CVE-2026-5588Network attack vector

Signature verification bypass via empty COMPOSITE sequence

Published Apr 15, 2026 · Updated Apr 15, 2026

Improper signature verification in affected Bouncy Castle Java releases allows attackers to pass an empty draft COMPOSITE signature as valid. JcaContentVerifierProviderBuilder's generic composite verifier iterates over ASN.1 sequence elements but returned true for an empty sequence because its failure flag remained false. Exploitation requires an application to verify attacker-controlled content with the early-draft test OID; acceptance bypasses authenticity validation for that content.

CVSS severity6.3
Medium
EPSS probability0.64%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper signature verification in affected Bouncy Castle Java releases allows attackers to pass an empty draft COMPOSITE signature as valid. JcaContentVerifierProviderBuilder's generic composite verifier iterates over ASN.1 sequence elements but returned true for an empty sequence because its failure flag remained false. Exploitation requires an application to verify attacker-controlled content with the early-draft test OID; acceptance bypasses authenticity validation for that content.

The record

CVE
CVE-2026-5588
Published
Apr 15, 2026
Updated
Apr 15, 2026
Vendor
Legion of the Bouncy Castle Inc.
Product
Bouncy Castle Java
Classifications
CWE-327, T1553
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=1.67 <1.84

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Legion of the Bouncy Castle Inc. · Bouncy Castle Javaversion=1.67 <1.84

Published CVSS scores

7.5Red Hat Product SecurityCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
6.3Legion of the Bouncy Castle Inc.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

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-327Use of a Broken or Risky Cryptographic Algorithm
T1553Subvert Trust Controls

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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