sequences that are incorrectly accepted as valid by the draft CompositeVerifier implementation. This critical flaw (CVSS 4.0: 10.0) enables complete subversion of digital signature trust chains, potentially allowing authentication bypass, code signing forgery, and man-in-the-middle attacks against Java ap

Earlier versions of this analysis (1)
v1 Apr 15, 2026 - 12:38 CVSS 6.3

Signature verification bypass in Bouncy Castle BC-JAVA (bcpkix module versions 1.49-1.83) allows remote unauthenticated attackers to forge cryptographic signatures by submitting empty signature sequences that are incorrectly accepted as valid by the draft CompositeVerifier implementation. This critical flaw (CVSS 4.0: 10.0) enables complete subversion of digital signature trust chains, potentially allowing authentication bypass, code signing forgery, and man-in-the-middle attacks against Java applications relying on Bouncy Castle for PKIX certificate validation. No public exploit identified at time of analysis, though the attack complexity is low (AC:L) requiring no special conditions.

RemediationAI

Immediately upgrade Bouncy Castle BC-JAVA to version 1.84 or later, which addresses the empty signature acceptance flaw in the CompositeVerifier implementation. Organizations should inventory all Java applications and dependencies for BC-JAVA versions 1.49-1.83, prioritizing internet-facing services and applications performing certificate validation or signature verification. Update dependencies in build configurations (Maven pom.xml, Gradle build files) to bc-java:1.84+ and redeploy affected applications. If immediate upgrade is blocked by compatibility testing requirements, implement temporary workaround by avoiding use of the draft CompositeVerifier API and reverting to production-grade signature verification methods in Bouncy Castle's standard PKIX implementation. Conduct post-remediation verification to confirm no historical signature forgery has occurred by reviewing authentication logs and certificate validation audit trails for anomalous patterns. Complete vendor security advisory and patch details available at https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%905588. No workarounds can fully mitigate the risk if CompositeVerifier is actively used in production code paths.

Affected VersionsENISA

BC-JAVA 1.49 <1.84

Affected Packages

maven org.bouncycastle:bcpkix-debug-jdk14 fix: 1.84
maven org.bouncycastle:bcpkix-debug-jdk15to18 fix: 1.84
maven org.bouncycastle:bcpkix-debug-jdk18on fix: 1.84
maven org.bouncycastle:bcpkix-jdk14 fix: 1.84
maven org.bouncycastle:bcpkix-jdk15on fix: 1.84
maven org.bouncycastle:bcpkix-jdk15to18 fix: 1.84
maven org.bouncycastle:bcpkix-jdk18on fix: 1.84
euvd bc_java fix: 1.84

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 8 packages in maven, euvd

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Technical ContextAI · show details

Bouncy Castle BC-JAVA is a widely-adopted cryptographic library providing Java implementations of cryptographic algorithms and PKIX (Public Key Infrastructure using X.509) certificate handling. The vulnerability resides in the bcpkix module's draft CompositeVerifier, which implements composite signature schemes per draft IETF standards. CWE-327 (Use of Broken or Risky Cryptographic Algorithm) classifies this as a cryptographic implementation flaw where the verifier incorrectly validates an empty signature sequence - a fundamental violation of cryptographic verification principles. In proper signature verification, an empty signature should always fail validation since it provides no cryptographic proof of origin or integrity. The CompositeVerifier's acceptance of null/empty signatures effectively bypasses the entire signature validation process, rendering cryptographic protections meaningless. Affected product CPE: cpe:2.3:a:legion_of_the_bouncy_castle_inc.:bc-java:*:*:*:*:*:*:*:* spanning versions 1.49 through 1.83 across all deployment platforms.

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Severity by source

Vendor (bcorg) PRIMARY 6.3 MEDIUM
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/U:Amber
SUSE 5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Red Hat 7.5 HIGH

Primary rating from Vendor (bcorg).

CVSS VectorVendor: bcorg

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
Present
Privileges Required
None
User Interaction
None
Vulnerable system: Confidentiality
Low
Vulnerable system: Integrity
None
Vulnerable system: Availability
None
Subsequent system: Confidentiality
None
Subsequent system: Integrity
None
Subsequent system: Availability
None

Lifecycle Timeline

5
Patch released
Apr 21, 2026 - 16:16 nvd
Patch available
Patch available
Apr 16, 2026 - 05:29 EUVD
1.84
Severity Changed
Apr 15, 2026 - 17:22 NVD
CRITICAL MEDIUM
CVSS changed
Apr 15, 2026 - 17:22 NVD
10.0 (CRITICAL) 6.3 (MEDIUM)
CVE Published
Apr 15, 2026 - 09:06 nvd
MEDIUM 6.3

DescriptionCVE.org

: Use of a Broken or Risky Cryptographic Algorithm vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA bcpkix on all (pkix modules).

PKIX draft CompositeVerifier accepts empty signature sequence as valid.

This issue affects BC-JAVA: from 1.49 before 1.84.

Executive ViewAI

Why It Matters

Bouncy Castle BC-JAVA (bcpkix versions 1.49-1.83) contains a critical signature verification bypass that allows attackers to forge cryptographic signatures and compromise digital trust chains in Java applications. Any organization using affected Bouncy Castle versions for certificate validation, code signing verification, or authentication faces immediate risk of signature forgery, authentication bypass, and man-in-the-middle attacks. This vulnerability requires urgent mitigation across all systems leveraging this library.

Business Impact: Severe
Compensating Controls

No vendor-released patch identified at time of analysis. Interim mitigations: (1) Isolate affected Java applications from untrusted networks until patch availability; (2) Implement secondary out-of-band signature validation mechanisms outside Bouncy Castle; (3) Deploy network segmentation to restrict certificate chain validation traffic; (4) Enable enhanced logging and alerting for all signature validation operations; (5) Consider temporary fallback to alternative PKIX libraries if operationally feasible; (6) Restrict deployment of new code signed by affected systems until patched.

Threat Intelligence Overview

EPSS Score
0.0%
Top 92% exploitable
Social Media
1
Sightings
Data Quality
8 sources
Confidence: High
EUVD NVD CVE.org EPSS Vulnrichment Vendor CIRCL GitHub VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity MEDIUM
CISA KEV No
Public POC No
EPSS 0.0%
Patch available Yes
Active threat No
Deciding rule
MEDIUM severity (CVSS 4.0-6.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision
Out-of-Cycle
Exploitation
None
System Exposure· derived
Open
Automatable
Yes
Human Impact
Very High
Technical Impact
Total

Priority Score

32
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +32
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.

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Vendor StatusVendor

SUSE

Severity: Medium
Product Status
SUSE Linux Enterprise High Performance Computing 15 SP4-ESPOS Affected
SUSE Linux Enterprise High Performance Computing 15 SP4-LTSS Affected
SUSE Linux Enterprise High Performance Computing 15 SP5-ESPOS Affected
SUSE Linux Enterprise High Performance Computing 15 SP5-LTSS Affected
SUSE Linux Enterprise Module for Development Tools 15 SP7 Affected

References

Advisories & patches
ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-5588 ADVISORY https://vuldb.com/vuln/357693 REF https://access.redhat.com/errata/RHSA-2026:11720 REF https://access.redhat.com/errata/RHSA-2026:11721 REF https://access.redhat.com/errata/RHSA-2026:13631 REF https://access.redhat.com/errata/RHSA-2026:14272 REF https://access.redhat.com/errata/RHSA-2026:14276 REF https://access.redhat.com/errata/RHSA-2026:17668 REF https://access.redhat.com/errata/RHSA-2026:18054 REF https://access.redhat.com/errata/RHSA-2026:18055 REF https://access.redhat.com/errata/RHSA-2026:18059 REF https://access.redhat.com/errata/RHSA-2026:21772 REF https://access.redhat.com/errata/RHSA-2026:53644 REF https://access.redhat.com/errata/RHSA-2026:53645 REF https://access.redhat.com/errata/RHSA-2026:53646 REF https://access.redhat.com/errata/RHSA-2026:53806 REF https://access.redhat.com/errata/RHSA-2026:60239 REF https://access.redhat.com/errata/RHSA-2026:60246 REF https://access.redhat.com/errata/RHSA-2026:60247 REF https://access.redhat.com/errata/RHSA-2026:60248 REF https://access.redhat.com/errata/RHSA-2026:60249 REF https://access.redhat.com/errata/RHSA-2026:60250 REF https://access.redhat.com/errata/RHSA-2026:60251 REF https://access.redhat.com/errata/RHSA-2026:60252 REF https://access.redhat.com/errata/RHSA-2026:60254 REF https://access.redhat.com/errata/RHSA-2026:60256 REF https://access.redhat.com/errata/RHSA-2026:60259 REF https://access.redhat.com/errata/RHSA-2026:66488 REF https://access.redhat.com/security/cve/CVE-2026-5588 REF https://bugzilla.redhat.com/show_bug.cgi?id=2458634 REF https://github.com/bcgit/bc-java/commit/656bae0dbd9b1521f840521ff786e78749fe3057 REF https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%905588 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-5588.json

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